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diff --git a/plugins/Pcre16/docs/doc/pcre.txt b/plugins/Pcre16/docs/doc/pcre.txt deleted file mode 100644 index 14cbb8bf2b..0000000000 --- a/plugins/Pcre16/docs/doc/pcre.txt +++ /dev/null @@ -1,10423 +0,0 @@ ------------------------------------------------------------------------------ -This file contains a concatenation of the PCRE man pages, converted to plain -text format for ease of searching with a text editor, or for use on systems -that do not have a man page processor. The small individual files that give -synopses of each function in the library have not been included. Neither has -the pcredemo program. There are separate text files for the pcregrep and -pcretest commands. ------------------------------------------------------------------------------ - - -PCRE(3) Library Functions Manual PCRE(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -INTRODUCTION - - The PCRE library is a set of functions that implement regular expres- - sion pattern matching using the same syntax and semantics as Perl, with - just a few differences. Some features that appeared in Python and PCRE - before they appeared in Perl are also available using the Python syn- - tax, there is some support for one or two .NET and Oniguruma syntax - items, and there is an option for requesting some minor changes that - give better JavaScript compatibility. - - Starting with release 8.30, it is possible to compile two separate PCRE - libraries: the original, which supports 8-bit character strings - (including UTF-8 strings), and a second library that supports 16-bit - character strings (including UTF-16 strings). The build process allows - either one or both to be built. The majority of the work to make this - possible was done by Zoltan Herczeg. - - Starting with release 8.32 it is possible to compile a third separate - PCRE library that supports 32-bit character strings (including UTF-32 - strings). The build process allows any combination of the 8-, 16- and - 32-bit libraries. The work to make this possible was done by Christian - Persch. - - The three libraries contain identical sets of functions, except that - the names in the 16-bit library start with pcre16_ instead of pcre_, - and the names in the 32-bit library start with pcre32_ instead of - pcre_. To avoid over-complication and reduce the documentation mainte- - nance load, most of the documentation describes the 8-bit library, with - the differences for the 16-bit and 32-bit libraries described sepa- - rately in the pcre16 and pcre32 pages. References to functions or - structures of the form pcre[16|32]_xxx should be read as meaning - "pcre_xxx when using the 8-bit library, pcre16_xxx when using the - 16-bit library, or pcre32_xxx when using the 32-bit library". - - The current implementation of PCRE corresponds approximately with Perl - 5.12, including support for UTF-8/16/32 encoded strings and Unicode - general category properties. However, UTF-8/16/32 and Unicode support - has to be explicitly enabled; it is not the default. The Unicode tables - correspond to Unicode release 6.3.0. - - In addition to the Perl-compatible matching function, PCRE contains an - alternative function that matches the same compiled patterns in a dif- - ferent way. In certain circumstances, the alternative function has some - advantages. For a discussion of the two matching algorithms, see the - pcrematching page. - - PCRE is written in C and released as a C library. A number of people - have written wrappers and interfaces of various kinds. In particular, - Google Inc. have provided a comprehensive C++ wrapper for the 8-bit - library. This is now included as part of the PCRE distribution. The - pcrecpp page has details of this interface. Other people's contribu- - tions can be found in the Contrib directory at the primary FTP site, - which is: - - ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre - - Details of exactly which Perl regular expression features are and are - not supported by PCRE are given in separate documents. See the pcrepat- - tern and pcrecompat pages. There is a syntax summary in the pcresyntax - page. - - Some features of PCRE can be included, excluded, or changed when the - library is built. The pcre_config() function makes it possible for a - client to discover which features are available. The features them- - selves are described in the pcrebuild page. Documentation about build- - ing PCRE for various operating systems can be found in the README and - NON-AUTOTOOLS_BUILD files in the source distribution. - - The libraries contains a number of undocumented internal functions and - data tables that are used by more than one of the exported external - functions, but which are not intended for use by external callers. - Their names all begin with "_pcre_" or "_pcre16_" or "_pcre32_", which - hopefully will not provoke any name clashes. In some environments, it - is possible to control which external symbols are exported when a - shared library is built, and in these cases the undocumented symbols - are not exported. - - -SECURITY CONSIDERATIONS - - If you are using PCRE in a non-UTF application that permits users to - supply arbitrary patterns for compilation, you should be aware of a - feature that allows users to turn on UTF support from within a pattern, - provided that PCRE was built with UTF support. For example, an 8-bit - pattern that begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, - which interprets patterns and subjects as strings of UTF-8 characters - instead of individual 8-bit characters. This causes both the pattern - and any data against which it is matched to be checked for UTF-8 valid- - ity. If the data string is very long, such a check might use suffi- - ciently many resources as to cause your application to lose perfor- - mance. - - One way of guarding against this possibility is to use the - pcre_fullinfo() function to check the compiled pattern's options for - UTF. Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF - option at compile time. This causes an compile time error if a pattern - contains a UTF-setting sequence. - - If your application is one that supports UTF, be aware that validity - checking can take time. If the same data string is to be matched many - times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the second - and subsequent matches to save redundant checks. - - Another way that performance can be hit is by running a pattern that - has a very large search tree against a string that will never match. - Nested unlimited repeats in a pattern are a common example. PCRE pro- - vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea- - ture in the pcreapi page. - - -USER DOCUMENTATION - - The user documentation for PCRE comprises a number of different sec- - tions. In the "man" format, each of these is a separate "man page". In - the HTML format, each is a separate page, linked from the index page. - In the plain text format, the descriptions of the pcregrep and pcretest - programs are in files called pcregrep.txt and pcretest.txt, respec- - tively. The remaining sections, except for the pcredemo section (which - is a program listing), are concatenated in pcre.txt, for ease of - searching. The sections are as follows: - - pcre this document - pcre-config show PCRE installation configuration information - pcre16 details of the 16-bit library - pcre32 details of the 32-bit library - pcreapi details of PCRE's native C API - pcrebuild building PCRE - pcrecallout details of the callout feature - pcrecompat discussion of Perl compatibility - pcrecpp details of the C++ wrapper for the 8-bit library - pcredemo a demonstration C program that uses PCRE - pcregrep description of the pcregrep command (8-bit only) - pcrejit discussion of the just-in-time optimization support - pcrelimits details of size and other limits - pcrematching discussion of the two matching algorithms - pcrepartial details of the partial matching facility - pcrepattern syntax and semantics of supported - regular expressions - pcreperform discussion of performance issues - pcreposix the POSIX-compatible C API for the 8-bit library - pcreprecompile details of saving and re-using precompiled patterns - pcresample discussion of the pcredemo program - pcrestack discussion of stack usage - pcresyntax quick syntax reference - pcretest description of the pcretest testing command - pcreunicode discussion of Unicode and UTF-8/16/32 support - - In the "man" and HTML formats, there is also a short page for each C - library function, listing its arguments and results. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - Putting an actual email address here seems to have been a spam magnet, - so I've taken it away. If you want to email me, use my two initials, - followed by the two digits 10, at the domain cam.ac.uk. - - -REVISION - - Last updated: 08 January 2014 - Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRE(3) Library Functions Manual PCRE(3) - - - -NAME - PCRE - Perl-compatible regular expressions - - #include <pcre.h> - - -PCRE 16-BIT API BASIC FUNCTIONS - - pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options, - int *errorcodeptr, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - pcre16_extra *pcre16_study(const pcre16 *code, int options, - const char **errptr); - - void pcre16_free_study(pcre16_extra *extra); - - int pcre16_exec(const pcre16 *code, const pcre16_extra *extra, - PCRE_SPTR16 subject, int length, int startoffset, - int options, int *ovector, int ovecsize); - - int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra, - PCRE_SPTR16 subject, int length, int startoffset, - int options, int *ovector, int ovecsize, - int *workspace, int wscount); - - -PCRE 16-BIT API STRING EXTRACTION FUNCTIONS - - int pcre16_copy_named_substring(const pcre16 *code, - PCRE_SPTR16 subject, int *ovector, - int stringcount, PCRE_SPTR16 stringname, - PCRE_UCHAR16 *buffer, int buffersize); - - int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector, - int stringcount, int stringnumber, PCRE_UCHAR16 *buffer, - int buffersize); - - int pcre16_get_named_substring(const pcre16 *code, - PCRE_SPTR16 subject, int *ovector, - int stringcount, PCRE_SPTR16 stringname, - PCRE_SPTR16 *stringptr); - - int pcre16_get_stringnumber(const pcre16 *code, - PCRE_SPTR16 name); - - int pcre16_get_stringtable_entries(const pcre16 *code, - PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last); - - int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector, - int stringcount, int stringnumber, - PCRE_SPTR16 *stringptr); - - int pcre16_get_substring_list(PCRE_SPTR16 subject, - int *ovector, int stringcount, PCRE_SPTR16 **listptr); - - void pcre16_free_substring(PCRE_SPTR16 stringptr); - - void pcre16_free_substring_list(PCRE_SPTR16 *stringptr); - - -PCRE 16-BIT API AUXILIARY FUNCTIONS - - pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize); - - void pcre16_jit_stack_free(pcre16_jit_stack *stack); - - void pcre16_assign_jit_stack(pcre16_extra *extra, - pcre16_jit_callback callback, void *data); - - const unsigned char *pcre16_maketables(void); - - int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra, - int what, void *where); - - int pcre16_refcount(pcre16 *code, int adjust); - - int pcre16_config(int what, void *where); - - const char *pcre16_version(void); - - int pcre16_pattern_to_host_byte_order(pcre16 *code, - pcre16_extra *extra, const unsigned char *tables); - - -PCRE 16-BIT API INDIRECTED FUNCTIONS - - void *(*pcre16_malloc)(size_t); - - void (*pcre16_free)(void *); - - void *(*pcre16_stack_malloc)(size_t); - - void (*pcre16_stack_free)(void *); - - int (*pcre16_callout)(pcre16_callout_block *); - - -PCRE 16-BIT API 16-BIT-ONLY FUNCTION - - int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output, - PCRE_SPTR16 input, int length, int *byte_order, - int keep_boms); - - -THE PCRE 16-BIT LIBRARY - - Starting with release 8.30, it is possible to compile a PCRE library - that supports 16-bit character strings, including UTF-16 strings, as - well as or instead of the original 8-bit library. The majority of the - work to make this possible was done by Zoltan Herczeg. The two - libraries contain identical sets of functions, used in exactly the same - way. Only the names of the functions and the data types of their argu- - ments and results are different. To avoid over-complication and reduce - the documentation maintenance load, most of the PCRE documentation - describes the 8-bit library, with only occasional references to the - 16-bit library. This page describes what is different when you use the - 16-bit library. - - WARNING: A single application can be linked with both libraries, but - you must take care when processing any particular pattern to use func- - tions from just one library. For example, if you want to study a pat- - tern that was compiled with pcre16_compile(), you must do so with - pcre16_study(), not pcre_study(), and you must free the study data with - pcre16_free_study(). - - -THE HEADER FILE - - There is only one header file, pcre.h. It contains prototypes for all - the functions in all libraries, as well as definitions of flags, struc- - tures, error codes, etc. - - -THE LIBRARY NAME - - In Unix-like systems, the 16-bit library is called libpcre16, and can - normally be accesss by adding -lpcre16 to the command for linking an - application that uses PCRE. - - -STRING TYPES - - In the 8-bit library, strings are passed to PCRE library functions as - vectors of bytes with the C type "char *". In the 16-bit library, - strings are passed as vectors of unsigned 16-bit quantities. The macro - PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is - defined as "const PCRE_UCHAR16 *". In very many environments, "short - int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16 - as "unsigned short int", but checks that it really is a 16-bit data - type. If it is not, the build fails with an error message telling the - maintainer to modify the definition appropriately. - - -STRUCTURE TYPES - - The types of the opaque structures that are used for compiled 16-bit - patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively. - The type of the user-accessible structure that is returned by - pcre16_study() is pcre16_extra, and the type of the structure that is - used for passing data to a callout function is pcre16_callout_block. - These structures contain the same fields, with the same names, as their - 8-bit counterparts. The only difference is that pointers to character - strings are 16-bit instead of 8-bit types. - - -16-BIT FUNCTIONS - - For every function in the 8-bit library there is a corresponding func- - tion in the 16-bit library with a name that starts with pcre16_ instead - of pcre_. The prototypes are listed above. In addition, there is one - extra function, pcre16_utf16_to_host_byte_order(). This is a utility - function that converts a UTF-16 character string to host byte order if - necessary. The other 16-bit functions expect the strings they are - passed to be in host byte order. - - The input and output arguments of pcre16_utf16_to_host_byte_order() may - point to the same address, that is, conversion in place is supported. - The output buffer must be at least as long as the input. - - The length argument specifies the number of 16-bit data units in the - input string; a negative value specifies a zero-terminated string. - - If byte_order is NULL, it is assumed that the string starts off in host - byte order. This may be changed by byte-order marks (BOMs) anywhere in - the string (commonly as the first character). - - If byte_order is not NULL, a non-zero value of the integer to which it - points means that the input starts off in host byte order, otherwise - the opposite order is assumed. Again, BOMs in the string can change - this. The final byte order is passed back at the end of processing. - - If keep_boms is not zero, byte-order mark characters (0xfeff) are - copied into the output string. Otherwise they are discarded. - - The result of the function is the number of 16-bit units placed into - the output buffer, including the zero terminator if the string was - zero-terminated. - - -SUBJECT STRING OFFSETS - - The lengths and starting offsets of subject strings must be specified - in 16-bit data units, and the offsets within subject strings that are - returned by the matching functions are in also 16-bit units rather than - bytes. - - -NAMED SUBPATTERNS - - The name-to-number translation table that is maintained for named sub- - patterns uses 16-bit characters. The pcre16_get_stringtable_entries() - function returns the length of each entry in the table as the number of - 16-bit data units. - - -OPTION NAMES - - There are two new general option names, PCRE_UTF16 and - PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and - PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options - define the same bits in the options word. There is a discussion about - the validity of UTF-16 strings in the pcreunicode page. - - For the pcre16_config() function there is an option PCRE_CONFIG_UTF16 - that returns 1 if UTF-16 support is configured, otherwise 0. If this - option is given to pcre_config() or pcre32_config(), or if the - PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF32 option is given to pcre16_con- - fig(), the result is the PCRE_ERROR_BADOPTION error. - - -CHARACTER CODES - - In 16-bit mode, when PCRE_UTF16 is not set, character values are - treated in the same way as in 8-bit, non UTF-8 mode, except, of course, - that they can range from 0 to 0xffff instead of 0 to 0xff. Character - types for characters less than 0xff can therefore be influenced by the - locale in the same way as before. Characters greater than 0xff have - only one case, and no "type" (such as letter or digit). - - In UTF-16 mode, the character code is Unicode, in the range 0 to - 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff - because those are "surrogate" values that are used in pairs to encode - values greater than 0xffff. - - A UTF-16 string can indicate its endianness by special code knows as a - byte-order mark (BOM). The PCRE functions do not handle this, expecting - strings to be in host byte order. A utility function called - pcre16_utf16_to_host_byte_order() is provided to help with this (see - above). - - -ERROR NAMES - - The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre- - spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is - given when a compiled pattern is passed to a function that processes - patterns in the other mode, for example, if a pattern compiled with - pcre_compile() is passed to pcre16_exec(). - - There are new error codes whose names begin with PCRE_UTF16_ERR for - invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for - UTF-8 strings that are described in the section entitled "Reason codes - for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors - are: - - PCRE_UTF16_ERR1 Missing low surrogate at end of string - PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate - PCRE_UTF16_ERR3 Isolated low surrogate - PCRE_UTF16_ERR4 Non-character - - -ERROR TEXTS - - If there is an error while compiling a pattern, the error text that is - passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit - character string, zero-terminated. - - -CALLOUTS - - The subject and mark fields in the callout block that is passed to a - callout function point to 16-bit vectors. - - -TESTING - - The pcretest program continues to operate with 8-bit input and output - files, but it can be used for testing the 16-bit library. If it is run - with the command line option -16, patterns and subject strings are con- - verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit - library functions are used instead of the 8-bit ones. Returned 16-bit - strings are converted to 8-bit for output. If both the 8-bit and the - 32-bit libraries were not compiled, pcretest defaults to 16-bit and the - -16 option is ignored. - - When PCRE is being built, the RunTest script that is called by "make - check" uses the pcretest -C option to discover which of the 8-bit, - 16-bit and 32-bit libraries has been built, and runs the tests appro- - priately. - - -NOT SUPPORTED IN 16-BIT MODE - - Not all the features of the 8-bit library are available with the 16-bit - library. The C++ and POSIX wrapper functions support only the 8-bit - library, and the pcregrep program is at present 8-bit only. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 12 May 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRE(3) Library Functions Manual PCRE(3) - - - -NAME - PCRE - Perl-compatible regular expressions - - #include <pcre.h> - - -PCRE 32-BIT API BASIC FUNCTIONS - - pcre32 *pcre32_compile(PCRE_SPTR32 pattern, int options, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - pcre32 *pcre32_compile2(PCRE_SPTR32 pattern, int options, - int *errorcodeptr, - const unsigned char *tableptr); - - pcre32_extra *pcre32_study(const pcre32 *code, int options, - const char **errptr); - - void pcre32_free_study(pcre32_extra *extra); - - int pcre32_exec(const pcre32 *code, const pcre32_extra *extra, - PCRE_SPTR32 subject, int length, int startoffset, - int options, int *ovector, int ovecsize); - - int pcre32_dfa_exec(const pcre32 *code, const pcre32_extra *extra, - PCRE_SPTR32 subject, int length, int startoffset, - int options, int *ovector, int ovecsize, - int *workspace, int wscount); - - -PCRE 32-BIT API STRING EXTRACTION FUNCTIONS - - int pcre32_copy_named_substring(const pcre32 *code, - PCRE_SPTR32 subject, int *ovector, - int stringcount, PCRE_SPTR32 stringname, - PCRE_UCHAR32 *buffer, int buffersize); - - int pcre32_copy_substring(PCRE_SPTR32 subject, int *ovector, - int stringcount, int stringnumber, PCRE_UCHAR32 *buffer, - int buffersize); - - int pcre32_get_named_substring(const pcre32 *code, - PCRE_SPTR32 subject, int *ovector, - int stringcount, PCRE_SPTR32 stringname, - PCRE_SPTR32 *stringptr); - - int pcre32_get_stringnumber(const pcre32 *code, - PCRE_SPTR32 name); - - int pcre32_get_stringtable_entries(const pcre32 *code, - PCRE_SPTR32 name, PCRE_UCHAR32 **first, PCRE_UCHAR32 **last); - - int pcre32_get_substring(PCRE_SPTR32 subject, int *ovector, - int stringcount, int stringnumber, - PCRE_SPTR32 *stringptr); - - int pcre32_get_substring_list(PCRE_SPTR32 subject, - int *ovector, int stringcount, PCRE_SPTR32 **listptr); - - void pcre32_free_substring(PCRE_SPTR32 stringptr); - - void pcre32_free_substring_list(PCRE_SPTR32 *stringptr); - - -PCRE 32-BIT API AUXILIARY FUNCTIONS - - pcre32_jit_stack *pcre32_jit_stack_alloc(int startsize, int maxsize); - - void pcre32_jit_stack_free(pcre32_jit_stack *stack); - - void pcre32_assign_jit_stack(pcre32_extra *extra, - pcre32_jit_callback callback, void *data); - - const unsigned char *pcre32_maketables(void); - - int pcre32_fullinfo(const pcre32 *code, const pcre32_extra *extra, - int what, void *where); - - int pcre32_refcount(pcre32 *code, int adjust); - - int pcre32_config(int what, void *where); - - const char *pcre32_version(void); - - int pcre32_pattern_to_host_byte_order(pcre32 *code, - pcre32_extra *extra, const unsigned char *tables); - - -PCRE 32-BIT API INDIRECTED FUNCTIONS - - void *(*pcre32_malloc)(size_t); - - void (*pcre32_free)(void *); - - void *(*pcre32_stack_malloc)(size_t); - - void (*pcre32_stack_free)(void *); - - int (*pcre32_callout)(pcre32_callout_block *); - - -PCRE 32-BIT API 32-BIT-ONLY FUNCTION - - int pcre32_utf32_to_host_byte_order(PCRE_UCHAR32 *output, - PCRE_SPTR32 input, int length, int *byte_order, - int keep_boms); - - -THE PCRE 32-BIT LIBRARY - - Starting with release 8.32, it is possible to compile a PCRE library - that supports 32-bit character strings, including UTF-32 strings, as - well as or instead of the original 8-bit library. This work was done by - Christian Persch, based on the work done by Zoltan Herczeg for the - 16-bit library. All three libraries contain identical sets of func- - tions, used in exactly the same way. Only the names of the functions - and the data types of their arguments and results are different. To - avoid over-complication and reduce the documentation maintenance load, - most of the PCRE documentation describes the 8-bit library, with only - occasional references to the 16-bit and 32-bit libraries. This page - describes what is different when you use the 32-bit library. - - WARNING: A single application can be linked with all or any of the - three libraries, but you must take care when processing any particular - pattern to use functions from just one library. For example, if you - want to study a pattern that was compiled with pcre32_compile(), you - must do so with pcre32_study(), not pcre_study(), and you must free the - study data with pcre32_free_study(). - - -THE HEADER FILE - - There is only one header file, pcre.h. It contains prototypes for all - the functions in all libraries, as well as definitions of flags, struc- - tures, error codes, etc. - - -THE LIBRARY NAME - - In Unix-like systems, the 32-bit library is called libpcre32, and can - normally be accesss by adding -lpcre32 to the command for linking an - application that uses PCRE. - - -STRING TYPES - - In the 8-bit library, strings are passed to PCRE library functions as - vectors of bytes with the C type "char *". In the 32-bit library, - strings are passed as vectors of unsigned 32-bit quantities. The macro - PCRE_UCHAR32 specifies an appropriate data type, and PCRE_SPTR32 is - defined as "const PCRE_UCHAR32 *". In very many environments, "unsigned - int" is a 32-bit data type. When PCRE is built, it defines PCRE_UCHAR32 - as "unsigned int", but checks that it really is a 32-bit data type. If - it is not, the build fails with an error message telling the maintainer - to modify the definition appropriately. - - -STRUCTURE TYPES - - The types of the opaque structures that are used for compiled 32-bit - patterns and JIT stacks are pcre32 and pcre32_jit_stack respectively. - The type of the user-accessible structure that is returned by - pcre32_study() is pcre32_extra, and the type of the structure that is - used for passing data to a callout function is pcre32_callout_block. - These structures contain the same fields, with the same names, as their - 8-bit counterparts. The only difference is that pointers to character - strings are 32-bit instead of 8-bit types. - - -32-BIT FUNCTIONS - - For every function in the 8-bit library there is a corresponding func- - tion in the 32-bit library with a name that starts with pcre32_ instead - of pcre_. The prototypes are listed above. In addition, there is one - extra function, pcre32_utf32_to_host_byte_order(). This is a utility - function that converts a UTF-32 character string to host byte order if - necessary. The other 32-bit functions expect the strings they are - passed to be in host byte order. - - The input and output arguments of pcre32_utf32_to_host_byte_order() may - point to the same address, that is, conversion in place is supported. - The output buffer must be at least as long as the input. - - The length argument specifies the number of 32-bit data units in the - input string; a negative value specifies a zero-terminated string. - - If byte_order is NULL, it is assumed that the string starts off in host - byte order. This may be changed by byte-order marks (BOMs) anywhere in - the string (commonly as the first character). - - If byte_order is not NULL, a non-zero value of the integer to which it - points means that the input starts off in host byte order, otherwise - the opposite order is assumed. Again, BOMs in the string can change - this. The final byte order is passed back at the end of processing. - - If keep_boms is not zero, byte-order mark characters (0xfeff) are - copied into the output string. Otherwise they are discarded. - - The result of the function is the number of 32-bit units placed into - the output buffer, including the zero terminator if the string was - zero-terminated. - - -SUBJECT STRING OFFSETS - - The lengths and starting offsets of subject strings must be specified - in 32-bit data units, and the offsets within subject strings that are - returned by the matching functions are in also 32-bit units rather than - bytes. - - -NAMED SUBPATTERNS - - The name-to-number translation table that is maintained for named sub- - patterns uses 32-bit characters. The pcre32_get_stringtable_entries() - function returns the length of each entry in the table as the number of - 32-bit data units. - - -OPTION NAMES - - There are two new general option names, PCRE_UTF32 and - PCRE_NO_UTF32_CHECK, which correspond to PCRE_UTF8 and - PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options - define the same bits in the options word. There is a discussion about - the validity of UTF-32 strings in the pcreunicode page. - - For the pcre32_config() function there is an option PCRE_CONFIG_UTF32 - that returns 1 if UTF-32 support is configured, otherwise 0. If this - option is given to pcre_config() or pcre16_config(), or if the - PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF16 option is given to pcre32_con- - fig(), the result is the PCRE_ERROR_BADOPTION error. - - -CHARACTER CODES - - In 32-bit mode, when PCRE_UTF32 is not set, character values are - treated in the same way as in 8-bit, non UTF-8 mode, except, of course, - that they can range from 0 to 0x7fffffff instead of 0 to 0xff. Charac- - ter types for characters less than 0xff can therefore be influenced by - the locale in the same way as before. Characters greater than 0xff - have only one case, and no "type" (such as letter or digit). - - In UTF-32 mode, the character code is Unicode, in the range 0 to - 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff - because those are "surrogate" values that are ill-formed in UTF-32. - - A UTF-32 string can indicate its endianness by special code knows as a - byte-order mark (BOM). The PCRE functions do not handle this, expecting - strings to be in host byte order. A utility function called - pcre32_utf32_to_host_byte_order() is provided to help with this (see - above). - - -ERROR NAMES - - The error PCRE_ERROR_BADUTF32 corresponds to its 8-bit counterpart. - The error PCRE_ERROR_BADMODE is given when a compiled pattern is passed - to a function that processes patterns in the other mode, for example, - if a pattern compiled with pcre_compile() is passed to pcre32_exec(). - - There are new error codes whose names begin with PCRE_UTF32_ERR for - invalid UTF-32 strings, corresponding to the PCRE_UTF8_ERR codes for - UTF-8 strings that are described in the section entitled "Reason codes - for invalid UTF-8 strings" in the main pcreapi page. The UTF-32 errors - are: - - PCRE_UTF32_ERR1 Surrogate character (range from 0xd800 to 0xdfff) - PCRE_UTF32_ERR2 Non-character - PCRE_UTF32_ERR3 Character > 0x10ffff - - -ERROR TEXTS - - If there is an error while compiling a pattern, the error text that is - passed back by pcre32_compile() or pcre32_compile2() is still an 8-bit - character string, zero-terminated. - - -CALLOUTS - - The subject and mark fields in the callout block that is passed to a - callout function point to 32-bit vectors. - - -TESTING - - The pcretest program continues to operate with 8-bit input and output - files, but it can be used for testing the 32-bit library. If it is run - with the command line option -32, patterns and subject strings are con- - verted from 8-bit to 32-bit before being passed to PCRE, and the 32-bit - library functions are used instead of the 8-bit ones. Returned 32-bit - strings are converted to 8-bit for output. If both the 8-bit and the - 16-bit libraries were not compiled, pcretest defaults to 32-bit and the - -32 option is ignored. - - When PCRE is being built, the RunTest script that is called by "make - check" uses the pcretest -C option to discover which of the 8-bit, - 16-bit and 32-bit libraries has been built, and runs the tests appro- - priately. - - -NOT SUPPORTED IN 32-BIT MODE - - Not all the features of the 8-bit library are available with the 32-bit - library. The C++ and POSIX wrapper functions support only the 8-bit - library, and the pcregrep program is at present 8-bit only. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 12 May 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREBUILD(3) Library Functions Manual PCREBUILD(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -BUILDING PCRE - - PCRE is distributed with a configure script that can be used to build - the library in Unix-like environments using the applications known as - Autotools. Also in the distribution are files to support building - using CMake instead of configure. The text file README contains general - information about building with Autotools (some of which is repeated - below), and also has some comments about building on various operating - systems. There is a lot more information about building PCRE without - using Autotools (including information about using CMake and building - "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should - consult this file as well as the README file if you are building in a - non-Unix-like environment. - - -PCRE BUILD-TIME OPTIONS - - The rest of this document describes the optional features of PCRE that - can be selected when the library is compiled. It assumes use of the - configure script, where the optional features are selected or dese- - lected by providing options to configure before running the make com- - mand. However, the same options can be selected in both Unix-like and - non-Unix-like environments using the GUI facility of cmake-gui if you - are using CMake instead of configure to build PCRE. - - If you are not using Autotools or CMake, option selection can be done - by editing the config.h file, or by passing parameter settings to the - compiler, as described in NON-AUTOTOOLS-BUILD. - - The complete list of options for configure (which includes the standard - ones such as the selection of the installation directory) can be - obtained by running - - ./configure --help - - The following sections include descriptions of options whose names - begin with --enable or --disable. These settings specify changes to the - defaults for the configure command. Because of the way that configure - works, --enable and --disable always come in pairs, so the complemen- - tary option always exists as well, but as it specifies the default, it - is not described. - - -BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES - - By default, a library called libpcre is built, containing functions - that take string arguments contained in vectors of bytes, either as - single-byte characters, or interpreted as UTF-8 strings. You can also - build a separate library, called libpcre16, in which strings are con- - tained in vectors of 16-bit data units and interpreted either as sin- - gle-unit characters or UTF-16 strings, by adding - - --enable-pcre16 - - to the configure command. You can also build yet another separate - library, called libpcre32, in which strings are contained in vectors of - 32-bit data units and interpreted either as single-unit characters or - UTF-32 strings, by adding - - --enable-pcre32 - - to the configure command. If you do not want the 8-bit library, add - - --disable-pcre8 - - as well. At least one of the three libraries must be built. Note that - the C++ and POSIX wrappers are for the 8-bit library only, and that - pcregrep is an 8-bit program. None of these are built if you select - only the 16-bit or 32-bit libraries. - - -BUILDING SHARED AND STATIC LIBRARIES - - The Autotools PCRE building process uses libtool to build both shared - and static libraries by default. You can suppress one of these by - adding one of - - --disable-shared - --disable-static - - to the configure command, as required. - - -C++ SUPPORT - - By default, if the 8-bit library is being built, the configure script - will search for a C++ compiler and C++ header files. If it finds them, - it automatically builds the C++ wrapper library (which supports only - 8-bit strings). You can disable this by adding - - --disable-cpp - - to the configure command. - - -UTF-8, UTF-16 AND UTF-32 SUPPORT - - To build PCRE with support for UTF Unicode character strings, add - - --enable-utf - - to the configure command. This setting applies to all three libraries, - adding support for UTF-8 to the 8-bit library, support for UTF-16 to - the 16-bit library, and support for UTF-32 to the to the 32-bit - library. There are no separate options for enabling UTF-8, UTF-16 and - UTF-32 independently because that would allow ridiculous settings such - as requesting UTF-16 support while building only the 8-bit library. It - is not possible to build one library with UTF support and another with- - out in the same configuration. (For backwards compatibility, --enable- - utf8 is a synonym of --enable-utf.) - - Of itself, this setting does not make PCRE treat strings as UTF-8, - UTF-16 or UTF-32. As well as compiling PCRE with this option, you also - have have to set the PCRE_UTF8, PCRE_UTF16 or PCRE_UTF32 option (as - appropriate) when you call one of the pattern compiling functions. - - If you set --enable-utf when compiling in an EBCDIC environment, PCRE - expects its input to be either ASCII or UTF-8 (depending on the run- - time option). It is not possible to support both EBCDIC and UTF-8 codes - in the same version of the library. Consequently, --enable-utf and - --enable-ebcdic are mutually exclusive. - - -UNICODE CHARACTER PROPERTY SUPPORT - - UTF support allows the libraries to process character codepoints up to - 0x10ffff in the strings that they handle. On its own, however, it does - not provide any facilities for accessing the properties of such charac- - ters. If you want to be able to use the pattern escapes \P, \p, and \X, - which refer to Unicode character properties, you must add - - --enable-unicode-properties - - to the configure command. This implies UTF support, even if you have - not explicitly requested it. - - Including Unicode property support adds around 30K of tables to the - PCRE library. Only the general category properties such as Lu and Nd - are supported. Details are given in the pcrepattern documentation. - - -JUST-IN-TIME COMPILER SUPPORT - - Just-in-time compiler support is included in the build by specifying - - --enable-jit - - This support is available only for certain hardware architectures. If - this option is set for an unsupported architecture, a compile time - error occurs. See the pcrejit documentation for a discussion of JIT - usage. When JIT support is enabled, pcregrep automatically makes use of - it, unless you add - - --disable-pcregrep-jit - - to the "configure" command. - - -CODE VALUE OF NEWLINE - - By default, PCRE interprets the linefeed (LF) character as indicating - the end of a line. This is the normal newline character on Unix-like - systems. You can compile PCRE to use carriage return (CR) instead, by - adding - - --enable-newline-is-cr - - to the configure command. There is also a --enable-newline-is-lf - option, which explicitly specifies linefeed as the newline character. - - Alternatively, you can specify that line endings are to be indicated by - the two character sequence CRLF. If you want this, add - - --enable-newline-is-crlf - - to the configure command. There is a fourth option, specified by - - --enable-newline-is-anycrlf - - which causes PCRE to recognize any of the three sequences CR, LF, or - CRLF as indicating a line ending. Finally, a fifth option, specified by - - --enable-newline-is-any - - causes PCRE to recognize any Unicode newline sequence. - - Whatever line ending convention is selected when PCRE is built can be - overridden when the library functions are called. At build time it is - conventional to use the standard for your operating system. - - -WHAT \R MATCHES - - By default, the sequence \R in a pattern matches any Unicode newline - sequence, whatever has been selected as the line ending sequence. If - you specify - - --enable-bsr-anycrlf - - the default is changed so that \R matches only CR, LF, or CRLF. What- - ever is selected when PCRE is built can be overridden when the library - functions are called. - - -POSIX MALLOC USAGE - - When the 8-bit library is called through the POSIX interface (see the - pcreposix documentation), additional working storage is required for - holding the pointers to capturing substrings, because PCRE requires - three integers per substring, whereas the POSIX interface provides only - two. If the number of expected substrings is small, the wrapper func- - tion uses space on the stack, because this is faster than using mal- - loc() for each call. The default threshold above which the stack is no - longer used is 10; it can be changed by adding a setting such as - - --with-posix-malloc-threshold=20 - - to the configure command. - - -HANDLING VERY LARGE PATTERNS - - Within a compiled pattern, offset values are used to point from one - part to another (for example, from an opening parenthesis to an alter- - nation metacharacter). By default, in the 8-bit and 16-bit libraries, - two-byte values are used for these offsets, leading to a maximum size - for a compiled pattern of around 64K. This is sufficient to handle all - but the most gigantic patterns. Nevertheless, some people do want to - process truly enormous patterns, so it is possible to compile PCRE to - use three-byte or four-byte offsets by adding a setting such as - - --with-link-size=3 - - to the configure command. The value given must be 2, 3, or 4. For the - 16-bit library, a value of 3 is rounded up to 4. In these libraries, - using longer offsets slows down the operation of PCRE because it has to - load additional data when handling them. For the 32-bit library the - value is always 4 and cannot be overridden; the value of --with-link- - size is ignored. - - -AVOIDING EXCESSIVE STACK USAGE - - When matching with the pcre_exec() function, PCRE implements backtrack- - ing by making recursive calls to an internal function called match(). - In environments where the size of the stack is limited, this can se- - verely limit PCRE's operation. (The Unix environment does not usually - suffer from this problem, but it may sometimes be necessary to increase - the maximum stack size. There is a discussion in the pcrestack docu- - mentation.) An alternative approach to recursion that uses memory from - the heap to remember data, instead of using recursive function calls, - has been implemented to work round the problem of limited stack size. - If you want to build a version of PCRE that works this way, add - - --disable-stack-for-recursion - - to the configure command. With this configuration, PCRE will use the - pcre_stack_malloc and pcre_stack_free variables to call memory manage- - ment functions. By default these point to malloc() and free(), but you - can replace the pointers so that your own functions are used instead. - - Separate functions are provided rather than using pcre_malloc and - pcre_free because the usage is very predictable: the block sizes - requested are always the same, and the blocks are always freed in - reverse order. A calling program might be able to implement optimized - functions that perform better than malloc() and free(). PCRE runs - noticeably more slowly when built in this way. This option affects only - the pcre_exec() function; it is not relevant for pcre_dfa_exec(). - - -LIMITING PCRE RESOURCE USAGE - - Internally, PCRE has a function called match(), which it calls repeat- - edly (sometimes recursively) when matching a pattern with the - pcre_exec() function. By controlling the maximum number of times this - function may be called during a single matching operation, a limit can - be placed on the resources used by a single call to pcre_exec(). The - limit can be changed at run time, as described in the pcreapi documen- - tation. The default is 10 million, but this can be changed by adding a - setting such as - - --with-match-limit=500000 - - to the configure command. This setting has no effect on the - pcre_dfa_exec() matching function. - - In some environments it is desirable to limit the depth of recursive - calls of match() more strictly than the total number of calls, in order - to restrict the maximum amount of stack (or heap, if --disable-stack- - for-recursion is specified) that is used. A second limit controls this; - it defaults to the value that is set for --with-match-limit, which - imposes no additional constraints. However, you can set a lower limit - by adding, for example, - - --with-match-limit-recursion=10000 - - to the configure command. This value can also be overridden at run - time. - - -CREATING CHARACTER TABLES AT BUILD TIME - - PCRE uses fixed tables for processing characters whose code values are - less than 256. By default, PCRE is built with a set of tables that are - distributed in the file pcre_chartables.c.dist. These tables are for - ASCII codes only. If you add - - --enable-rebuild-chartables - - to the configure command, the distributed tables are no longer used. - Instead, a program called dftables is compiled and run. This outputs - the source for new set of tables, created in the default locale of your - C run-time system. (This method of replacing the tables does not work - if you are cross compiling, because dftables is run on the local host. - If you need to create alternative tables when cross compiling, you will - have to do so "by hand".) - - -USING EBCDIC CODE - - PCRE assumes by default that it will run in an environment where the - character code is ASCII (or Unicode, which is a superset of ASCII). - This is the case for most computer operating systems. PCRE can, how- - ever, be compiled to run in an EBCDIC environment by adding - - --enable-ebcdic - - to the configure command. This setting implies --enable-rebuild-charta- - bles. You should only use it if you know that you are in an EBCDIC - environment (for example, an IBM mainframe operating system). The - --enable-ebcdic option is incompatible with --enable-utf. - - The EBCDIC character that corresponds to an ASCII LF is assumed to have - the value 0x15 by default. However, in some EBCDIC environments, 0x25 - is used. In such an environment you should use - - --enable-ebcdic-nl25 - - as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR - has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and - 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- - acter (which, in Unicode, is 0x85). - - The options that select newline behaviour, such as --enable-newline-is- - cr, and equivalent run-time options, refer to these character values in - an EBCDIC environment. - - -PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT - - By default, pcregrep reads all files as plain text. You can build it so - that it recognizes files whose names end in .gz or .bz2, and reads them - with libz or libbz2, respectively, by adding one or both of - - --enable-pcregrep-libz - --enable-pcregrep-libbz2 - - to the configure command. These options naturally require that the rel- - evant libraries are installed on your system. Configuration will fail - if they are not. - - -PCREGREP BUFFER SIZE - - pcregrep uses an internal buffer to hold a "window" on the file it is - scanning, in order to be able to output "before" and "after" lines when - it finds a match. The size of the buffer is controlled by a parameter - whose default value is 20K. The buffer itself is three times this size, - but because of the way it is used for holding "before" lines, the long- - est line that is guaranteed to be processable is the parameter size. - You can change the default parameter value by adding, for example, - - --with-pcregrep-bufsize=50K - - to the configure command. The caller of pcregrep can, however, override - this value by specifying a run-time option. - - -PCRETEST OPTION FOR LIBREADLINE SUPPORT - - If you add - - --enable-pcretest-libreadline - - to the configure command, pcretest is linked with the libreadline - library, and when its input is from a terminal, it reads it using the - readline() function. This provides line-editing and history facilities. - Note that libreadline is GPL-licensed, so if you distribute a binary of - pcretest linked in this way, there may be licensing issues. - - Setting this option causes the -lreadline option to be added to the - pcretest build. In many operating environments with a sytem-installed - libreadline this is sufficient. However, in some environments (e.g. if - an unmodified distribution version of readline is in use), some extra - configuration may be necessary. The INSTALL file for libreadline says - this: - - "Readline uses the termcap functions, but does not link with the - termcap or curses library itself, allowing applications which link - with readline the to choose an appropriate library." - - If your environment has not been set up so that an appropriate library - is automatically included, you may need to add something like - - LIBS="-ncurses" - - immediately before the configure command. - - -DEBUGGING WITH VALGRIND SUPPORT - - By adding the - - --enable-valgrind - - option to to the configure command, PCRE will use valgrind annotations - to mark certain memory regions as unaddressable. This allows it to - detect invalid memory accesses, and is mostly useful for debugging PCRE - itself. - - -CODE COVERAGE REPORTING - - If your C compiler is gcc, you can build a version of PCRE that can - generate a code coverage report for its test suite. To enable this, you - must install lcov version 1.6 or above. Then specify - - --enable-coverage - - to the configure command and build PCRE in the usual way. - - Note that using ccache (a caching C compiler) is incompatible with code - coverage reporting. If you have configured ccache to run automatically - on your system, you must set the environment variable - - CCACHE_DISABLE=1 - - before running make to build PCRE, so that ccache is not used. - - When --enable-coverage is used, the following addition targets are - added to the Makefile: - - make coverage - - This creates a fresh coverage report for the PCRE test suite. It is - equivalent to running "make coverage-reset", "make coverage-baseline", - "make check", and then "make coverage-report". - - make coverage-reset - - This zeroes the coverage counters, but does nothing else. - - make coverage-baseline - - This captures baseline coverage information. - - make coverage-report - - This creates the coverage report. - - make coverage-clean-report - - This removes the generated coverage report without cleaning the cover- - age data itself. - - make coverage-clean-data - - This removes the captured coverage data without removing the coverage - files created at compile time (*.gcno). - - make coverage-clean - - This cleans all coverage data including the generated coverage report. - For more information about code coverage, see the gcov and lcov docu- - mentation. - - -SEE ALSO - - pcreapi(3), pcre16, pcre32, pcre_config(3). - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 12 May 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREMATCHING(3) Library Functions Manual PCREMATCHING(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE MATCHING ALGORITHMS - - This document describes the two different algorithms that are available - in PCRE for matching a compiled regular expression against a given sub- - ject string. The "standard" algorithm is the one provided by the - pcre_exec(), pcre16_exec() and pcre32_exec() functions. These work in - the same as as Perl's matching function, and provide a Perl-compatible - matching operation. The just-in-time (JIT) optimization that is - described in the pcrejit documentation is compatible with these func- - tions. - - An alternative algorithm is provided by the pcre_dfa_exec(), - pcre16_dfa_exec() and pcre32_dfa_exec() functions; they operate in a - different way, and are not Perl-compatible. This alternative has advan- - tages and disadvantages compared with the standard algorithm, and these - are described below. - - When there is only one possible way in which a given subject string can - match a pattern, the two algorithms give the same answer. A difference - arises, however, when there are multiple possibilities. For example, if - the pattern - - ^<.*> - - is matched against the string - - <something> <something else> <something further> - - there are three possible answers. The standard algorithm finds only one - of them, whereas the alternative algorithm finds all three. - - -REGULAR EXPRESSIONS AS TREES - - The set of strings that are matched by a regular expression can be rep- - resented as a tree structure. An unlimited repetition in the pattern - makes the tree of infinite size, but it is still a tree. Matching the - pattern to a given subject string (from a given starting point) can be - thought of as a search of the tree. There are two ways to search a - tree: depth-first and breadth-first, and these correspond to the two - matching algorithms provided by PCRE. - - -THE STANDARD MATCHING ALGORITHM - - In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- - sions", the standard algorithm is an "NFA algorithm". It conducts a - depth-first search of the pattern tree. That is, it proceeds along a - single path through the tree, checking that the subject matches what is - required. When there is a mismatch, the algorithm tries any alterna- - tives at the current point, and if they all fail, it backs up to the - previous branch point in the tree, and tries the next alternative - branch at that level. This often involves backing up (moving to the - left) in the subject string as well. The order in which repetition - branches are tried is controlled by the greedy or ungreedy nature of - the quantifier. - - If a leaf node is reached, a matching string has been found, and at - that point the algorithm stops. Thus, if there is more than one possi- - ble match, this algorithm returns the first one that it finds. Whether - this is the shortest, the longest, or some intermediate length depends - on the way the greedy and ungreedy repetition quantifiers are specified - in the pattern. - - Because it ends up with a single path through the tree, it is rela- - tively straightforward for this algorithm to keep track of the sub- - strings that are matched by portions of the pattern in parentheses. - This provides support for capturing parentheses and back references. - - -THE ALTERNATIVE MATCHING ALGORITHM - - This algorithm conducts a breadth-first search of the tree. Starting - from the first matching point in the subject, it scans the subject - string from left to right, once, character by character, and as it does - this, it remembers all the paths through the tree that represent valid - matches. In Friedl's terminology, this is a kind of "DFA algorithm", - though it is not implemented as a traditional finite state machine (it - keeps multiple states active simultaneously). - - Although the general principle of this matching algorithm is that it - scans the subject string only once, without backtracking, there is one - exception: when a lookaround assertion is encountered, the characters - following or preceding the current point have to be independently - inspected. - - The scan continues until either the end of the subject is reached, or - there are no more unterminated paths. At this point, terminated paths - represent the different matching possibilities (if there are none, the - match has failed). Thus, if there is more than one possible match, - this algorithm finds all of them, and in particular, it finds the long- - est. The matches are returned in decreasing order of length. There is - an option to stop the algorithm after the first match (which is neces- - sarily the shortest) is found. - - Note that all the matches that are found start at the same point in the - subject. If the pattern - - cat(er(pillar)?)? - - is matched against the string "the caterpillar catchment", the result - will be the three strings "caterpillar", "cater", and "cat" that start - at the fifth character of the subject. The algorithm does not automati- - cally move on to find matches that start at later positions. - - PCRE's "auto-possessification" optimization usually applies to charac- - ter repeats at the end of a pattern (as well as internally). For exam- - ple, the pattern "a\d+" is compiled as if it were "a\d++" because there - is no point even considering the possibility of backtracking into the - repeated digits. For DFA matching, this means that only one possible - match is found. If you really do want multiple matches in such cases, - either use an ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS - option when compiling. - - There are a number of features of PCRE regular expressions that are not - supported by the alternative matching algorithm. They are as follows: - - 1. Because the algorithm finds all possible matches, the greedy or - ungreedy nature of repetition quantifiers is not relevant. Greedy and - ungreedy quantifiers are treated in exactly the same way. However, pos- - sessive quantifiers can make a difference when what follows could also - match what is quantified, for example in a pattern like this: - - ^a++\w! - - This pattern matches "aaab!" but not "aaa!", which would be matched by - a non-possessive quantifier. Similarly, if an atomic group is present, - it is matched as if it were a standalone pattern at the current point, - and the longest match is then "locked in" for the rest of the overall - pattern. - - 2. When dealing with multiple paths through the tree simultaneously, it - is not straightforward to keep track of captured substrings for the - different matching possibilities, and PCRE's implementation of this - algorithm does not attempt to do this. This means that no captured sub- - strings are available. - - 3. Because no substrings are captured, back references within the pat- - tern are not supported, and cause errors if encountered. - - 4. For the same reason, conditional expressions that use a backrefer- - ence as the condition or test for a specific group recursion are not - supported. - - 5. Because many paths through the tree may be active, the \K escape - sequence, which resets the start of the match when encountered (but may - be on some paths and not on others), is not supported. It causes an - error if encountered. - - 6. Callouts are supported, but the value of the capture_top field is - always 1, and the value of the capture_last field is always -1. - - 7. The \C escape sequence, which (in the standard algorithm) always - matches a single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is - not supported in these modes, because the alternative algorithm moves - through the subject string one character (not data unit) at a time, for - all active paths through the tree. - - 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) - are not supported. (*FAIL) is supported, and behaves like a failing - negative assertion. - - -ADVANTAGES OF THE ALTERNATIVE ALGORITHM - - Using the alternative matching algorithm provides the following advan- - tages: - - 1. All possible matches (at a single point in the subject) are automat- - ically found, and in particular, the longest match is found. To find - more than one match using the standard algorithm, you have to do kludgy - things with callouts. - - 2. Because the alternative algorithm scans the subject string just - once, and never needs to backtrack (except for lookbehinds), it is pos- - sible to pass very long subject strings to the matching function in - several pieces, checking for partial matching each time. Although it is - possible to do multi-segment matching using the standard algorithm by - retaining partially matched substrings, it is more complicated. The - pcrepartial documentation gives details of partial matching and dis- - cusses multi-segment matching. - - -DISADVANTAGES OF THE ALTERNATIVE ALGORITHM - - The alternative algorithm suffers from a number of disadvantages: - - 1. It is substantially slower than the standard algorithm. This is - partly because it has to search for all possible matches, but is also - because it is less susceptible to optimization. - - 2. Capturing parentheses and back references are not supported. - - 3. Although atomic groups are supported, their use does not provide the - performance advantage that it does for the standard algorithm. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 12 November 2013 - Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREAPI(3) Library Functions Manual PCREAPI(3) - - - -NAME - PCRE - Perl-compatible regular expressions - - #include <pcre.h> - - -PCRE NATIVE API BASIC FUNCTIONS - - pcre *pcre_compile(const char *pattern, int options, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - pcre *pcre_compile2(const char *pattern, int options, - int *errorcodeptr, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - pcre_extra *pcre_study(const pcre *code, int options, - const char **errptr); - - void pcre_free_study(pcre_extra *extra); - - int pcre_exec(const pcre *code, const pcre_extra *extra, - const char *subject, int length, int startoffset, - int options, int *ovector, int ovecsize); - - int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, - const char *subject, int length, int startoffset, - int options, int *ovector, int ovecsize, - int *workspace, int wscount); - - -PCRE NATIVE API STRING EXTRACTION FUNCTIONS - - int pcre_copy_named_substring(const pcre *code, - const char *subject, int *ovector, - int stringcount, const char *stringname, - char *buffer, int buffersize); - - int pcre_copy_substring(const char *subject, int *ovector, - int stringcount, int stringnumber, char *buffer, - int buffersize); - - int pcre_get_named_substring(const pcre *code, - const char *subject, int *ovector, - int stringcount, const char *stringname, - const char **stringptr); - - int pcre_get_stringnumber(const pcre *code, - const char *name); - - int pcre_get_stringtable_entries(const pcre *code, - const char *name, char **first, char **last); - - int pcre_get_substring(const char *subject, int *ovector, - int stringcount, int stringnumber, - const char **stringptr); - - int pcre_get_substring_list(const char *subject, - int *ovector, int stringcount, const char ***listptr); - - void pcre_free_substring(const char *stringptr); - - void pcre_free_substring_list(const char **stringptr); - - -PCRE NATIVE API AUXILIARY FUNCTIONS - - int pcre_jit_exec(const pcre *code, const pcre_extra *extra, - const char *subject, int length, int startoffset, - int options, int *ovector, int ovecsize, - pcre_jit_stack *jstack); - - pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize); - - void pcre_jit_stack_free(pcre_jit_stack *stack); - - void pcre_assign_jit_stack(pcre_extra *extra, - pcre_jit_callback callback, void *data); - - const unsigned char *pcre_maketables(void); - - int pcre_fullinfo(const pcre *code, const pcre_extra *extra, - int what, void *where); - - int pcre_refcount(pcre *code, int adjust); - - int pcre_config(int what, void *where); - - const char *pcre_version(void); - - int pcre_pattern_to_host_byte_order(pcre *code, - pcre_extra *extra, const unsigned char *tables); - - -PCRE NATIVE API INDIRECTED FUNCTIONS - - void *(*pcre_malloc)(size_t); - - void (*pcre_free)(void *); - - void *(*pcre_stack_malloc)(size_t); - - void (*pcre_stack_free)(void *); - - int (*pcre_callout)(pcre_callout_block *); - - int (*pcre_stack_guard)(void); - - -PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES - - As well as support for 8-bit character strings, PCRE also supports - 16-bit strings (from release 8.30) and 32-bit strings (from release - 8.32), by means of two additional libraries. They can be built as well - as, or instead of, the 8-bit library. To avoid too much complication, - this document describes the 8-bit versions of the functions, with only - occasional references to the 16-bit and 32-bit libraries. - - The 16-bit and 32-bit functions operate in the same way as their 8-bit - counterparts; they just use different data types for their arguments - and results, and their names start with pcre16_ or pcre32_ instead of - pcre_. For every option that has UTF8 in its name (for example, - PCRE_UTF8), there are corresponding 16-bit and 32-bit names with UTF8 - replaced by UTF16 or UTF32, respectively. This facility is in fact just - cosmetic; the 16-bit and 32-bit option names define the same bit val- - ues. - - References to bytes and UTF-8 in this document should be read as refer- - ences to 16-bit data units and UTF-16 when using the 16-bit library, or - 32-bit data units and UTF-32 when using the 32-bit library, unless - specified otherwise. More details of the specific differences for the - 16-bit and 32-bit libraries are given in the pcre16 and pcre32 pages. - - -PCRE API OVERVIEW - - PCRE has its own native API, which is described in this document. There - are also some wrapper functions (for the 8-bit library only) that cor- - respond to the POSIX regular expression API, but they do not give - access to all the functionality. They are described in the pcreposix - documentation. Both of these APIs define a set of C function calls. A - C++ wrapper (again for the 8-bit library only) is also distributed with - PCRE. It is documented in the pcrecpp page. - - The native API C function prototypes are defined in the header file - pcre.h, and on Unix-like systems the (8-bit) library itself is called - libpcre. It can normally be accessed by adding -lpcre to the command - for linking an application that uses PCRE. The header file defines the - macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release - numbers for the library. Applications can use these to include support - for different releases of PCRE. - - In a Windows environment, if you want to statically link an application - program against a non-dll pcre.a file, you must define PCRE_STATIC - before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- - loc() and pcre_free() exported functions will be declared - __declspec(dllimport), with unwanted results. - - The functions pcre_compile(), pcre_compile2(), pcre_study(), and - pcre_exec() are used for compiling and matching regular expressions in - a Perl-compatible manner. A sample program that demonstrates the sim- - plest way of using them is provided in the file called pcredemo.c in - the PCRE source distribution. A listing of this program is given in the - pcredemo documentation, and the pcresample documentation describes how - to compile and run it. - - Just-in-time compiler support is an optional feature of PCRE that can - be built in appropriate hardware environments. It greatly speeds up the - matching performance of many patterns. Simple programs can easily - request that it be used if available, by setting an option that is - ignored when it is not relevant. More complicated programs might need - to make use of the functions pcre_jit_stack_alloc(), - pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control - the JIT code's memory usage. - - From release 8.32 there is also a direct interface for JIT execution, - which gives improved performance. The JIT-specific functions are dis- - cussed in the pcrejit documentation. - - A second matching function, pcre_dfa_exec(), which is not Perl-compati- - ble, is also provided. This uses a different algorithm for the match- - ing. The alternative algorithm finds all possible matches (at a given - point in the subject), and scans the subject just once (unless there - are lookbehind assertions). However, this algorithm does not return - captured substrings. A description of the two matching algorithms and - their advantages and disadvantages is given in the pcrematching docu- - mentation. - - In addition to the main compiling and matching functions, there are - convenience functions for extracting captured substrings from a subject - string that is matched by pcre_exec(). They are: - - pcre_copy_substring() - pcre_copy_named_substring() - pcre_get_substring() - pcre_get_named_substring() - pcre_get_substring_list() - pcre_get_stringnumber() - pcre_get_stringtable_entries() - - pcre_free_substring() and pcre_free_substring_list() are also provided, - to free the memory used for extracted strings. - - The function pcre_maketables() is used to build a set of character - tables in the current locale for passing to pcre_compile(), - pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is - provided for specialist use. Most commonly, no special tables are - passed, in which case internal tables that are generated when PCRE is - built are used. - - The function pcre_fullinfo() is used to find out information about a - compiled pattern. The function pcre_version() returns a pointer to a - string containing the version of PCRE and its date of release. - - The function pcre_refcount() maintains a reference count in a data - block containing a compiled pattern. This is provided for the benefit - of object-oriented applications. - - The global variables pcre_malloc and pcre_free initially contain the - entry points of the standard malloc() and free() functions, respec- - tively. PCRE calls the memory management functions via these variables, - so a calling program can replace them if it wishes to intercept the - calls. This should be done before calling any PCRE functions. - - The global variables pcre_stack_malloc and pcre_stack_free are also - indirections to memory management functions. These special functions - are used only when PCRE is compiled to use the heap for remembering - data, instead of recursive function calls, when running the pcre_exec() - function. See the pcrebuild documentation for details of how to do - this. It is a non-standard way of building PCRE, for use in environ- - ments that have limited stacks. Because of the greater use of memory - management, it runs more slowly. Separate functions are provided so - that special-purpose external code can be used for this case. When - used, these functions are always called in a stack-like manner (last - obtained, first freed), and always for memory blocks of the same size. - There is a discussion about PCRE's stack usage in the pcrestack docu- - mentation. - - The global variable pcre_callout initially contains NULL. It can be set - by the caller to a "callout" function, which PCRE will then call at - specified points during a matching operation. Details are given in the - pcrecallout documentation. - - The global variable pcre_stack_guard initially contains NULL. It can be - set by the caller to a function that is called by PCRE whenever it - starts to compile a parenthesized part of a pattern. When parentheses - are nested, PCRE uses recursive function calls, which use up the system - stack. This function is provided so that applications with restricted - stacks can force a compilation error if the stack runs out. The func- - tion should return zero if all is well, or non-zero to force an error. - - -NEWLINES - - PCRE supports five different conventions for indicating line breaks in - strings: a single CR (carriage return) character, a single LF (line- - feed) character, the two-character sequence CRLF, any of the three pre- - ceding, or any Unicode newline sequence. The Unicode newline sequences - are the three just mentioned, plus the single characters VT (vertical - tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line - separator, U+2028), and PS (paragraph separator, U+2029). - - Each of the first three conventions is used by at least one operating - system as its standard newline sequence. When PCRE is built, a default - can be specified. The default default is LF, which is the Unix stan- - dard. When PCRE is run, the default can be overridden, either when a - pattern is compiled, or when it is matched. - - At compile time, the newline convention can be specified by the options - argument of pcre_compile(), or it can be specified by special text at - the start of the pattern itself; this overrides any other settings. See - the pcrepattern page for details of the special character sequences. - - In the PCRE documentation the word "newline" is used to mean "the char- - acter or pair of characters that indicate a line break". The choice of - newline convention affects the handling of the dot, circumflex, and - dollar metacharacters, the handling of #-comments in /x mode, and, when - CRLF is a recognized line ending sequence, the match position advance- - ment for a non-anchored pattern. There is more detail about this in the - section on pcre_exec() options below. - - The choice of newline convention does not affect the interpretation of - the \n or \r escape sequences, nor does it affect what \R matches, - which is controlled in a similar way, but by separate options. - - -MULTITHREADING - - The PCRE functions can be used in multi-threading applications, with - the proviso that the memory management functions pointed to by - pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the - callout and stack-checking functions pointed to by pcre_callout and - pcre_stack_guard, are shared by all threads. - - The compiled form of a regular expression is not altered during match- - ing, so the same compiled pattern can safely be used by several threads - at once. - - If the just-in-time optimization feature is being used, it needs sepa- - rate memory stack areas for each thread. See the pcrejit documentation - for more details. - - -SAVING PRECOMPILED PATTERNS FOR LATER USE - - The compiled form of a regular expression can be saved and re-used at a - later time, possibly by a different program, and even on a host other - than the one on which it was compiled. Details are given in the - pcreprecompile documentation, which includes a description of the - pcre_pattern_to_host_byte_order() function. However, compiling a regu- - lar expression with one version of PCRE for use with a different ver- - sion is not guaranteed to work and may cause crashes. - - -CHECKING BUILD-TIME OPTIONS - - int pcre_config(int what, void *where); - - The function pcre_config() makes it possible for a PCRE client to dis- - cover which optional features have been compiled into the PCRE library. - The pcrebuild documentation has more details about these optional fea- - tures. - - The first argument for pcre_config() is an integer, specifying which - information is required; the second argument is a pointer to a variable - into which the information is placed. The returned value is zero on - success, or the negative error code PCRE_ERROR_BADOPTION if the value - in the first argument is not recognized. The following information is - available: - - PCRE_CONFIG_UTF8 - - The output is an integer that is set to one if UTF-8 support is avail- - able; otherwise it is set to zero. This value should normally be given - to the 8-bit version of this function, pcre_config(). If it is given to - the 16-bit or 32-bit version of this function, the result is - PCRE_ERROR_BADOPTION. - - PCRE_CONFIG_UTF16 - - The output is an integer that is set to one if UTF-16 support is avail- - able; otherwise it is set to zero. This value should normally be given - to the 16-bit version of this function, pcre16_config(). If it is given - to the 8-bit or 32-bit version of this function, the result is - PCRE_ERROR_BADOPTION. - - PCRE_CONFIG_UTF32 - - The output is an integer that is set to one if UTF-32 support is avail- - able; otherwise it is set to zero. This value should normally be given - to the 32-bit version of this function, pcre32_config(). If it is given - to the 8-bit or 16-bit version of this function, the result is - PCRE_ERROR_BADOPTION. - - PCRE_CONFIG_UNICODE_PROPERTIES - - The output is an integer that is set to one if support for Unicode - character properties is available; otherwise it is set to zero. - - PCRE_CONFIG_JIT - - The output is an integer that is set to one if support for just-in-time - compiling is available; otherwise it is set to zero. - - PCRE_CONFIG_JITTARGET - - The output is a pointer to a zero-terminated "const char *" string. If - JIT support is available, the string contains the name of the architec- - ture for which the JIT compiler is configured, for example "x86 32bit - (little endian + unaligned)". If JIT support is not available, the - result is NULL. - - PCRE_CONFIG_NEWLINE - - The output is an integer whose value specifies the default character - sequence that is recognized as meaning "newline". The values that are - supported in ASCII/Unicode environments are: 10 for LF, 13 for CR, 3338 - for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC environments, CR, - ANYCRLF, and ANY yield the same values. However, the value for LF is - normally 21, though some EBCDIC environments use 37. The corresponding - values for CRLF are 3349 and 3365. The default should normally corre- - spond to the standard sequence for your operating system. - - PCRE_CONFIG_BSR - - The output is an integer whose value indicates what character sequences - the \R escape sequence matches by default. A value of 0 means that \R - matches any Unicode line ending sequence; a value of 1 means that \R - matches only CR, LF, or CRLF. The default can be overridden when a pat- - tern is compiled or matched. - - PCRE_CONFIG_LINK_SIZE - - The output is an integer that contains the number of bytes used for - internal linkage in compiled regular expressions. For the 8-bit - library, the value can be 2, 3, or 4. For the 16-bit library, the value - is either 2 or 4 and is still a number of bytes. For the 32-bit - library, the value is either 2 or 4 and is still a number of bytes. The - default value of 2 is sufficient for all but the most massive patterns, - since it allows the compiled pattern to be up to 64K in size. Larger - values allow larger regular expressions to be compiled, at the expense - of slower matching. - - PCRE_CONFIG_POSIX_MALLOC_THRESHOLD - - The output is an integer that contains the threshold above which the - POSIX interface uses malloc() for output vectors. Further details are - given in the pcreposix documentation. - - PCRE_CONFIG_PARENS_LIMIT - - The output is a long integer that gives the maximum depth of nesting of - parentheses (of any kind) in a pattern. This limit is imposed to cap - the amount of system stack used when a pattern is compiled. It is spec- - ified when PCRE is built; the default is 250. This limit does not take - into account the stack that may already be used by the calling applica- - tion. For finer control over compilation stack usage, you can set a - pointer to an external checking function in pcre_stack_guard. - - PCRE_CONFIG_MATCH_LIMIT - - The output is a long integer that gives the default limit for the num- - ber of internal matching function calls in a pcre_exec() execution. - Further details are given with pcre_exec() below. - - PCRE_CONFIG_MATCH_LIMIT_RECURSION - - The output is a long integer that gives the default limit for the depth - of recursion when calling the internal matching function in a - pcre_exec() execution. Further details are given with pcre_exec() - below. - - PCRE_CONFIG_STACKRECURSE - - The output is an integer that is set to one if internal recursion when - running pcre_exec() is implemented by recursive function calls that use - the stack to remember their state. This is the usual way that PCRE is - compiled. The output is zero if PCRE was compiled to use blocks of data - on the heap instead of recursive function calls. In this case, - pcre_stack_malloc and pcre_stack_free are called to manage memory - blocks on the heap, thus avoiding the use of the stack. - - -COMPILING A PATTERN - - pcre *pcre_compile(const char *pattern, int options, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - pcre *pcre_compile2(const char *pattern, int options, - int *errorcodeptr, - const char **errptr, int *erroffset, - const unsigned char *tableptr); - - Either of the functions pcre_compile() or pcre_compile2() can be called - to compile a pattern into an internal form. The only difference between - the two interfaces is that pcre_compile2() has an additional argument, - errorcodeptr, via which a numerical error code can be returned. To - avoid too much repetition, we refer just to pcre_compile() below, but - the information applies equally to pcre_compile2(). - - The pattern is a C string terminated by a binary zero, and is passed in - the pattern argument. A pointer to a single block of memory that is - obtained via pcre_malloc is returned. This contains the compiled code - and related data. The pcre type is defined for the returned block; this - is a typedef for a structure whose contents are not externally defined. - It is up to the caller to free the memory (via pcre_free) when it is no - longer required. - - Although the compiled code of a PCRE regex is relocatable, that is, it - does not depend on memory location, the complete pcre data block is not - fully relocatable, because it may contain a copy of the tableptr argu- - ment, which is an address (see below). - - The options argument contains various bit settings that affect the com- - pilation. It should be zero if no options are required. The available - options are described below. Some of them (in particular, those that - are compatible with Perl, but some others as well) can also be set and - unset from within the pattern (see the detailed description in the - pcrepattern documentation). For those options that can be different in - different parts of the pattern, the contents of the options argument - specifies their settings at the start of compilation and execution. The - PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and - PCRE_NO_START_OPTIMIZE options can be set at the time of matching as - well as at compile time. - - If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, - if compilation of a pattern fails, pcre_compile() returns NULL, and - sets the variable pointed to by errptr to point to a textual error mes- - sage. This is a static string that is part of the library. You must not - try to free it. Normally, the offset from the start of the pattern to - the data unit that was being processed when the error was discovered is - placed in the variable pointed to by erroffset, which must not be NULL - (if it is, an immediate error is given). However, for an invalid UTF-8 - or UTF-16 string, the offset is that of the first data unit of the - failing character. - - Some errors are not detected until the whole pattern has been scanned; - in these cases, the offset passed back is the length of the pattern. - Note that the offset is in data units, not characters, even in a UTF - mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- - acter. - - If pcre_compile2() is used instead of pcre_compile(), and the error- - codeptr argument is not NULL, a non-zero error code number is returned - via this argument in the event of an error. This is in addition to the - textual error message. Error codes and messages are listed below. - - If the final argument, tableptr, is NULL, PCRE uses a default set of - character tables that are built when PCRE is compiled, using the - default C locale. Otherwise, tableptr must be an address that is the - result of a call to pcre_maketables(). This value is stored with the - compiled pattern, and used again by pcre_exec() and pcre_dfa_exec() - when the pattern is matched. For more discussion, see the section on - locale support below. - - This code fragment shows a typical straightforward call to pcre_com- - pile(): - - pcre *re; - const char *error; - int erroffset; - re = pcre_compile( - "^A.*Z", /* the pattern */ - 0, /* default options */ - &error, /* for error message */ - &erroffset, /* for error offset */ - NULL); /* use default character tables */ - - The following names for option bits are defined in the pcre.h header - file: - - PCRE_ANCHORED - - If this bit is set, the pattern is forced to be "anchored", that is, it - is constrained to match only at the first matching point in the string - that is being searched (the "subject string"). This effect can also be - achieved by appropriate constructs in the pattern itself, which is the - only way to do it in Perl. - - PCRE_AUTO_CALLOUT - - If this bit is set, pcre_compile() automatically inserts callout items, - all with number 255, before each pattern item. For discussion of the - callout facility, see the pcrecallout documentation. - - PCRE_BSR_ANYCRLF - PCRE_BSR_UNICODE - - These options (which are mutually exclusive) control what the \R escape - sequence matches. The choice is either to match only CR, LF, or CRLF, - or to match any Unicode newline sequence. The default is specified when - PCRE is built. It can be overridden from within the pattern, or by set- - ting an option when a compiled pattern is matched. - - PCRE_CASELESS - - If this bit is set, letters in the pattern match both upper and lower - case letters. It is equivalent to Perl's /i option, and it can be - changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE - always understands the concept of case for characters whose values are - less than 128, so caseless matching is always possible. For characters - with higher values, the concept of case is supported if PCRE is com- - piled with Unicode property support, but not otherwise. If you want to - use caseless matching for characters 128 and above, you must ensure - that PCRE is compiled with Unicode property support as well as with - UTF-8 support. - - PCRE_DOLLAR_ENDONLY - - If this bit is set, a dollar metacharacter in the pattern matches only - at the end of the subject string. Without this option, a dollar also - matches immediately before a newline at the end of the string (but not - before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored - if PCRE_MULTILINE is set. There is no equivalent to this option in - Perl, and no way to set it within a pattern. - - PCRE_DOTALL - - If this bit is set, a dot metacharacter in the pattern matches a char- - acter of any value, including one that indicates a newline. However, it - only ever matches one character, even if newlines are coded as CRLF. - Without this option, a dot does not match when the current position is - at a newline. This option is equivalent to Perl's /s option, and it can - be changed within a pattern by a (?s) option setting. A negative class - such as [^a] always matches newline characters, independent of the set- - ting of this option. - - PCRE_DUPNAMES - - If this bit is set, names used to identify capturing subpatterns need - not be unique. This can be helpful for certain types of pattern when it - is known that only one instance of the named subpattern can ever be - matched. There are more details of named subpatterns below; see also - the pcrepattern documentation. - - PCRE_EXTENDED - - If this bit is set, most white space characters in the pattern are - totally ignored except when escaped or inside a character class. How- - ever, white space is not allowed within sequences such as (?> that - introduce various parenthesized subpatterns, nor within a numerical - quantifier such as {1,3}. However, ignorable white space is permitted - between an item and a following quantifier and between a quantifier and - a following + that indicates possessiveness. - - White space did not used to include the VT character (code 11), because - Perl did not treat this character as white space. However, Perl changed - at release 5.18, so PCRE followed at release 8.34, and VT is now - treated as white space. - - PCRE_EXTENDED also causes characters between an unescaped # outside a - character class and the next newline, inclusive, to be ignored. - PCRE_EXTENDED is equivalent to Perl's /x option, and it can be changed - within a pattern by a (?x) option setting. - - Which characters are interpreted as newlines is controlled by the - options passed to pcre_compile() or by a special sequence at the start - of the pattern, as described in the section entitled "Newline conven- - tions" in the pcrepattern documentation. Note that the end of this type - of comment is a literal newline sequence in the pattern; escape - sequences that happen to represent a newline do not count. - - This option makes it possible to include comments inside complicated - patterns. Note, however, that this applies only to data characters. - White space characters may never appear within special character - sequences in a pattern, for example within the sequence (?( that intro- - duces a conditional subpattern. - - PCRE_EXTRA - - This option was invented in order to turn on additional functionality - of PCRE that is incompatible with Perl, but it is currently of very - little use. When set, any backslash in a pattern that is followed by a - letter that has no special meaning causes an error, thus reserving - these combinations for future expansion. By default, as in Perl, a - backslash followed by a letter with no special meaning is treated as a - literal. (Perl can, however, be persuaded to give an error for this, by - running it with the -w option.) There are at present no other features - controlled by this option. It can also be set by a (?X) option setting - within a pattern. - - PCRE_FIRSTLINE - - If this option is set, an unanchored pattern is required to match - before or at the first newline in the subject string, though the - matched text may continue over the newline. - - PCRE_JAVASCRIPT_COMPAT - - If this option is set, PCRE's behaviour is changed in some ways so that - it is compatible with JavaScript rather than Perl. The changes are as - follows: - - (1) A lone closing square bracket in a pattern causes a compile-time - error, because this is illegal in JavaScript (by default it is treated - as a data character). Thus, the pattern AB]CD becomes illegal when this - option is set. - - (2) At run time, a back reference to an unset subpattern group matches - an empty string (by default this causes the current matching alterna- - tive to fail). A pattern such as (\1)(a) succeeds when this option is - set (assuming it can find an "a" in the subject), whereas it fails by - default, for Perl compatibility. - - (3) \U matches an upper case "U" character; by default \U causes a com- - pile time error (Perl uses \U to upper case subsequent characters). - - (4) \u matches a lower case "u" character unless it is followed by four - hexadecimal digits, in which case the hexadecimal number defines the - code point to match. By default, \u causes a compile time error (Perl - uses it to upper case the following character). - - (5) \x matches a lower case "x" character unless it is followed by two - hexadecimal digits, in which case the hexadecimal number defines the - code point to match. By default, as in Perl, a hexadecimal number is - always expected after \x, but it may have zero, one, or two digits (so, - for example, \xz matches a binary zero character followed by z). - - PCRE_MULTILINE - - By default, for the purposes of matching "start of line" and "end of - line", PCRE treats the subject string as consisting of a single line of - characters, even if it actually contains newlines. The "start of line" - metacharacter (^) matches only at the start of the string, and the "end - of line" metacharacter ($) matches only at the end of the string, or - before a terminating newline (except when PCRE_DOLLAR_ENDONLY is set). - Note, however, that unless PCRE_DOTALL is set, the "any character" - metacharacter (.) does not match at a newline. This behaviour (for ^, - $, and dot) is the same as Perl. - - When PCRE_MULTILINE it is set, the "start of line" and "end of line" - constructs match immediately following or immediately before internal - newlines in the subject string, respectively, as well as at the very - start and end. This is equivalent to Perl's /m option, and it can be - changed within a pattern by a (?m) option setting. If there are no new- - lines in a subject string, or no occurrences of ^ or $ in a pattern, - setting PCRE_MULTILINE has no effect. - - PCRE_NEVER_UTF - - This option locks out interpretation of the pattern as UTF-8 (or UTF-16 - or UTF-32 in the 16-bit and 32-bit libraries). In particular, it pre- - vents the creator of the pattern from switching to UTF interpretation - by starting the pattern with (*UTF). This may be useful in applications - that process patterns from external sources. The combination of - PCRE_UTF8 and PCRE_NEVER_UTF also causes an error. - - PCRE_NEWLINE_CR - PCRE_NEWLINE_LF - PCRE_NEWLINE_CRLF - PCRE_NEWLINE_ANYCRLF - PCRE_NEWLINE_ANY - - These options override the default newline definition that was chosen - when PCRE was built. Setting the first or the second specifies that a - newline is indicated by a single character (CR or LF, respectively). - Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the - two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies - that any of the three preceding sequences should be recognized. Setting - PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be - recognized. - - In an ASCII/Unicode environment, the Unicode newline sequences are the - three just mentioned, plus the single characters VT (vertical tab, - U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line sep- - arator, U+2028), and PS (paragraph separator, U+2029). For the 8-bit - library, the last two are recognized only in UTF-8 mode. - - When PCRE is compiled to run in an EBCDIC (mainframe) environment, the - code for CR is 0x0d, the same as ASCII. However, the character code for - LF is normally 0x15, though in some EBCDIC environments 0x25 is used. - Whichever of these is not LF is made to correspond to Unicode's NEL - character. EBCDIC codes are all less than 256. For more details, see - the pcrebuild documentation. - - The newline setting in the options word uses three bits that are - treated as a number, giving eight possibilities. Currently only six are - used (default plus the five values above). This means that if you set - more than one newline option, the combination may or may not be sensi- - ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to - PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and - cause an error. - - The only time that a line break in a pattern is specially recognized - when compiling is when PCRE_EXTENDED is set. CR and LF are white space - characters, and so are ignored in this mode. Also, an unescaped # out- - side a character class indicates a comment that lasts until after the - next line break sequence. In other circumstances, line break sequences - in patterns are treated as literal data. - - The newline option that is set at compile time becomes the default that - is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. - - PCRE_NO_AUTO_CAPTURE - - If this option is set, it disables the use of numbered capturing paren- - theses in the pattern. Any opening parenthesis that is not followed by - ? behaves as if it were followed by ?: but named parentheses can still - be used for capturing (and they acquire numbers in the usual way). - There is no equivalent of this option in Perl. - - PCRE_NO_AUTO_POSSESS - - If this option is set, it disables "auto-possessification". This is an - optimization that, for example, turns a+b into a++b in order to avoid - backtracks into a+ that can never be successful. However, if callouts - are in use, auto-possessification means that some of them are never - taken. You can set this option if you want the matching functions to do - a full unoptimized search and run all the callouts, but it is mainly - provided for testing purposes. - - PCRE_NO_START_OPTIMIZE - - This is an option that acts at matching time; that is, it is really an - option for pcre_exec() or pcre_dfa_exec(). If it is set at compile - time, it is remembered with the compiled pattern and assumed at match- - ing time. This is necessary if you want to use JIT execution, because - the JIT compiler needs to know whether or not this option is set. For - details see the discussion of PCRE_NO_START_OPTIMIZE below. - - PCRE_UCP - - This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, - \w, and some of the POSIX character classes. By default, only ASCII - characters are recognized, but if PCRE_UCP is set, Unicode properties - are used instead to classify characters. More details are given in the - section on generic character types in the pcrepattern page. If you set - PCRE_UCP, matching one of the items it affects takes much longer. The - option is available only if PCRE has been compiled with Unicode prop- - erty support. - - PCRE_UNGREEDY - - This option inverts the "greediness" of the quantifiers so that they - are not greedy by default, but become greedy if followed by "?". It is - not compatible with Perl. It can also be set by a (?U) option setting - within the pattern. - - PCRE_UTF8 - - This option causes PCRE to regard both the pattern and the subject as - strings of UTF-8 characters instead of single-byte strings. However, it - is available only when PCRE is built to include UTF support. If not, - the use of this option provokes an error. Details of how this option - changes the behaviour of PCRE are given in the pcreunicode page. - - PCRE_NO_UTF8_CHECK - - When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is - automatically checked. There is a discussion about the validity of - UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is - found, pcre_compile() returns an error. If you already know that your - pattern is valid, and you want to skip this check for performance rea- - sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the - effect of passing an invalid UTF-8 string as a pattern is undefined. It - may cause your program to crash or loop. Note that this option can also - be passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity - checking of subject strings only. If the same string is being matched - many times, the option can be safely set for the second and subsequent - matchings to improve performance. - - -COMPILATION ERROR CODES - - The following table lists the error codes than may be returned by - pcre_compile2(), along with the error messages that may be returned by - both compiling functions. Note that error messages are always 8-bit - ASCII strings, even in 16-bit or 32-bit mode. As PCRE has developed, - some error codes have fallen out of use. To avoid confusion, they have - not been re-used. - - 0 no error - 1 \ at end of pattern - 2 \c at end of pattern - 3 unrecognized character follows \ - 4 numbers out of order in {} quantifier - 5 number too big in {} quantifier - 6 missing terminating ] for character class - 7 invalid escape sequence in character class - 8 range out of order in character class - 9 nothing to repeat - 10 [this code is not in use] - 11 internal error: unexpected repeat - 12 unrecognized character after (? or (?- - 13 POSIX named classes are supported only within a class - 14 missing ) - 15 reference to non-existent subpattern - 16 erroffset passed as NULL - 17 unknown option bit(s) set - 18 missing ) after comment - 19 [this code is not in use] - 20 regular expression is too large - 21 failed to get memory - 22 unmatched parentheses - 23 internal error: code overflow - 24 unrecognized character after (?< - 25 lookbehind assertion is not fixed length - 26 malformed number or name after (?( - 27 conditional group contains more than two branches - 28 assertion expected after (?( - 29 (?R or (?[+-]digits must be followed by ) - 30 unknown POSIX class name - 31 POSIX collating elements are not supported - 32 this version of PCRE is compiled without UTF support - 33 [this code is not in use] - 34 character value in \x{} or \o{} is too large - 35 invalid condition (?(0) - 36 \C not allowed in lookbehind assertion - 37 PCRE does not support \L, \l, \N{name}, \U, or \u - 38 number after (?C is > 255 - 39 closing ) for (?C expected - 40 recursive call could loop indefinitely - 41 unrecognized character after (?P - 42 syntax error in subpattern name (missing terminator) - 43 two named subpatterns have the same name - 44 invalid UTF-8 string (specifically UTF-8) - 45 support for \P, \p, and \X has not been compiled - 46 malformed \P or \p sequence - 47 unknown property name after \P or \p - 48 subpattern name is too long (maximum 32 characters) - 49 too many named subpatterns (maximum 10000) - 50 [this code is not in use] - 51 octal value is greater than \377 in 8-bit non-UTF-8 mode - 52 internal error: overran compiling workspace - 53 internal error: previously-checked referenced subpattern - not found - 54 DEFINE group contains more than one branch - 55 repeating a DEFINE group is not allowed - 56 inconsistent NEWLINE options - 57 \g is not followed by a braced, angle-bracketed, or quoted - name/number or by a plain number - 58 a numbered reference must not be zero - 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) - 60 (*VERB) not recognized or malformed - 61 number is too big - 62 subpattern name expected - 63 digit expected after (?+ - 64 ] is an invalid data character in JavaScript compatibility mode - 65 different names for subpatterns of the same number are - not allowed - 66 (*MARK) must have an argument - 67 this version of PCRE is not compiled with Unicode property - support - 68 \c must be followed by an ASCII character - 69 \k is not followed by a braced, angle-bracketed, or quoted name - 70 internal error: unknown opcode in find_fixedlength() - 71 \N is not supported in a class - 72 too many forward references - 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff) - 74 invalid UTF-16 string (specifically UTF-16) - 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN) - 76 character value in \u.... sequence is too large - 77 invalid UTF-32 string (specifically UTF-32) - 78 setting UTF is disabled by the application - 79 non-hex character in \x{} (closing brace missing?) - 80 non-octal character in \o{} (closing brace missing?) - 81 missing opening brace after \o - 82 parentheses are too deeply nested - 83 invalid range in character class - 84 group name must start with a non-digit - 85 parentheses are too deeply nested (stack check) - - The numbers 32 and 10000 in errors 48 and 49 are defaults; different - values may be used if the limits were changed when PCRE was built. - - -STUDYING A PATTERN - - pcre_extra *pcre_study(const pcre *code, int options, - const char **errptr); - - If a compiled pattern is going to be used several times, it is worth - spending more time analyzing it in order to speed up the time taken for - matching. The function pcre_study() takes a pointer to a compiled pat- - tern as its first argument. If studying the pattern produces additional - information that will help speed up matching, pcre_study() returns a - pointer to a pcre_extra block, in which the study_data field points to - the results of the study. - - The returned value from pcre_study() can be passed directly to - pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- - tains other fields that can be set by the caller before the block is - passed; these are described below in the section on matching a pattern. - - If studying the pattern does not produce any useful information, - pcre_study() returns NULL by default. In that circumstance, if the - calling program wants to pass any of the other fields to pcre_exec() or - pcre_dfa_exec(), it must set up its own pcre_extra block. However, if - pcre_study() is called with the PCRE_STUDY_EXTRA_NEEDED option, it - returns a pcre_extra block even if studying did not find any additional - information. It may still return NULL, however, if an error occurs in - pcre_study(). - - The second argument of pcre_study() contains option bits. There are - three further options in addition to PCRE_STUDY_EXTRA_NEEDED: - - PCRE_STUDY_JIT_COMPILE - PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE - PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE - - If any of these are set, and the just-in-time compiler is available, - the pattern is further compiled into machine code that executes much - faster than the pcre_exec() interpretive matching function. If the - just-in-time compiler is not available, these options are ignored. All - undefined bits in the options argument must be zero. - - JIT compilation is a heavyweight optimization. It can take some time - for patterns to be analyzed, and for one-off matches and simple pat- - terns the benefit of faster execution might be offset by a much slower - study time. Not all patterns can be optimized by the JIT compiler. For - those that cannot be handled, matching automatically falls back to the - pcre_exec() interpreter. For more details, see the pcrejit documenta- - tion. - - The third argument for pcre_study() is a pointer for an error message. - If studying succeeds (even if no data is returned), the variable it - points to is set to NULL. Otherwise it is set to point to a textual - error message. This is a static string that is part of the library. You - must not try to free it. You should test the error pointer for NULL - after calling pcre_study(), to be sure that it has run successfully. - - When you are finished with a pattern, you can free the memory used for - the study data by calling pcre_free_study(). This function was added to - the API for release 8.20. For earlier versions, the memory could be - freed with pcre_free(), just like the pattern itself. This will still - work in cases where JIT optimization is not used, but it is advisable - to change to the new function when convenient. - - This is a typical way in which pcre_study() is used (except that in a - real application there should be tests for errors): - - int rc; - pcre *re; - pcre_extra *sd; - re = pcre_compile("pattern", 0, &error, &erroroffset, NULL); - sd = pcre_study( - re, /* result of pcre_compile() */ - 0, /* no options */ - &error); /* set to NULL or points to a message */ - rc = pcre_exec( /* see below for details of pcre_exec() options */ - re, sd, "subject", 7, 0, 0, ovector, 30); - ... - pcre_free_study(sd); - pcre_free(re); - - Studying a pattern does two things: first, a lower bound for the length - of subject string that is needed to match the pattern is computed. This - does not mean that there are any strings of that length that match, but - it does guarantee that no shorter strings match. The value is used to - avoid wasting time by trying to match strings that are shorter than the - lower bound. You can find out the value in a calling program via the - pcre_fullinfo() function. - - Studying a pattern is also useful for non-anchored patterns that do not - have a single fixed starting character. A bitmap of possible starting - bytes is created. This speeds up finding a position in the subject at - which to start matching. (In 16-bit mode, the bitmap is used for 16-bit - values less than 256. In 32-bit mode, the bitmap is used for 32-bit - values less than 256.) - - These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), - and the information is also used by the JIT compiler. The optimiza- - tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option. - You might want to do this if your pattern contains callouts or (*MARK) - and you want to make use of these facilities in cases where matching - fails. - - PCRE_NO_START_OPTIMIZE can be specified at either compile time or exe- - cution time. However, if PCRE_NO_START_OPTIMIZE is passed to - pcre_exec(), (that is, after any JIT compilation has happened) JIT exe- - cution is disabled. For JIT execution to work with PCRE_NO_START_OPTI- - MIZE, the option must be set at compile time. - - There is a longer discussion of PCRE_NO_START_OPTIMIZE below. - - -LOCALE SUPPORT - - PCRE handles caseless matching, and determines whether characters are - letters, digits, or whatever, by reference to a set of tables, indexed - by character code point. When running in UTF-8 mode, or in the 16- or - 32-bit libraries, this applies only to characters with code points less - than 256. By default, higher-valued code points never match escapes - such as \w or \d. However, if PCRE is built with Unicode property sup- - port, all characters can be tested with \p and \P, or, alternatively, - the PCRE_UCP option can be set when a pattern is compiled; this causes - \w and friends to use Unicode property support instead of the built-in - tables. - - The use of locales with Unicode is discouraged. If you are handling - characters with code points greater than 128, you should either use - Unicode support, or use locales, but not try to mix the two. - - PCRE contains an internal set of tables that are used when the final - argument of pcre_compile() is NULL. These are sufficient for many - applications. Normally, the internal tables recognize only ASCII char- - acters. However, when PCRE is built, it is possible to cause the inter- - nal tables to be rebuilt in the default "C" locale of the local system, - which may cause them to be different. - - The internal tables can always be overridden by tables supplied by the - application that calls PCRE. These may be created in a different locale - from the default. As more and more applications change to using Uni- - code, the need for this locale support is expected to die away. - - External tables are built by calling the pcre_maketables() function, - which has no arguments, in the relevant locale. The result can then be - passed to pcre_compile() as often as necessary. For example, to build - and use tables that are appropriate for the French locale (where - accented characters with values greater than 128 are treated as let- - ters), the following code could be used: - - setlocale(LC_CTYPE, "fr_FR"); - tables = pcre_maketables(); - re = pcre_compile(..., tables); - - The locale name "fr_FR" is used on Linux and other Unix-like systems; - if you are using Windows, the name for the French locale is "french". - - When pcre_maketables() runs, the tables are built in memory that is - obtained via pcre_malloc. It is the caller's responsibility to ensure - that the memory containing the tables remains available for as long as - it is needed. - - The pointer that is passed to pcre_compile() is saved with the compiled - pattern, and the same tables are used via this pointer by pcre_study() - and also by pcre_exec() and pcre_dfa_exec(). Thus, for any single pat- - tern, compilation, studying and matching all happen in the same locale, - but different patterns can be processed in different locales. - - It is possible to pass a table pointer or NULL (indicating the use of - the internal tables) to pcre_exec() or pcre_dfa_exec() (see the discus- - sion below in the section on matching a pattern). This facility is pro- - vided for use with pre-compiled patterns that have been saved and - reloaded. Character tables are not saved with patterns, so if a non- - standard table was used at compile time, it must be provided again when - the reloaded pattern is matched. Attempting to use this facility to - match a pattern in a different locale from the one in which it was com- - piled is likely to lead to anomalous (usually incorrect) results. - - -INFORMATION ABOUT A PATTERN - - int pcre_fullinfo(const pcre *code, const pcre_extra *extra, - int what, void *where); - - The pcre_fullinfo() function returns information about a compiled pat- - tern. It replaces the pcre_info() function, which was removed from the - library at version 8.30, after more than 10 years of obsolescence. - - The first argument for pcre_fullinfo() is a pointer to the compiled - pattern. The second argument is the result of pcre_study(), or NULL if - the pattern was not studied. The third argument specifies which piece - of information is required, and the fourth argument is a pointer to a - variable to receive the data. The yield of the function is zero for - success, or one of the following negative numbers: - - PCRE_ERROR_NULL the argument code was NULL - the argument where was NULL - PCRE_ERROR_BADMAGIC the "magic number" was not found - PCRE_ERROR_BADENDIANNESS the pattern was compiled with different - endianness - PCRE_ERROR_BADOPTION the value of what was invalid - PCRE_ERROR_UNSET the requested field is not set - - The "magic number" is placed at the start of each compiled pattern as - an simple check against passing an arbitrary memory pointer. The endi- - anness error can occur if a compiled pattern is saved and reloaded on a - different host. Here is a typical call of pcre_fullinfo(), to obtain - the length of the compiled pattern: - - int rc; - size_t length; - rc = pcre_fullinfo( - re, /* result of pcre_compile() */ - sd, /* result of pcre_study(), or NULL */ - PCRE_INFO_SIZE, /* what is required */ - &length); /* where to put the data */ - - The possible values for the third argument are defined in pcre.h, and - are as follows: - - PCRE_INFO_BACKREFMAX - - Return the number of the highest back reference in the pattern. The - fourth argument should point to an int variable. Zero is returned if - there are no back references. - - PCRE_INFO_CAPTURECOUNT - - Return the number of capturing subpatterns in the pattern. The fourth - argument should point to an int variable. - - PCRE_INFO_DEFAULT_TABLES - - Return a pointer to the internal default character tables within PCRE. - The fourth argument should point to an unsigned char * variable. This - information call is provided for internal use by the pcre_study() func- - tion. External callers can cause PCRE to use its internal tables by - passing a NULL table pointer. - - PCRE_INFO_FIRSTBYTE (deprecated) - - Return information about the first data unit of any matched string, for - a non-anchored pattern. The name of this option refers to the 8-bit - library, where data units are bytes. The fourth argument should point - to an int variable. Negative values are used for special cases. How- - ever, this means that when the 32-bit library is in non-UTF-32 mode, - the full 32-bit range of characters cannot be returned. For this rea- - son, this value is deprecated; use PCRE_INFO_FIRSTCHARACTERFLAGS and - PCRE_INFO_FIRSTCHARACTER instead. - - If there is a fixed first value, for example, the letter "c" from a - pattern such as (cat|cow|coyote), its value is returned. In the 8-bit - library, the value is always less than 256. In the 16-bit library the - value can be up to 0xffff. In the 32-bit library the value can be up to - 0x10ffff. - - If there is no fixed first value, and if either - - (a) the pattern was compiled with the PCRE_MULTILINE option, and every - branch starts with "^", or - - (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not - set (if it were set, the pattern would be anchored), - - -1 is returned, indicating that the pattern matches only at the start - of a subject string or after any newline within the string. Otherwise - -2 is returned. For anchored patterns, -2 is returned. - - PCRE_INFO_FIRSTCHARACTER - - Return the value of the first data unit (non-UTF character) of any - matched string in the situation where PCRE_INFO_FIRSTCHARACTERFLAGS - returns 1; otherwise return 0. The fourth argument should point to an - uint_t variable. - - In the 8-bit library, the value is always less than 256. In the 16-bit - library the value can be up to 0xffff. In the 32-bit library in UTF-32 - mode the value can be up to 0x10ffff, and up to 0xffffffff when not - using UTF-32 mode. - - PCRE_INFO_FIRSTCHARACTERFLAGS - - Return information about the first data unit of any matched string, for - a non-anchored pattern. The fourth argument should point to an int - variable. - - If there is a fixed first value, for example, the letter "c" from a - pattern such as (cat|cow|coyote), 1 is returned, and the character - value can be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no - fixed first value, and if either - - (a) the pattern was compiled with the PCRE_MULTILINE option, and every - branch starts with "^", or - - (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not - set (if it were set, the pattern would be anchored), - - 2 is returned, indicating that the pattern matches only at the start of - a subject string or after any newline within the string. Otherwise 0 is - returned. For anchored patterns, 0 is returned. - - PCRE_INFO_FIRSTTABLE - - If the pattern was studied, and this resulted in the construction of a - 256-bit table indicating a fixed set of values for the first data unit - in any matching string, a pointer to the table is returned. Otherwise - NULL is returned. The fourth argument should point to an unsigned char - * variable. - - PCRE_INFO_HASCRORLF - - Return 1 if the pattern contains any explicit matches for CR or LF - characters, otherwise 0. The fourth argument should point to an int - variable. An explicit match is either a literal CR or LF character, or - \r or \n. - - PCRE_INFO_JCHANGED - - Return 1 if the (?J) or (?-J) option setting is used in the pattern, - otherwise 0. The fourth argument should point to an int variable. (?J) - and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. - - PCRE_INFO_JIT - - Return 1 if the pattern was studied with one of the JIT options, and - just-in-time compiling was successful. The fourth argument should point - to an int variable. A return value of 0 means that JIT support is not - available in this version of PCRE, or that the pattern was not studied - with a JIT option, or that the JIT compiler could not handle this par- - ticular pattern. See the pcrejit documentation for details of what can - and cannot be handled. - - PCRE_INFO_JITSIZE - - If the pattern was successfully studied with a JIT option, return the - size of the JIT compiled code, otherwise return zero. The fourth argu- - ment should point to a size_t variable. - - PCRE_INFO_LASTLITERAL - - Return the value of the rightmost literal data unit that must exist in - any matched string, other than at its start, if such a value has been - recorded. The fourth argument should point to an int variable. If there - is no such value, -1 is returned. For anchored patterns, a last literal - value is recorded only if it follows something of variable length. For - example, for the pattern /^a\d+z\d+/ the returned value is "z", but for - /^a\dz\d/ the returned value is -1. - - Since for the 32-bit library using the non-UTF-32 mode, this function - is unable to return the full 32-bit range of characters, this value is - deprecated; instead the PCRE_INFO_REQUIREDCHARFLAGS and - PCRE_INFO_REQUIREDCHAR values should be used. - - PCRE_INFO_MATCH_EMPTY - - Return 1 if the pattern can match an empty string, otherwise 0. The - fourth argument should point to an int variable. - - PCRE_INFO_MATCHLIMIT - - If the pattern set a match limit by including an item of the form - (*LIMIT_MATCH=nnnn) at the start, the value is returned. The fourth - argument should point to an unsigned 32-bit integer. If no such value - has been set, the call to pcre_fullinfo() returns the error - PCRE_ERROR_UNSET. - - PCRE_INFO_MAXLOOKBEHIND - - Return the number of characters (NB not data units) in the longest - lookbehind assertion in the pattern. This information is useful when - doing multi-segment matching using the partial matching facilities. - Note that the simple assertions \b and \B require a one-character look- - behind. \A also registers a one-character lookbehind, though it does - not actually inspect the previous character. This is to ensure that at - least one character from the old segment is retained when a new segment - is processed. Otherwise, if there are no lookbehinds in the pattern, \A - might match incorrectly at the start of a new segment. - - PCRE_INFO_MINLENGTH - - If the pattern was studied and a minimum length for matching subject - strings was computed, its value is returned. Otherwise the returned - value is -1. The value is a number of characters, which in UTF mode may - be different from the number of data units. The fourth argument should - point to an int variable. A non-negative value is a lower bound to the - length of any matching string. There may not be any strings of that - length that do actually match, but every string that does match is at - least that long. - - PCRE_INFO_NAMECOUNT - PCRE_INFO_NAMEENTRYSIZE - PCRE_INFO_NAMETABLE - - PCRE supports the use of named as well as numbered capturing parenthe- - ses. The names are just an additional way of identifying the parenthe- - ses, which still acquire numbers. Several convenience functions such as - pcre_get_named_substring() are provided for extracting captured sub- - strings by name. It is also possible to extract the data directly, by - first converting the name to a number in order to access the correct - pointers in the output vector (described with pcre_exec() below). To do - the conversion, you need to use the name-to-number map, which is - described by these three values. - - The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT - gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size - of each entry; both of these return an int value. The entry size - depends on the length of the longest name. PCRE_INFO_NAMETABLE returns - a pointer to the first entry of the table. This is a pointer to char in - the 8-bit library, where the first two bytes of each entry are the num- - ber of the capturing parenthesis, most significant byte first. In the - 16-bit library, the pointer points to 16-bit data units, the first of - which contains the parenthesis number. In the 32-bit library, the - pointer points to 32-bit data units, the first of which contains the - parenthesis number. The rest of the entry is the corresponding name, - zero terminated. - - The names are in alphabetical order. If (?| is used to create multiple - groups with the same number, as described in the section on duplicate - subpattern numbers in the pcrepattern page, the groups may be given the - same name, but there is only one entry in the table. Different names - for groups of the same number are not permitted. Duplicate names for - subpatterns with different numbers are permitted, but only if PCRE_DUP- - NAMES is set. They appear in the table in the order in which they were - found in the pattern. In the absence of (?| this is the order of - increasing number; when (?| is used this is not necessarily the case - because later subpatterns may have lower numbers. - - As a simple example of the name/number table, consider the following - pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is - set, so white space - including newlines - is ignored): - - (?<date> (?<year>(\d\d)?\d\d) - - (?<month>\d\d) - (?<day>\d\d) ) - - There are four named subpatterns, so the table has four entries, and - each entry in the table is eight bytes long. The table is as follows, - with non-printing bytes shows in hexadecimal, and undefined bytes shown - as ??: - - 00 01 d a t e 00 ?? - 00 05 d a y 00 ?? ?? - 00 04 m o n t h 00 - 00 02 y e a r 00 ?? - - When writing code to extract data from named subpatterns using the - name-to-number map, remember that the length of the entries is likely - to be different for each compiled pattern. - - PCRE_INFO_OKPARTIAL - - Return 1 if the pattern can be used for partial matching with - pcre_exec(), otherwise 0. The fourth argument should point to an int - variable. From release 8.00, this always returns 1, because the - restrictions that previously applied to partial matching have been - lifted. The pcrepartial documentation gives details of partial match- - ing. - - PCRE_INFO_OPTIONS - - Return a copy of the options with which the pattern was compiled. The - fourth argument should point to an unsigned long int variable. These - option bits are those specified in the call to pcre_compile(), modified - by any top-level option settings at the start of the pattern itself. In - other words, they are the options that will be in force when matching - starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with - the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, - and PCRE_EXTENDED. - - A pattern is automatically anchored by PCRE if all of its top-level - alternatives begin with one of the following: - - ^ unless PCRE_MULTILINE is set - \A always - \G always - .* if PCRE_DOTALL is set and there are no back - references to the subpattern in which .* appears - - For such patterns, the PCRE_ANCHORED bit is set in the options returned - by pcre_fullinfo(). - - PCRE_INFO_RECURSIONLIMIT - - If the pattern set a recursion limit by including an item of the form - (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The fourth - argument should point to an unsigned 32-bit integer. If no such value - has been set, the call to pcre_fullinfo() returns the error - PCRE_ERROR_UNSET. - - PCRE_INFO_SIZE - - Return the size of the compiled pattern in bytes (for all three - libraries). The fourth argument should point to a size_t variable. This - value does not include the size of the pcre structure that is returned - by pcre_compile(). The value that is passed as the argument to - pcre_malloc() when pcre_compile() is getting memory in which to place - the compiled data is the value returned by this option plus the size of - the pcre structure. Studying a compiled pattern, with or without JIT, - does not alter the value returned by this option. - - PCRE_INFO_STUDYSIZE - - Return the size in bytes (for all three libraries) of the data block - pointed to by the study_data field in a pcre_extra block. If pcre_extra - is NULL, or there is no study data, zero is returned. The fourth argu- - ment should point to a size_t variable. The study_data field is set by - pcre_study() to record information that will speed up matching (see the - section entitled "Studying a pattern" above). The format of the - study_data block is private, but its length is made available via this - option so that it can be saved and restored (see the pcreprecompile - documentation for details). - - PCRE_INFO_REQUIREDCHARFLAGS - - Returns 1 if there is a rightmost literal data unit that must exist in - any matched string, other than at its start. The fourth argument should - point to an int variable. If there is no such value, 0 is returned. If - returning 1, the character value itself can be retrieved using - PCRE_INFO_REQUIREDCHAR. - - For anchored patterns, a last literal value is recorded only if it fol- - lows something of variable length. For example, for the pattern - /^a\d+z\d+/ the returned value 1 (with "z" returned from - PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0. - - PCRE_INFO_REQUIREDCHAR - - Return the value of the rightmost literal data unit that must exist in - any matched string, other than at its start, if such a value has been - recorded. The fourth argument should point to an uint32_t variable. If - there is no such value, 0 is returned. - - -REFERENCE COUNTS - - int pcre_refcount(pcre *code, int adjust); - - The pcre_refcount() function is used to maintain a reference count in - the data block that contains a compiled pattern. It is provided for the - benefit of applications that operate in an object-oriented manner, - where different parts of the application may be using the same compiled - pattern, but you want to free the block when they are all done. - - When a pattern is compiled, the reference count field is initialized to - zero. It is changed only by calling this function, whose action is to - add the adjust value (which may be positive or negative) to it. The - yield of the function is the new value. However, the value of the count - is constrained to lie between 0 and 65535, inclusive. If the new value - is outside these limits, it is forced to the appropriate limit value. - - Except when it is zero, the reference count is not correctly preserved - if a pattern is compiled on one host and then transferred to a host - whose byte-order is different. (This seems a highly unlikely scenario.) - - -MATCHING A PATTERN: THE TRADITIONAL FUNCTION - - int pcre_exec(const pcre *code, const pcre_extra *extra, - const char *subject, int length, int startoffset, - int options, int *ovector, int ovecsize); - - The function pcre_exec() is called to match a subject string against a - compiled pattern, which is passed in the code argument. If the pattern - was studied, the result of the study should be passed in the extra - argument. You can call pcre_exec() with the same code and extra argu- - ments as many times as you like, in order to match different subject - strings with the same pattern. - - This function is the main matching facility of the library, and it - operates in a Perl-like manner. For specialist use there is also an - alternative matching function, which is described below in the section - about the pcre_dfa_exec() function. - - In most applications, the pattern will have been compiled (and option- - ally studied) in the same process that calls pcre_exec(). However, it - is possible to save compiled patterns and study data, and then use them - later in different processes, possibly even on different hosts. For a - discussion about this, see the pcreprecompile documentation. - - Here is an example of a simple call to pcre_exec(): - - int rc; - int ovector[30]; - rc = pcre_exec( - re, /* result of pcre_compile() */ - NULL, /* we didn't study the pattern */ - "some string", /* the subject string */ - 11, /* the length of the subject string */ - 0, /* start at offset 0 in the subject */ - 0, /* default options */ - ovector, /* vector of integers for substring information */ - 30); /* number of elements (NOT size in bytes) */ - - Extra data for pcre_exec() - - If the extra argument is not NULL, it must point to a pcre_extra data - block. The pcre_study() function returns such a block (when it doesn't - return NULL), but you can also create one for yourself, and pass addi- - tional information in it. The pcre_extra block contains the following - fields (not necessarily in this order): - - unsigned long int flags; - void *study_data; - void *executable_jit; - unsigned long int match_limit; - unsigned long int match_limit_recursion; - void *callout_data; - const unsigned char *tables; - unsigned char **mark; - - In the 16-bit version of this structure, the mark field has type - "PCRE_UCHAR16 **". - - In the 32-bit version of this structure, the mark field has type - "PCRE_UCHAR32 **". - - The flags field is used to specify which of the other fields are set. - The flag bits are: - - PCRE_EXTRA_CALLOUT_DATA - PCRE_EXTRA_EXECUTABLE_JIT - PCRE_EXTRA_MARK - PCRE_EXTRA_MATCH_LIMIT - PCRE_EXTRA_MATCH_LIMIT_RECURSION - PCRE_EXTRA_STUDY_DATA - PCRE_EXTRA_TABLES - - Other flag bits should be set to zero. The study_data field and some- - times the executable_jit field are set in the pcre_extra block that is - returned by pcre_study(), together with the appropriate flag bits. You - should not set these yourself, but you may add to the block by setting - other fields and their corresponding flag bits. - - The match_limit field provides a means of preventing PCRE from using up - a vast amount of resources when running patterns that are not going to - match, but which have a very large number of possibilities in their - search trees. The classic example is a pattern that uses nested unlim- - ited repeats. - - Internally, pcre_exec() uses a function called match(), which it calls - repeatedly (sometimes recursively). The limit set by match_limit is - imposed on the number of times this function is called during a match, - which has the effect of limiting the amount of backtracking that can - take place. For patterns that are not anchored, the count restarts from - zero for each position in the subject string. - - When pcre_exec() is called with a pattern that was successfully studied - with a JIT option, the way that the matching is executed is entirely - different. However, there is still the possibility of runaway matching - that goes on for a very long time, and so the match_limit value is also - used in this case (but in a different way) to limit how long the match- - ing can continue. - - The default value for the limit can be set when PCRE is built; the - default default is 10 million, which handles all but the most extreme - cases. You can override the default by suppling pcre_exec() with a - pcre_extra block in which match_limit is set, and - PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is - exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. - - A value for the match limit may also be supplied by an item at the - start of a pattern of the form - - (*LIMIT_MATCH=d) - - where d is a decimal number. However, such a setting is ignored unless - d is less than the limit set by the caller of pcre_exec() or, if no - such limit is set, less than the default. - - The match_limit_recursion field is similar to match_limit, but instead - of limiting the total number of times that match() is called, it limits - the depth of recursion. The recursion depth is a smaller number than - the total number of calls, because not all calls to match() are recur- - sive. This limit is of use only if it is set smaller than match_limit. - - Limiting the recursion depth limits the amount of machine stack that - can be used, or, when PCRE has been compiled to use memory on the heap - instead of the stack, the amount of heap memory that can be used. This - limit is not relevant, and is ignored, when matching is done using JIT - compiled code. - - The default value for match_limit_recursion can be set when PCRE is - built; the default default is the same value as the default for - match_limit. You can override the default by suppling pcre_exec() with - a pcre_extra block in which match_limit_recursion is set, and - PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the - limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. - - A value for the recursion limit may also be supplied by an item at the - start of a pattern of the form - - (*LIMIT_RECURSION=d) - - where d is a decimal number. However, such a setting is ignored unless - d is less than the limit set by the caller of pcre_exec() or, if no - such limit is set, less than the default. - - The callout_data field is used in conjunction with the "callout" fea- - ture, and is described in the pcrecallout documentation. - - The tables field is provided for use with patterns that have been pre- - compiled using custom character tables, saved to disc or elsewhere, and - then reloaded, because the tables that were used to compile a pattern - are not saved with it. See the pcreprecompile documentation for a dis- - cussion of saving compiled patterns for later use. If NULL is passed - using this mechanism, it forces PCRE's internal tables to be used. - - Warning: The tables that pcre_exec() uses must be the same as those - that were used when the pattern was compiled. If this is not the case, - the behaviour of pcre_exec() is undefined. Therefore, when a pattern is - compiled and matched in the same process, this field should never be - set. In this (the most common) case, the correct table pointer is auto- - matically passed with the compiled pattern from pcre_compile() to - pcre_exec(). - - If PCRE_EXTRA_MARK is set in the flags field, the mark field must be - set to point to a suitable variable. If the pattern contains any back- - tracking control verbs such as (*MARK:NAME), and the execution ends up - with a name to pass back, a pointer to the name string (zero termi- - nated) is placed in the variable pointed to by the mark field. The - names are within the compiled pattern; if you wish to retain such a - name you must copy it before freeing the memory of a compiled pattern. - If there is no name to pass back, the variable pointed to by the mark - field is set to NULL. For details of the backtracking control verbs, - see the section entitled "Backtracking control" in the pcrepattern doc- - umentation. - - Option bits for pcre_exec() - - The unused bits of the options argument for pcre_exec() must be zero. - The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, - PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, - PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and - PCRE_PARTIAL_SOFT. - - If the pattern was successfully studied with one of the just-in-time - (JIT) compile options, the only supported options for JIT execution are - PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, - PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an - unsupported option is used, JIT execution is disabled and the normal - interpretive code in pcre_exec() is run. - - PCRE_ANCHORED - - The PCRE_ANCHORED option limits pcre_exec() to matching at the first - matching position. If a pattern was compiled with PCRE_ANCHORED, or - turned out to be anchored by virtue of its contents, it cannot be made - unachored at matching time. - - PCRE_BSR_ANYCRLF - PCRE_BSR_UNICODE - - These options (which are mutually exclusive) control what the \R escape - sequence matches. The choice is either to match only CR, LF, or CRLF, - or to match any Unicode newline sequence. These options override the - choice that was made or defaulted when the pattern was compiled. - - PCRE_NEWLINE_CR - PCRE_NEWLINE_LF - PCRE_NEWLINE_CRLF - PCRE_NEWLINE_ANYCRLF - PCRE_NEWLINE_ANY - - These options override the newline definition that was chosen or - defaulted when the pattern was compiled. For details, see the descrip- - tion of pcre_compile() above. During matching, the newline choice - affects the behaviour of the dot, circumflex, and dollar metacharac- - ters. It may also alter the way the match position is advanced after a - match failure for an unanchored pattern. - - When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is - set, and a match attempt for an unanchored pattern fails when the cur- - rent position is at a CRLF sequence, and the pattern contains no - explicit matches for CR or LF characters, the match position is - advanced by two characters instead of one, in other words, to after the - CRLF. - - The above rule is a compromise that makes the most common cases work as - expected. For example, if the pattern is .+A (and the PCRE_DOTALL - option is not set), it does not match the string "\r\nA" because, after - failing at the start, it skips both the CR and the LF before retrying. - However, the pattern [\r\n]A does match that string, because it con- - tains an explicit CR or LF reference, and so advances only by one char- - acter after the first failure. - - An explicit match for CR of LF is either a literal appearance of one of - those characters, or one of the \r or \n escape sequences. Implicit - matches such as [^X] do not count, nor does \s (which includes CR and - LF in the characters that it matches). - - Notwithstanding the above, anomalous effects may still occur when CRLF - is a valid newline sequence and explicit \r or \n escapes appear in the - pattern. - - PCRE_NOTBOL - - This option specifies that first character of the subject string is not - the beginning of a line, so the circumflex metacharacter should not - match before it. Setting this without PCRE_MULTILINE (at compile time) - causes circumflex never to match. This option affects only the behav- - iour of the circumflex metacharacter. It does not affect \A. - - PCRE_NOTEOL - - This option specifies that the end of the subject string is not the end - of a line, so the dollar metacharacter should not match it nor (except - in multiline mode) a newline immediately before it. Setting this with- - out PCRE_MULTILINE (at compile time) causes dollar never to match. This - option affects only the behaviour of the dollar metacharacter. It does - not affect \Z or \z. - - PCRE_NOTEMPTY - - An empty string is not considered to be a valid match if this option is - set. If there are alternatives in the pattern, they are tried. If all - the alternatives match the empty string, the entire match fails. For - example, if the pattern - - a?b? - - is applied to a string not beginning with "a" or "b", it matches an - empty string at the start of the subject. With PCRE_NOTEMPTY set, this - match is not valid, so PCRE searches further into the string for occur- - rences of "a" or "b". - - PCRE_NOTEMPTY_ATSTART - - This is like PCRE_NOTEMPTY, except that an empty string match that is - not at the start of the subject is permitted. If the pattern is - anchored, such a match can occur only if the pattern contains \K. - - Perl has no direct equivalent of PCRE_NOTEMPTY or - PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern - match of the empty string within its split() function, and when using - the /g modifier. It is possible to emulate Perl's behaviour after - matching a null string by first trying the match again at the same off- - set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that - fails, by advancing the starting offset (see below) and trying an ordi- - nary match again. There is some code that demonstrates how to do this - in the pcredemo sample program. In the most general case, you have to - check to see if the newline convention recognizes CRLF as a newline, - and if so, and the current character is CR followed by LF, advance the - starting offset by two characters instead of one. - - PCRE_NO_START_OPTIMIZE - - There are a number of optimizations that pcre_exec() uses at the start - of a match, in order to speed up the process. For example, if it is - known that an unanchored match must start with a specific character, it - searches the subject for that character, and fails immediately if it - cannot find it, without actually running the main matching function. - This means that a special item such as (*COMMIT) at the start of a pat- - tern is not considered until after a suitable starting point for the - match has been found. Also, when callouts or (*MARK) items are in use, - these "start-up" optimizations can cause them to be skipped if the pat- - tern is never actually used. The start-up optimizations are in effect a - pre-scan of the subject that takes place before the pattern is run. - - The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, - possibly causing performance to suffer, but ensuring that in cases - where the result is "no match", the callouts do occur, and that items - such as (*COMMIT) and (*MARK) are considered at every possible starting - position in the subject string. If PCRE_NO_START_OPTIMIZE is set at - compile time, it cannot be unset at matching time. The use of - PCRE_NO_START_OPTIMIZE at matching time (that is, passing it to - pcre_exec()) disables JIT execution; in this situation, matching is - always done using interpretively. - - Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching - operation. Consider the pattern - - (*COMMIT)ABC - - When this is compiled, PCRE records the fact that a match must start - with the character "A". Suppose the subject string is "DEFABC". The - start-up optimization scans along the subject, finds "A" and runs the - first match attempt from there. The (*COMMIT) item means that the pat- - tern must match the current starting position, which in this case, it - does. However, if the same match is run with PCRE_NO_START_OPTIMIZE - set, the initial scan along the subject string does not happen. The - first match attempt is run starting from "D" and when this fails, - (*COMMIT) prevents any further matches being tried, so the overall - result is "no match". If the pattern is studied, more start-up opti- - mizations may be used. For example, a minimum length for the subject - may be recorded. Consider the pattern - - (*MARK:A)(X|Y) - - The minimum length for a match is one character. If the subject is - "ABC", there will be attempts to match "ABC", "BC", "C", and then - finally an empty string. If the pattern is studied, the final attempt - does not take place, because PCRE knows that the subject is too short, - and so the (*MARK) is never encountered. In this case, studying the - pattern does not affect the overall match result, which is still "no - match", but it does affect the auxiliary information that is returned. - - PCRE_NO_UTF8_CHECK - - When PCRE_UTF8 is set at compile time, the validity of the subject as a - UTF-8 string is automatically checked when pcre_exec() is subsequently - called. The entire string is checked before any other processing takes - place. The value of startoffset is also checked to ensure that it - points to the start of a UTF-8 character. There is a discussion about - the validity of UTF-8 strings in the pcreunicode page. If an invalid - sequence of bytes is found, pcre_exec() returns the error - PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a - truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In - both cases, information about the precise nature of the error may also - be returned (see the descriptions of these errors in the section enti- - tled Error return values from pcre_exec() below). If startoffset con- - tains a value that does not point to the start of a UTF-8 character (or - to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned. - - If you already know that your subject is valid, and you want to skip - these checks for performance reasons, you can set the - PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to - do this for the second and subsequent calls to pcre_exec() if you are - making repeated calls to find all the matches in a single subject - string. However, you should be sure that the value of startoffset - points to the start of a character (or the end of the subject). When - PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a - subject or an invalid value of startoffset is undefined. Your program - may crash or loop. - - PCRE_PARTIAL_HARD - PCRE_PARTIAL_SOFT - - These options turn on the partial matching feature. For backwards com- - patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial - match occurs if the end of the subject string is reached successfully, - but there are not enough subject characters to complete the match. If - this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, - matching continues by testing any remaining alternatives. Only if no - complete match can be found is PCRE_ERROR_PARTIAL returned instead of - PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the - caller is prepared to handle a partial match, but only if no complete - match can be found. - - If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this - case, if a partial match is found, pcre_exec() immediately returns - PCRE_ERROR_PARTIAL, without considering any other alternatives. In - other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- - ered to be more important that an alternative complete match. - - In both cases, the portion of the string that was inspected when the - partial match was found is set as the first matching string. There is a - more detailed discussion of partial and multi-segment matching, with - examples, in the pcrepartial documentation. - - The string to be matched by pcre_exec() - - The subject string is passed to pcre_exec() as a pointer in subject, a - length in length, and a starting offset in startoffset. The units for - length and startoffset are bytes for the 8-bit library, 16-bit data - items for the 16-bit library, and 32-bit data items for the 32-bit - library. - - If startoffset is negative or greater than the length of the subject, - pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is - zero, the search for a match starts at the beginning of the subject, - and this is by far the most common case. In UTF-8 or UTF-16 mode, the - offset must point to the start of a character, or the end of the sub- - ject (in UTF-32 mode, one data unit equals one character, so all off- - sets are valid). Unlike the pattern string, the subject may contain - binary zeroes. - - A non-zero starting offset is useful when searching for another match - in the same subject by calling pcre_exec() again after a previous suc- - cess. Setting startoffset differs from just passing over a shortened - string and setting PCRE_NOTBOL in the case of a pattern that begins - with any kind of lookbehind. For example, consider the pattern - - \Biss\B - - which finds occurrences of "iss" in the middle of words. (\B matches - only if the current position in the subject is not a word boundary.) - When applied to the string "Mississipi" the first call to pcre_exec() - finds the first occurrence. If pcre_exec() is called again with just - the remainder of the subject, namely "issipi", it does not match, - because \B is always false at the start of the subject, which is deemed - to be a word boundary. However, if pcre_exec() is passed the entire - string again, but with startoffset set to 4, it finds the second occur- - rence of "iss" because it is able to look behind the starting point to - discover that it is preceded by a letter. - - Finding all the matches in a subject is tricky when the pattern can - match an empty string. It is possible to emulate Perl's /g behaviour by - first trying the match again at the same offset, with the - PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that - fails, advancing the starting offset and trying an ordinary match - again. There is some code that demonstrates how to do this in the pcre- - demo sample program. In the most general case, you have to check to see - if the newline convention recognizes CRLF as a newline, and if so, and - the current character is CR followed by LF, advance the starting offset - by two characters instead of one. - - If a non-zero starting offset is passed when the pattern is anchored, - one attempt to match at the given offset is made. This can only succeed - if the pattern does not require the match to be at the start of the - subject. - - How pcre_exec() returns captured substrings - - In general, a pattern matches a certain portion of the subject, and in - addition, further substrings from the subject may be picked out by - parts of the pattern. Following the usage in Jeffrey Friedl's book, - this is called "capturing" in what follows, and the phrase "capturing - subpattern" is used for a fragment of a pattern that picks out a sub- - string. PCRE supports several other kinds of parenthesized subpattern - that do not cause substrings to be captured. - - Captured substrings are returned to the caller via a vector of integers - whose address is passed in ovector. The number of elements in the vec- - tor is passed in ovecsize, which must be a non-negative number. Note: - this argument is NOT the size of ovector in bytes. - - The first two-thirds of the vector is used to pass back captured sub- - strings, each substring using a pair of integers. The remaining third - of the vector is used as workspace by pcre_exec() while matching cap- - turing subpatterns, and is not available for passing back information. - The number passed in ovecsize should always be a multiple of three. If - it is not, it is rounded down. - - When a match is successful, information about captured substrings is - returned in pairs of integers, starting at the beginning of ovector, - and continuing up to two-thirds of its length at the most. The first - element of each pair is set to the offset of the first character in a - substring, and the second is set to the offset of the first character - after the end of a substring. These values are always data unit off- - sets, even in UTF mode. They are byte offsets in the 8-bit library, - 16-bit data item offsets in the 16-bit library, and 32-bit data item - offsets in the 32-bit library. Note: they are not character counts. - - The first pair of integers, ovector[0] and ovector[1], identify the - portion of the subject string matched by the entire pattern. The next - pair is used for the first capturing subpattern, and so on. The value - returned by pcre_exec() is one more than the highest numbered pair that - has been set. For example, if two substrings have been captured, the - returned value is 3. If there are no capturing subpatterns, the return - value from a successful match is 1, indicating that just the first pair - of offsets has been set. - - If a capturing subpattern is matched repeatedly, it is the last portion - of the string that it matched that is returned. - - If the vector is too small to hold all the captured substring offsets, - it is used as far as possible (up to two-thirds of its length), and the - function returns a value of zero. If neither the actual string matched - nor any captured substrings are of interest, pcre_exec() may be called - with ovector passed as NULL and ovecsize as zero. However, if the pat- - tern contains back references and the ovector is not big enough to - remember the related substrings, PCRE has to get additional memory for - use during matching. Thus it is usually advisable to supply an ovector - of reasonable size. - - There are some cases where zero is returned (indicating vector over- - flow) when in fact the vector is exactly the right size for the final - match. For example, consider the pattern - - (a)(?:(b)c|bd) - - If a vector of 6 elements (allowing for only 1 captured substring) is - given with subject string "abd", pcre_exec() will try to set the second - captured string, thereby recording a vector overflow, before failing to - match "c" and backing up to try the second alternative. The zero - return, however, does correctly indicate that the maximum number of - slots (namely 2) have been filled. In similar cases where there is tem- - porary overflow, but the final number of used slots is actually less - than the maximum, a non-zero value is returned. - - The pcre_fullinfo() function can be used to find out how many capturing - subpatterns there are in a compiled pattern. The smallest size for - ovector that will allow for n captured substrings, in addition to the - offsets of the substring matched by the whole pattern, is (n+1)*3. - - It is possible for capturing subpattern number n+1 to match some part - of the subject when subpattern n has not been used at all. For example, - if the string "abc" is matched against the pattern (a|(z))(bc) the - return from the function is 4, and subpatterns 1 and 3 are matched, but - 2 is not. When this happens, both values in the offset pairs corre- - sponding to unused subpatterns are set to -1. - - Offset values that correspond to unused subpatterns at the end of the - expression are also set to -1. For example, if the string "abc" is - matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not - matched. The return from the function is 2, because the highest used - capturing subpattern number is 1, and the offsets for for the second - and third capturing subpatterns (assuming the vector is large enough, - of course) are set to -1. - - Note: Elements in the first two-thirds of ovector that do not corre- - spond to capturing parentheses in the pattern are never changed. That - is, if a pattern contains n capturing parentheses, no more than ovec- - tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in - the first two-thirds) retain whatever values they previously had. - - Some convenience functions are provided for extracting the captured - substrings as separate strings. These are described below. - - Error return values from pcre_exec() - - If pcre_exec() fails, it returns a negative number. The following are - defined in the header file: - - PCRE_ERROR_NOMATCH (-1) - - The subject string did not match the pattern. - - PCRE_ERROR_NULL (-2) - - Either code or subject was passed as NULL, or ovector was NULL and - ovecsize was not zero. - - PCRE_ERROR_BADOPTION (-3) - - An unrecognized bit was set in the options argument. - - PCRE_ERROR_BADMAGIC (-4) - - PCRE stores a 4-byte "magic number" at the start of the compiled code, - to catch the case when it is passed a junk pointer and to detect when a - pattern that was compiled in an environment of one endianness is run in - an environment with the other endianness. This is the error that PCRE - gives when the magic number is not present. - - PCRE_ERROR_UNKNOWN_OPCODE (-5) - - While running the pattern match, an unknown item was encountered in the - compiled pattern. This error could be caused by a bug in PCRE or by - overwriting of the compiled pattern. - - PCRE_ERROR_NOMEMORY (-6) - - If a pattern contains back references, but the ovector that is passed - to pcre_exec() is not big enough to remember the referenced substrings, - PCRE gets a block of memory at the start of matching to use for this - purpose. If the call via pcre_malloc() fails, this error is given. The - memory is automatically freed at the end of matching. - - This error is also given if pcre_stack_malloc() fails in pcre_exec(). - This can happen only when PCRE has been compiled with --disable-stack- - for-recursion. - - PCRE_ERROR_NOSUBSTRING (-7) - - This error is used by the pcre_copy_substring(), pcre_get_substring(), - and pcre_get_substring_list() functions (see below). It is never - returned by pcre_exec(). - - PCRE_ERROR_MATCHLIMIT (-8) - - The backtracking limit, as specified by the match_limit field in a - pcre_extra structure (or defaulted) was reached. See the description - above. - - PCRE_ERROR_CALLOUT (-9) - - This error is never generated by pcre_exec() itself. It is provided for - use by callout functions that want to yield a distinctive error code. - See the pcrecallout documentation for details. - - PCRE_ERROR_BADUTF8 (-10) - - A string that contains an invalid UTF-8 byte sequence was passed as a - subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of - the output vector (ovecsize) is at least 2, the byte offset to the - start of the the invalid UTF-8 character is placed in the first ele- - ment, and a reason code is placed in the second element. The reason - codes are listed in the following section. For backward compatibility, - if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char- - acter at the end of the subject (reason codes 1 to 5), - PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8. - - PCRE_ERROR_BADUTF8_OFFSET (-11) - - The UTF-8 byte sequence that was passed as a subject was checked and - found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the - value of startoffset did not point to the beginning of a UTF-8 charac- - ter or the end of the subject. - - PCRE_ERROR_PARTIAL (-12) - - The subject string did not match, but it did match partially. See the - pcrepartial documentation for details of partial matching. - - PCRE_ERROR_BADPARTIAL (-13) - - This code is no longer in use. It was formerly returned when the - PCRE_PARTIAL option was used with a compiled pattern containing items - that were not supported for partial matching. From release 8.00 - onwards, there are no restrictions on partial matching. - - PCRE_ERROR_INTERNAL (-14) - - An unexpected internal error has occurred. This error could be caused - by a bug in PCRE or by overwriting of the compiled pattern. - - PCRE_ERROR_BADCOUNT (-15) - - This error is given if the value of the ovecsize argument is negative. - - PCRE_ERROR_RECURSIONLIMIT (-21) - - The internal recursion limit, as specified by the match_limit_recursion - field in a pcre_extra structure (or defaulted) was reached. See the - description above. - - PCRE_ERROR_BADNEWLINE (-23) - - An invalid combination of PCRE_NEWLINE_xxx options was given. - - PCRE_ERROR_BADOFFSET (-24) - - The value of startoffset was negative or greater than the length of the - subject, that is, the value in length. - - PCRE_ERROR_SHORTUTF8 (-25) - - This error is returned instead of PCRE_ERROR_BADUTF8 when the subject - string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD - option is set. Information about the failure is returned as for - PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but - this special error code for PCRE_PARTIAL_HARD precedes the implementa- - tion of returned information; it is retained for backwards compatibil- - ity. - - PCRE_ERROR_RECURSELOOP (-26) - - This error is returned when pcre_exec() detects a recursion loop within - the pattern. Specifically, it means that either the whole pattern or a - subpattern has been called recursively for the second time at the same - position in the subject string. Some simple patterns that might do this - are detected and faulted at compile time, but more complicated cases, - in particular mutual recursions between two different subpatterns, can- - not be detected until run time. - - PCRE_ERROR_JIT_STACKLIMIT (-27) - - This error is returned when a pattern that was successfully studied - using a JIT compile option is being matched, but the memory available - for the just-in-time processing stack is not large enough. See the - pcrejit documentation for more details. - - PCRE_ERROR_BADMODE (-28) - - This error is given if a pattern that was compiled by the 8-bit library - is passed to a 16-bit or 32-bit library function, or vice versa. - - PCRE_ERROR_BADENDIANNESS (-29) - - This error is given if a pattern that was compiled and saved is - reloaded on a host with different endianness. The utility function - pcre_pattern_to_host_byte_order() can be used to convert such a pattern - so that it runs on the new host. - - PCRE_ERROR_JIT_BADOPTION - - This error is returned when a pattern that was successfully studied - using a JIT compile option is being matched, but the matching mode - (partial or complete match) does not correspond to any JIT compilation - mode. When the JIT fast path function is used, this error may be also - given for invalid options. See the pcrejit documentation for more - details. - - PCRE_ERROR_BADLENGTH (-32) - - This error is given if pcre_exec() is called with a negative value for - the length argument. - - Error numbers -16 to -20, -22, and 30 are not used by pcre_exec(). - - Reason codes for invalid UTF-8 strings - - This section applies only to the 8-bit library. The corresponding - information for the 16-bit and 32-bit libraries is given in the pcre16 - and pcre32 pages. - - When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT- - UTF8, and the size of the output vector (ovecsize) is at least 2, the - offset of the start of the invalid UTF-8 character is placed in the - first output vector element (ovector[0]) and a reason code is placed in - the second element (ovector[1]). The reason codes are given names in - the pcre.h header file: - - PCRE_UTF8_ERR1 - PCRE_UTF8_ERR2 - PCRE_UTF8_ERR3 - PCRE_UTF8_ERR4 - PCRE_UTF8_ERR5 - - The string ends with a truncated UTF-8 character; the code specifies - how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 - characters to be no longer than 4 bytes, the encoding scheme (origi- - nally defined by RFC 2279) allows for up to 6 bytes, and this is - checked first; hence the possibility of 4 or 5 missing bytes. - - PCRE_UTF8_ERR6 - PCRE_UTF8_ERR7 - PCRE_UTF8_ERR8 - PCRE_UTF8_ERR9 - PCRE_UTF8_ERR10 - - The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of - the character do not have the binary value 0b10 (that is, either the - most significant bit is 0, or the next bit is 1). - - PCRE_UTF8_ERR11 - PCRE_UTF8_ERR12 - - A character that is valid by the RFC 2279 rules is either 5 or 6 bytes - long; these code points are excluded by RFC 3629. - - PCRE_UTF8_ERR13 - - A 4-byte character has a value greater than 0x10fff; these code points - are excluded by RFC 3629. - - PCRE_UTF8_ERR14 - - A 3-byte character has a value in the range 0xd800 to 0xdfff; this - range of code points are reserved by RFC 3629 for use with UTF-16, and - so are excluded from UTF-8. - - PCRE_UTF8_ERR15 - PCRE_UTF8_ERR16 - PCRE_UTF8_ERR17 - PCRE_UTF8_ERR18 - PCRE_UTF8_ERR19 - - A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes - for a value that can be represented by fewer bytes, which is invalid. - For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- - rect coding uses just one byte. - - PCRE_UTF8_ERR20 - - The two most significant bits of the first byte of a character have the - binary value 0b10 (that is, the most significant bit is 1 and the sec- - ond is 0). Such a byte can only validly occur as the second or subse- - quent byte of a multi-byte character. - - PCRE_UTF8_ERR21 - - The first byte of a character has the value 0xfe or 0xff. These values - can never occur in a valid UTF-8 string. - - PCRE_UTF8_ERR22 - - This error code was formerly used when the presence of a so-called - "non-character" caused an error. Unicode corrigendum #9 makes it clear - that such characters should not cause a string to be rejected, and so - this code is no longer in use and is never returned. - - -EXTRACTING CAPTURED SUBSTRINGS BY NUMBER - - int pcre_copy_substring(const char *subject, int *ovector, - int stringcount, int stringnumber, char *buffer, - int buffersize); - - int pcre_get_substring(const char *subject, int *ovector, - int stringcount, int stringnumber, - const char **stringptr); - - int pcre_get_substring_list(const char *subject, - int *ovector, int stringcount, const char ***listptr); - - Captured substrings can be accessed directly by using the offsets - returned by pcre_exec() in ovector. For convenience, the functions - pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- - string_list() are provided for extracting captured substrings as new, - separate, zero-terminated strings. These functions identify substrings - by number. The next section describes functions for extracting named - substrings. - - A substring that contains a binary zero is correctly extracted and has - a further zero added on the end, but the result is not, of course, a C - string. However, you can process such a string by referring to the - length that is returned by pcre_copy_substring() and pcre_get_sub- - string(). Unfortunately, the interface to pcre_get_substring_list() is - not adequate for handling strings containing binary zeros, because the - end of the final string is not independently indicated. - - The first three arguments are the same for all three of these func- - tions: subject is the subject string that has just been successfully - matched, ovector is a pointer to the vector of integer offsets that was - passed to pcre_exec(), and stringcount is the number of substrings that - were captured by the match, including the substring that matched the - entire regular expression. This is the value returned by pcre_exec() if - it is greater than zero. If pcre_exec() returned zero, indicating that - it ran out of space in ovector, the value passed as stringcount should - be the number of elements in the vector divided by three. - - The functions pcre_copy_substring() and pcre_get_substring() extract a - single substring, whose number is given as stringnumber. A value of - zero extracts the substring that matched the entire pattern, whereas - higher values extract the captured substrings. For pcre_copy_sub- - string(), the string is placed in buffer, whose length is given by - buffersize, while for pcre_get_substring() a new block of memory is - obtained via pcre_malloc, and its address is returned via stringptr. - The yield of the function is the length of the string, not including - the terminating zero, or one of these error codes: - - PCRE_ERROR_NOMEMORY (-6) - - The buffer was too small for pcre_copy_substring(), or the attempt to - get memory failed for pcre_get_substring(). - - PCRE_ERROR_NOSUBSTRING (-7) - - There is no substring whose number is stringnumber. - - The pcre_get_substring_list() function extracts all available sub- - strings and builds a list of pointers to them. All this is done in a - single block of memory that is obtained via pcre_malloc. The address of - the memory block is returned via listptr, which is also the start of - the list of string pointers. The end of the list is marked by a NULL - pointer. The yield of the function is zero if all went well, or the - error code - - PCRE_ERROR_NOMEMORY (-6) - - if the attempt to get the memory block failed. - - When any of these functions encounter a substring that is unset, which - can happen when capturing subpattern number n+1 matches some part of - the subject, but subpattern n has not been used at all, they return an - empty string. This can be distinguished from a genuine zero-length sub- - string by inspecting the appropriate offset in ovector, which is nega- - tive for unset substrings. - - The two convenience functions pcre_free_substring() and pcre_free_sub- - string_list() can be used to free the memory returned by a previous - call of pcre_get_substring() or pcre_get_substring_list(), respec- - tively. They do nothing more than call the function pointed to by - pcre_free, which of course could be called directly from a C program. - However, PCRE is used in some situations where it is linked via a spe- - cial interface to another programming language that cannot use - pcre_free directly; it is for these cases that the functions are pro- - vided. - - -EXTRACTING CAPTURED SUBSTRINGS BY NAME - - int pcre_get_stringnumber(const pcre *code, - const char *name); - - int pcre_copy_named_substring(const pcre *code, - const char *subject, int *ovector, - int stringcount, const char *stringname, - char *buffer, int buffersize); - - int pcre_get_named_substring(const pcre *code, - const char *subject, int *ovector, - int stringcount, const char *stringname, - const char **stringptr); - - To extract a substring by name, you first have to find associated num- - ber. For example, for this pattern - - (a+)b(?<xxx>\d+)... - - the number of the subpattern called "xxx" is 2. If the name is known to - be unique (PCRE_DUPNAMES was not set), you can find the number from the - name by calling pcre_get_stringnumber(). The first argument is the com- - piled pattern, and the second is the name. The yield of the function is - the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no - subpattern of that name. - - Given the number, you can extract the substring directly, or use one of - the functions described in the previous section. For convenience, there - are also two functions that do the whole job. - - Most of the arguments of pcre_copy_named_substring() and - pcre_get_named_substring() are the same as those for the similarly - named functions that extract by number. As these are described in the - previous section, they are not re-described here. There are just two - differences: - - First, instead of a substring number, a substring name is given. Sec- - ond, there is an extra argument, given at the start, which is a pointer - to the compiled pattern. This is needed in order to gain access to the - name-to-number translation table. - - These functions call pcre_get_stringnumber(), and if it succeeds, they - then call pcre_copy_substring() or pcre_get_substring(), as appropri- - ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the - behaviour may not be what you want (see the next section). - - Warning: If the pattern uses the (?| feature to set up multiple subpat- - terns with the same number, as described in the section on duplicate - subpattern numbers in the pcrepattern page, you cannot use names to - distinguish the different subpatterns, because names are not included - in the compiled code. The matching process uses only numbers. For this - reason, the use of different names for subpatterns of the same number - causes an error at compile time. - - -DUPLICATE SUBPATTERN NAMES - - int pcre_get_stringtable_entries(const pcre *code, - const char *name, char **first, char **last); - - When a pattern is compiled with the PCRE_DUPNAMES option, names for - subpatterns are not required to be unique. (Duplicate names are always - allowed for subpatterns with the same number, created by using the (?| - feature. Indeed, if such subpatterns are named, they are required to - use the same names.) - - Normally, patterns with duplicate names are such that in any one match, - only one of the named subpatterns participates. An example is shown in - the pcrepattern documentation. - - When duplicates are present, pcre_copy_named_substring() and - pcre_get_named_substring() return the first substring corresponding to - the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING - (-7) is returned; no data is returned. The pcre_get_stringnumber() - function returns one of the numbers that are associated with the name, - but it is not defined which it is. - - If you want to get full details of all captured substrings for a given - name, you must use the pcre_get_stringtable_entries() function. The - first argument is the compiled pattern, and the second is the name. The - third and fourth are pointers to variables which are updated by the - function. After it has run, they point to the first and last entries in - the name-to-number table for the given name. The function itself - returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if - there are none. The format of the table is described above in the sec- - tion entitled Information about a pattern above. Given all the rele- - vant entries for the name, you can extract each of their numbers, and - hence the captured data, if any. - - -FINDING ALL POSSIBLE MATCHES - - The traditional matching function uses a similar algorithm to Perl, - which stops when it finds the first match, starting at a given point in - the subject. If you want to find all possible matches, or the longest - possible match, consider using the alternative matching function (see - below) instead. If you cannot use the alternative function, but still - need to find all possible matches, you can kludge it up by making use - of the callout facility, which is described in the pcrecallout documen- - tation. - - What you have to do is to insert a callout right at the end of the pat- - tern. When your callout function is called, extract and save the cur- - rent matched substring. Then return 1, which forces pcre_exec() to - backtrack and try other alternatives. Ultimately, when it runs out of - matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. - - -OBTAINING AN ESTIMATE OF STACK USAGE - - Matching certain patterns using pcre_exec() can use a lot of process - stack, which in certain environments can be rather limited in size. - Some users find it helpful to have an estimate of the amount of stack - that is used by pcre_exec(), to help them set recursion limits, as - described in the pcrestack documentation. The estimate that is output - by pcretest when called with the -m and -C options is obtained by call- - ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its - first five arguments. - - Normally, if its first argument is NULL, pcre_exec() immediately - returns the negative error code PCRE_ERROR_NULL, but with this special - combination of arguments, it returns instead a negative number whose - absolute value is the approximate stack frame size in bytes. (A nega- - tive number is used so that it is clear that no match has happened.) - The value is approximate because in some cases, recursive calls to - pcre_exec() occur when there are one or two additional variables on the - stack. - - If PCRE has been compiled to use the heap instead of the stack for - recursion, the value returned is the size of each block that is - obtained from the heap. - - -MATCHING A PATTERN: THE ALTERNATIVE FUNCTION - - int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, - const char *subject, int length, int startoffset, - int options, int *ovector, int ovecsize, - int *workspace, int wscount); - - The function pcre_dfa_exec() is called to match a subject string - against a compiled pattern, using a matching algorithm that scans the - subject string just once, and does not backtrack. This has different - characteristics to the normal algorithm, and is not compatible with - Perl. Some of the features of PCRE patterns are not supported. Never- - theless, there are times when this kind of matching can be useful. For - a discussion of the two matching algorithms, and a list of features - that pcre_dfa_exec() does not support, see the pcrematching documenta- - tion. - - The arguments for the pcre_dfa_exec() function are the same as for - pcre_exec(), plus two extras. The ovector argument is used in a differ- - ent way, and this is described below. The other common arguments are - used in the same way as for pcre_exec(), so their description is not - repeated here. - - The two additional arguments provide workspace for the function. The - workspace vector should contain at least 20 elements. It is used for - keeping track of multiple paths through the pattern tree. More - workspace will be needed for patterns and subjects where there are a - lot of potential matches. - - Here is an example of a simple call to pcre_dfa_exec(): - - int rc; - int ovector[10]; - int wspace[20]; - rc = pcre_dfa_exec( - re, /* result of pcre_compile() */ - NULL, /* we didn't study the pattern */ - "some string", /* the subject string */ - 11, /* the length of the subject string */ - 0, /* start at offset 0 in the subject */ - 0, /* default options */ - ovector, /* vector of integers for substring information */ - 10, /* number of elements (NOT size in bytes) */ - wspace, /* working space vector */ - 20); /* number of elements (NOT size in bytes) */ - - Option bits for pcre_dfa_exec() - - The unused bits of the options argument for pcre_dfa_exec() must be - zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- - LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, - PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, - PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- - TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last - four of these are exactly the same as for pcre_exec(), so their - description is not repeated here. - - PCRE_PARTIAL_HARD - PCRE_PARTIAL_SOFT - - These have the same general effect as they do for pcre_exec(), but the - details are slightly different. When PCRE_PARTIAL_HARD is set for - pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- - ject is reached and there is still at least one matching possibility - that requires additional characters. This happens even if some complete - matches have also been found. When PCRE_PARTIAL_SOFT is set, the return - code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end - of the subject is reached, there have been no complete matches, but - there is still at least one matching possibility. The portion of the - string that was inspected when the longest partial match was found is - set as the first matching string in both cases. There is a more - detailed discussion of partial and multi-segment matching, with exam- - ples, in the pcrepartial documentation. - - PCRE_DFA_SHORTEST - - Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to - stop as soon as it has found one match. Because of the way the alterna- - tive algorithm works, this is necessarily the shortest possible match - at the first possible matching point in the subject string. - - PCRE_DFA_RESTART - - When pcre_dfa_exec() returns a partial match, it is possible to call it - again, with additional subject characters, and have it continue with - the same match. The PCRE_DFA_RESTART option requests this action; when - it is set, the workspace and wscount options must reference the same - vector as before because data about the match so far is left in them - after a partial match. There is more discussion of this facility in the - pcrepartial documentation. - - Successful returns from pcre_dfa_exec() - - When pcre_dfa_exec() succeeds, it may have matched more than one sub- - string in the subject. Note, however, that all the matches from one run - of the function start at the same point in the subject. The shorter - matches are all initial substrings of the longer matches. For example, - if the pattern - - <.*> - - is matched against the string - - This is <something> <something else> <something further> no more - - the three matched strings are - - <something> - <something> <something else> - <something> <something else> <something further> - - On success, the yield of the function is a number greater than zero, - which is the number of matched substrings. The substrings themselves - are returned in ovector. Each string uses two elements; the first is - the offset to the start, and the second is the offset to the end. In - fact, all the strings have the same start offset. (Space could have - been saved by giving this only once, but it was decided to retain some - compatibility with the way pcre_exec() returns data, even though the - meaning of the strings is different.) - - The strings are returned in reverse order of length; that is, the long- - est matching string is given first. If there were too many matches to - fit into ovector, the yield of the function is zero, and the vector is - filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() - can use the entire ovector for returning matched strings. - - NOTE: PCRE's "auto-possessification" optimization usually applies to - character repeats at the end of a pattern (as well as internally). For - example, the pattern "a\d+" is compiled as if it were "a\d++" because - there is no point even considering the possibility of backtracking into - the repeated digits. For DFA matching, this means that only one possi- - ble match is found. If you really do want multiple matches in such - cases, either use an ungreedy repeat ("a\d+?") or set the - PCRE_NO_AUTO_POSSESS option when compiling. - - Error returns from pcre_dfa_exec() - - The pcre_dfa_exec() function returns a negative number when it fails. - Many of the errors are the same as for pcre_exec(), and these are - described above. There are in addition the following errors that are - specific to pcre_dfa_exec(): - - PCRE_ERROR_DFA_UITEM (-16) - - This return is given if pcre_dfa_exec() encounters an item in the pat- - tern that it does not support, for instance, the use of \C or a back - reference. - - PCRE_ERROR_DFA_UCOND (-17) - - This return is given if pcre_dfa_exec() encounters a condition item - that uses a back reference for the condition, or a test for recursion - in a specific group. These are not supported. - - PCRE_ERROR_DFA_UMLIMIT (-18) - - This return is given if pcre_dfa_exec() is called with an extra block - that contains a setting of the match_limit or match_limit_recursion - fields. This is not supported (these fields are meaningless for DFA - matching). - - PCRE_ERROR_DFA_WSSIZE (-19) - - This return is given if pcre_dfa_exec() runs out of space in the - workspace vector. - - PCRE_ERROR_DFA_RECURSE (-20) - - When a recursive subpattern is processed, the matching function calls - itself recursively, using private vectors for ovector and workspace. - This error is given if the output vector is not large enough. This - should be extremely rare, as a vector of size 1000 is used. - - PCRE_ERROR_DFA_BADRESTART (-30) - - When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some - plausibility checks are made on the contents of the workspace, which - should contain data about the previous partial match. If any of these - checks fail, this error is given. - - -SEE ALSO - - pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), - pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcre- - sample(3), pcrestack(3). - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 09 February 2014 - Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -SYNOPSIS - - #include <pcre.h> - - int (*pcre_callout)(pcre_callout_block *); - - int (*pcre16_callout)(pcre16_callout_block *); - - int (*pcre32_callout)(pcre32_callout_block *); - - -DESCRIPTION - - PCRE provides a feature called "callout", which is a means of temporar- - ily passing control to the caller of PCRE in the middle of pattern - matching. The caller of PCRE provides an external function by putting - its entry point in the global variable pcre_callout (pcre16_callout for - the 16-bit library, pcre32_callout for the 32-bit library). By default, - this variable contains NULL, which disables all calling out. - - Within a regular expression, (?C) indicates the points at which the - external function is to be called. Different callout points can be - identified by putting a number less than 256 after the letter C. The - default value is zero. For example, this pattern has two callout - points: - - (?C1)abc(?C2)def - - If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, - PCRE automatically inserts callouts, all with number 255, before each - item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the - pattern - - A(\d{2}|--) - - it is processed as if it were - - (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) - - Notice that there is a callout before and after each parenthesis and - alternation bar. If the pattern contains a conditional group whose con- - dition is an assertion, an automatic callout is inserted immediately - before the condition. Such a callout may also be inserted explicitly, - for example: - - (?(?C9)(?=a)ab|de) - - This applies only to assertion conditions (because they are themselves - independent groups). - - Automatic callouts can be used for tracking the progress of pattern - matching. The pcretest program has a pattern qualifier (/C) that sets - automatic callouts; when it is used, the output indicates how the pat- - tern is being matched. This is useful information when you are trying - to optimize the performance of a particular pattern. - - -MISSING CALLOUTS - - You should be aware that, because of optimizations in the way PCRE com- - piles and matches patterns, callouts sometimes do not happen exactly as - you might expect. - - At compile time, PCRE "auto-possessifies" repeated items when it knows - that what follows cannot be part of the repeat. For example, a+[bc] is - compiled as if it were a++[bc]. The pcretest output when this pattern - is anchored and then applied with automatic callouts to the string - "aaaa" is: - - --->aaaa - +0 ^ ^ - +1 ^ a+ - +3 ^ ^ [bc] - No match - - This indicates that when matching [bc] fails, there is no backtracking - into a+ and therefore the callouts that would be taken for the back- - tracks do not occur. You can disable the auto-possessify feature by - passing PCRE_NO_AUTO_POSSESS to pcre_compile(), or starting the pattern - with (*NO_AUTO_POSSESS). If this is done in pcretest (using the /O - qualifier), the output changes to this: - - --->aaaa - +0 ^ ^ - +1 ^ a+ - +3 ^ ^ [bc] - +3 ^ ^ [bc] - +3 ^ ^ [bc] - +3 ^^ [bc] - No match - - This time, when matching [bc] fails, the matcher backtracks into a+ and - tries again, repeatedly, until a+ itself fails. - - Other optimizations that provide fast "no match" results also affect - callouts. For example, if the pattern is - - ab(?C4)cd - - PCRE knows that any matching string must contain the letter "d". If the - subject string is "abyz", the lack of "d" means that matching doesn't - ever start, and the callout is never reached. However, with "abyd", - though the result is still no match, the callout is obeyed. - - If the pattern is studied, PCRE knows the minimum length of a matching - string, and will immediately give a "no match" return without actually - running a match if the subject is not long enough, or, for unanchored - patterns, if it has been scanned far enough. - - You can disable these optimizations by passing the PCRE_NO_START_OPTI- - MIZE option to the matching function, or by starting the pattern with - (*NO_START_OPT). This slows down the matching process, but does ensure - that callouts such as the example above are obeyed. - - -THE CALLOUT INTERFACE - - During matching, when PCRE reaches a callout point, the external func- - tion defined by pcre_callout or pcre[16|32]_callout is called (if it is - set). This applies to both normal and DFA matching. The only argument - to the callout function is a pointer to a pcre_callout or - pcre[16|32]_callout block. These structures contains the following - fields: - - int version; - int callout_number; - int *offset_vector; - const char *subject; (8-bit version) - PCRE_SPTR16 subject; (16-bit version) - PCRE_SPTR32 subject; (32-bit version) - int subject_length; - int start_match; - int current_position; - int capture_top; - int capture_last; - void *callout_data; - int pattern_position; - int next_item_length; - const unsigned char *mark; (8-bit version) - const PCRE_UCHAR16 *mark; (16-bit version) - const PCRE_UCHAR32 *mark; (32-bit version) - - The version field is an integer containing the version number of the - block format. The initial version was 0; the current version is 2. The - version number will change again in future if additional fields are - added, but the intention is never to remove any of the existing fields. - - The callout_number field contains the number of the callout, as com- - piled into the pattern (that is, the number after ?C for manual call- - outs, and 255 for automatically generated callouts). - - The offset_vector field is a pointer to the vector of offsets that was - passed by the caller to the matching function. When pcre_exec() or - pcre[16|32]_exec() is used, the contents can be inspected, in order to - extract substrings that have been matched so far, in the same way as - for extracting substrings after a match has completed. For the DFA - matching functions, this field is not useful. - - The subject and subject_length fields contain copies of the values that - were passed to the matching function. - - The start_match field normally contains the offset within the subject - at which the current match attempt started. However, if the escape - sequence \K has been encountered, this value is changed to reflect the - modified starting point. If the pattern is not anchored, the callout - function may be called several times from the same point in the pattern - for different starting points in the subject. - - The current_position field contains the offset within the subject of - the current match pointer. - - When the pcre_exec() or pcre[16|32]_exec() is used, the capture_top - field contains one more than the number of the highest numbered cap- - tured substring so far. If no substrings have been captured, the value - of capture_top is one. This is always the case when the DFA functions - are used, because they do not support captured substrings. - - The capture_last field contains the number of the most recently cap- - tured substring. However, when a recursion exits, the value reverts to - what it was outside the recursion, as do the values of all captured - substrings. If no substrings have been captured, the value of cap- - ture_last is -1. This is always the case for the DFA matching func- - tions. - - The callout_data field contains a value that is passed to a matching - function specifically so that it can be passed back in callouts. It is - passed in the callout_data field of a pcre_extra or pcre[16|32]_extra - data structure. If no such data was passed, the value of callout_data - in a callout block is NULL. There is a description of the pcre_extra - structure in the pcreapi documentation. - - The pattern_position field is present from version 1 of the callout - structure. It contains the offset to the next item to be matched in the - pattern string. - - The next_item_length field is present from version 1 of the callout - structure. It contains the length of the next item to be matched in the - pattern string. When the callout immediately precedes an alternation - bar, a closing parenthesis, or the end of the pattern, the length is - zero. When the callout precedes an opening parenthesis, the length is - that of the entire subpattern. - - The pattern_position and next_item_length fields are intended to help - in distinguishing between different automatic callouts, which all have - the same callout number. However, they are set for all callouts. - - The mark field is present from version 2 of the callout structure. In - callouts from pcre_exec() or pcre[16|32]_exec() it contains a pointer - to the zero-terminated name of the most recently passed (*MARK), - (*PRUNE), or (*THEN) item in the match, or NULL if no such items have - been passed. Instances of (*PRUNE) or (*THEN) without a name do not - obliterate a previous (*MARK). In callouts from the DFA matching func- - tions this field always contains NULL. - - -RETURN VALUES - - The external callout function returns an integer to PCRE. If the value - is zero, matching proceeds as normal. If the value is greater than - zero, matching fails at the current point, but the testing of other - matching possibilities goes ahead, just as if a lookahead assertion had - failed. If the value is less than zero, the match is abandoned, the - matching function returns the negative value. - - Negative values should normally be chosen from the set of - PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- - dard "no match" failure. The error number PCRE_ERROR_CALLOUT is - reserved for use by callout functions; it will never be used by PCRE - itself. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 12 November 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRECOMPAT(3) Library Functions Manual PCRECOMPAT(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -DIFFERENCES BETWEEN PCRE AND PERL - - This document describes the differences in the ways that PCRE and Perl - handle regular expressions. The differences described here are with - respect to Perl versions 5.10 and above. - - 1. PCRE has only a subset of Perl's Unicode support. Details of what it - does have are given in the pcreunicode page. - - 2. PCRE allows repeat quantifiers only on parenthesized assertions, but - they do not mean what you might think. For example, (?!a){3} does not - assert that the next three characters are not "a". It just asserts that - the next character is not "a" three times (in principle: PCRE optimizes - this to run the assertion just once). Perl allows repeat quantifiers on - other assertions such as \b, but these do not seem to have any use. - - 3. Capturing subpatterns that occur inside negative lookahead asser- - tions are counted, but their entries in the offsets vector are never - set. Perl sometimes (but not always) sets its numerical variables from - inside negative assertions. - - 4. Though binary zero characters are supported in the subject string, - they are not allowed in a pattern string because it is passed as a nor- - mal C string, terminated by zero. The escape sequence \0 can be used in - the pattern to represent a binary zero. - - 5. The following Perl escape sequences are not supported: \l, \u, \L, - \U, and \N when followed by a character name or Unicode value. (\N on - its own, matching a non-newline character, is supported.) In fact these - are implemented by Perl's general string-handling and are not part of - its pattern matching engine. If any of these are encountered by PCRE, - an error is generated by default. However, if the PCRE_JAVASCRIPT_COM- - PAT option is set, \U and \u are interpreted as JavaScript interprets - them. - - 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE - is built with Unicode character property support. The properties that - can be tested with \p and \P are limited to the general category prop- - erties such as Lu and Nd, script names such as Greek or Han, and the - derived properties Any and L&. PCRE does support the Cs (surrogate) - property, which Perl does not; the Perl documentation says "Because - Perl hides the need for the user to understand the internal representa- - tion of Unicode characters, there is no need to implement the somewhat - messy concept of surrogates." - - 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- - ters in between are treated as literals. This is slightly different - from Perl in that $ and @ are also handled as literals inside the - quotes. In Perl, they cause variable interpolation (but of course PCRE - does not have variables). Note the following examples: - - Pattern PCRE matches Perl matches - - \Qabc$xyz\E abc$xyz abc followed by the - contents of $xyz - \Qabc\$xyz\E abc\$xyz abc\$xyz - \Qabc\E\$\Qxyz\E abc$xyz abc$xyz - - The \Q...\E sequence is recognized both inside and outside character - classes. - - 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) - constructions. However, there is support for recursive patterns. This - is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE - "callout" feature allows an external function to be called during pat- - tern matching. See the pcrecallout documentation for details. - - 9. Subpatterns that are called as subroutines (whether or not recur- - sively) are always treated as atomic groups in PCRE. This is like - Python, but unlike Perl. Captured values that are set outside a sub- - routine call can be reference from inside in PCRE, but not in Perl. - There is a discussion that explains these differences in more detail in - the section on recursion differences from Perl in the pcrepattern page. - - 10. If any of the backtracking control verbs are used in a subpattern - that is called as a subroutine (whether or not recursively), their - effect is confined to that subpattern; it does not extend to the sur- - rounding pattern. This is not always the case in Perl. In particular, - if (*THEN) is present in a group that is called as a subroutine, its - action is limited to that group, even if the group does not contain any - | characters. Note that such subpatterns are processed as anchored at - the point where they are tested. - - 11. If a pattern contains more than one backtracking control verb, the - first one that is backtracked onto acts. For example, in the pattern - A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure - in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases - it is the same as PCRE, but there are examples where it differs. - - 12. Most backtracking verbs in assertions have their normal actions. - They are not confined to the assertion. - - 13. There are some differences that are concerned with the settings of - captured strings when part of a pattern is repeated. For example, - matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 - unset, but in PCRE it is set to "b". - - 14. PCRE's handling of duplicate subpattern numbers and duplicate sub- - pattern names is not as general as Perl's. This is a consequence of the - fact the PCRE works internally just with numbers, using an external ta- - ble to translate between numbers and names. In particular, a pattern - such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have - the same number but different names, is not supported, and causes an - error at compile time. If it were allowed, it would not be possible to - distinguish which parentheses matched, because both names map to cap- - turing subpattern number 1. To avoid this confusing situation, an error - is given at compile time. - - 15. Perl recognizes comments in some places that PCRE does not, for - example, between the ( and ? at the start of a subpattern. If the /x - modifier is set, Perl allows white space between ( and ? (though cur- - rent Perls warn that this is deprecated) but PCRE never does, even if - the PCRE_EXTENDED option is set. - - 16. Perl, when in warning mode, gives warnings for character classes - such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- - als. PCRE has no warning features, so it gives an error in these cases - because they are almost certainly user mistakes. - - 17. In PCRE, the upper/lower case character properties Lu and Ll are - not affected when case-independent matching is specified. For example, - \p{Lu} always matches an upper case letter. I think Perl has changed in - this respect; in the release at the time of writing (5.16), \p{Lu} and - \p{Ll} match all letters, regardless of case, when case independence is - specified. - - 18. PCRE provides some extensions to the Perl regular expression facil- - ities. Perl 5.10 includes new features that are not in earlier ver- - sions of Perl, some of which (such as named parentheses) have been in - PCRE for some time. This list is with respect to Perl 5.10: - - (a) Although lookbehind assertions in PCRE must match fixed length - strings, each alternative branch of a lookbehind assertion can match a - different length of string. Perl requires them all to have the same - length. - - (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ - meta-character matches only at the very end of the string. - - (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- - cial meaning is faulted. Otherwise, like Perl, the backslash is quietly - ignored. (Perl can be made to issue a warning.) - - (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- - fiers is inverted, that is, by default they are not greedy, but if fol- - lowed by a question mark they are. - - (e) PCRE_ANCHORED can be used at matching time to force a pattern to be - tried only at the first matching position in the subject string. - - (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, - and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- - lents. - - (g) The \R escape sequence can be restricted to match only CR, LF, or - CRLF by the PCRE_BSR_ANYCRLF option. - - (h) The callout facility is PCRE-specific. - - (i) The partial matching facility is PCRE-specific. - - (j) Patterns compiled by PCRE can be saved and re-used at a later time, - even on different hosts that have the other endianness. However, this - does not apply to optimized data created by the just-in-time compiler. - - (k) The alternative matching functions (pcre_dfa_exec(), - pcre16_dfa_exec() and pcre32_dfa_exec(),) match in a different way and - are not Perl-compatible. - - (l) PCRE recognizes some special sequences such as (*CR) at the start - of a pattern that set overall options that cannot be changed within the - pattern. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 10 November 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREPATTERN(3) Library Functions Manual PCREPATTERN(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE REGULAR EXPRESSION DETAILS - - The syntax and semantics of the regular expressions that are supported - by PCRE are described in detail below. There is a quick-reference syn- - tax summary in the pcresyntax page. PCRE tries to match Perl syntax and - semantics as closely as it can. PCRE also supports some alternative - regular expression syntax (which does not conflict with the Perl syn- - tax) in order to provide some compatibility with regular expressions in - Python, .NET, and Oniguruma. - - Perl's regular expressions are described in its own documentation, and - regular expressions in general are covered in a number of books, some - of which have copious examples. Jeffrey Friedl's "Mastering Regular - Expressions", published by O'Reilly, covers regular expressions in - great detail. This description of PCRE's regular expressions is - intended as reference material. - - This document discusses the patterns that are supported by PCRE when - one its main matching functions, pcre_exec() (8-bit) or - pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has alternative - matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which - match using a different algorithm that is not Perl-compatible. Some of - the features discussed below are not available when DFA matching is - used. The advantages and disadvantages of the alternative functions, - and how they differ from the normal functions, are discussed in the - pcrematching page. - - -SPECIAL START-OF-PATTERN ITEMS - - A number of options that can be passed to pcre_compile() can also be - set by special items at the start of a pattern. These are not Perl-com- - patible, but are provided to make these options accessible to pattern - writers who are not able to change the program that processes the pat- - tern. Any number of these items may appear, but they must all be - together right at the start of the pattern string, and the letters must - be in upper case. - - UTF support - - The original operation of PCRE was on strings of one-byte characters. - However, there is now also support for UTF-8 strings in the original - library, an extra library that supports 16-bit and UTF-16 character - strings, and a third library that supports 32-bit and UTF-32 character - strings. To use these features, PCRE must be built to include appropri- - ate support. When using UTF strings you must either call the compiling - function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the - pattern must start with one of these special sequences: - - (*UTF8) - (*UTF16) - (*UTF32) - (*UTF) - - (*UTF) is a generic sequence that can be used with any of the - libraries. Starting a pattern with such a sequence is equivalent to - setting the relevant option. How setting a UTF mode affects pattern - matching is mentioned in several places below. There is also a summary - of features in the pcreunicode page. - - Some applications that allow their users to supply patterns may wish to - restrict them to non-UTF data for security reasons. If the - PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not - allowed, and their appearance causes an error. - - Unicode property support - - Another special sequence that may appear at the start of a pattern is - (*UCP). This has the same effect as setting the PCRE_UCP option: it - causes sequences such as \d and \w to use Unicode properties to deter- - mine character types, instead of recognizing only characters with codes - less than 128 via a lookup table. - - Disabling auto-possessification - - If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as - setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops - PCRE from making quantifiers possessive when what follows cannot match - the repeated item. For example, by default a+b is treated as a++b. For - more details, see the pcreapi documentation. - - Disabling start-up optimizations - - If a pattern starts with (*NO_START_OPT), it has the same effect as - setting the PCRE_NO_START_OPTIMIZE option either at compile or matching - time. This disables several optimizations for quickly reaching "no - match" results. For more details, see the pcreapi documentation. - - Newline conventions - - PCRE supports five different conventions for indicating line breaks in - strings: a single CR (carriage return) character, a single LF (line- - feed) character, the two-character sequence CRLF, any of the three pre- - ceding, or any Unicode newline sequence. The pcreapi page has further - discussion about newlines, and shows how to set the newline convention - in the options arguments for the compiling and matching functions. - - It is also possible to specify a newline convention by starting a pat- - tern string with one of the following five sequences: - - (*CR) carriage return - (*LF) linefeed - (*CRLF) carriage return, followed by linefeed - (*ANYCRLF) any of the three above - (*ANY) all Unicode newline sequences - - These override the default and the options given to the compiling func- - tion. For example, on a Unix system where LF is the default newline - sequence, the pattern - - (*CR)a.b - - changes the convention to CR. That pattern matches "a\nb" because LF is - no longer a newline. If more than one of these settings is present, the - last one is used. - - The newline convention affects where the circumflex and dollar asser- - tions are true. It also affects the interpretation of the dot metachar- - acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it - does not affect what the \R escape sequence matches. By default, this - is any Unicode newline sequence, for Perl compatibility. However, this - can be changed; see the description of \R in the section entitled "New- - line sequences" below. A change of \R setting can be combined with a - change of newline convention. - - Setting match and recursion limits - - The caller of pcre_exec() can set a limit on the number of times the - internal match() function is called and on the maximum depth of recur- - sive calls. These facilities are provided to catch runaway matches that - are provoked by patterns with huge matching trees (a typical example is - a pattern with nested unlimited repeats) and to avoid running out of - system stack by too much recursion. When one of these limits is - reached, pcre_exec() gives an error return. The limits can also be set - by items at the start of the pattern of the form - - (*LIMIT_MATCH=d) - (*LIMIT_RECURSION=d) - - where d is any number of decimal digits. However, the value of the set- - ting must be less than the value set (or defaulted) by the caller of - pcre_exec() for it to have any effect. In other words, the pattern - writer can lower the limits set by the programmer, but not raise them. - If there is more than one setting of one of these limits, the lower - value is used. - - -EBCDIC CHARACTER CODES - - PCRE can be compiled to run in an environment that uses EBCDIC as its - character code rather than ASCII or Unicode (typically a mainframe sys- - tem). In the sections below, character code values are ASCII or Uni- - code; in an EBCDIC environment these characters may have different code - values, and there are no code points greater than 255. - - -CHARACTERS AND METACHARACTERS - - A regular expression is a pattern that is matched against a subject - string from left to right. Most characters stand for themselves in a - pattern, and match the corresponding characters in the subject. As a - trivial example, the pattern - - The quick brown fox - - matches a portion of a subject string that is identical to itself. When - caseless matching is specified (the PCRE_CASELESS option), letters are - matched independently of case. In a UTF mode, PCRE always understands - the concept of case for characters whose values are less than 128, so - caseless matching is always possible. For characters with higher val- - ues, the concept of case is supported if PCRE is compiled with Unicode - property support, but not otherwise. If you want to use caseless - matching for characters 128 and above, you must ensure that PCRE is - compiled with Unicode property support as well as with UTF support. - - The power of regular expressions comes from the ability to include - alternatives and repetitions in the pattern. These are encoded in the - pattern by the use of metacharacters, which do not stand for themselves - but instead are interpreted in some special way. - - There are two different sets of metacharacters: those that are recog- - nized anywhere in the pattern except within square brackets, and those - that are recognized within square brackets. Outside square brackets, - the metacharacters are as follows: - - \ general escape character with several uses - ^ assert start of string (or line, in multiline mode) - $ assert end of string (or line, in multiline mode) - . match any character except newline (by default) - [ start character class definition - | start of alternative branch - ( start subpattern - ) end subpattern - ? extends the meaning of ( - also 0 or 1 quantifier - also quantifier minimizer - * 0 or more quantifier - + 1 or more quantifier - also "possessive quantifier" - { start min/max quantifier - - Part of a pattern that is in square brackets is called a "character - class". In a character class the only metacharacters are: - - \ general escape character - ^ negate the class, but only if the first character - - indicates character range - [ POSIX character class (only if followed by POSIX - syntax) - ] terminates the character class - - The following sections describe the use of each of the metacharacters. - - -BACKSLASH - - The backslash character has several uses. Firstly, if it is followed by - a character that is not a number or a letter, it takes away any special - meaning that character may have. This use of backslash as an escape - character applies both inside and outside character classes. - - For example, if you want to match a * character, you write \* in the - pattern. This escaping action applies whether or not the following - character would otherwise be interpreted as a metacharacter, so it is - always safe to precede a non-alphanumeric with backslash to specify - that it stands for itself. In particular, if you want to match a back- - slash, you write \\. - - In a UTF mode, only ASCII numbers and letters have any special meaning - after a backslash. All other characters (in particular, those whose - codepoints are greater than 127) are treated as literals. - - If a pattern is compiled with the PCRE_EXTENDED option, most white - space in the pattern (other than in a character class), and characters - between a # outside a character class and the next newline, inclusive, - are ignored. An escaping backslash can be used to include a white space - or # character as part of the pattern. - - If you want to remove the special meaning from a sequence of charac- - ters, you can do so by putting them between \Q and \E. This is differ- - ent from Perl in that $ and @ are handled as literals in \Q...\E - sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- - tion. Note the following examples: - - Pattern PCRE matches Perl matches - - \Qabc$xyz\E abc$xyz abc followed by the - contents of $xyz - \Qabc\$xyz\E abc\$xyz abc\$xyz - \Qabc\E\$\Qxyz\E abc$xyz abc$xyz - - The \Q...\E sequence is recognized both inside and outside character - classes. An isolated \E that is not preceded by \Q is ignored. If \Q - is not followed by \E later in the pattern, the literal interpretation - continues to the end of the pattern (that is, \E is assumed at the - end). If the isolated \Q is inside a character class, this causes an - error, because the character class is not terminated. - - Non-printing characters - - A second use of backslash provides a way of encoding non-printing char- - acters in patterns in a visible manner. There is no restriction on the - appearance of non-printing characters, apart from the binary zero that - terminates a pattern, but when a pattern is being prepared by text - editing, it is often easier to use one of the following escape - sequences than the binary character it represents: - - \a alarm, that is, the BEL character (hex 07) - \cx "control-x", where x is any ASCII character - \e escape (hex 1B) - \f form feed (hex 0C) - \n linefeed (hex 0A) - \r carriage return (hex 0D) - \t tab (hex 09) - \0dd character with octal code 0dd - \ddd character with octal code ddd, or back reference - \o{ddd..} character with octal code ddd.. - \xhh character with hex code hh - \x{hhh..} character with hex code hhh.. (non-JavaScript mode) - \uhhhh character with hex code hhhh (JavaScript mode only) - - The precise effect of \cx on ASCII characters is as follows: if x is a - lower case letter, it is converted to upper case. Then bit 6 of the - character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A - (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes - hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c - has a value greater than 127, a compile-time error occurs. This locks - out non-ASCII characters in all modes. - - The \c facility was designed for use with ASCII characters, but with - the extension to Unicode it is even less useful than it once was. It - is, however, recognized when PCRE is compiled in EBCDIC mode, where - data items are always bytes. In this mode, all values are valid after - \c. If the next character is a lower case letter, it is converted to - upper case. Then the 0xc0 bits of the byte are inverted. Thus \cA - becomes hex 01, as in ASCII (A is C1), but because the EBCDIC letters - are disjoint, \cZ becomes hex 29 (Z is E9), and other characters also - generate different values. - - After \0 up to two further octal digits are read. If there are fewer - than two digits, just those that are present are used. Thus the - sequence \0\x\07 specifies two binary zeros followed by a BEL character - (code value 7). Make sure you supply two digits after the initial zero - if the pattern character that follows is itself an octal digit. - - The escape \o must be followed by a sequence of octal digits, enclosed - in braces. An error occurs if this is not the case. This escape is a - recent addition to Perl; it provides way of specifying character code - points as octal numbers greater than 0777, and it also allows octal - numbers and back references to be unambiguously specified. - - For greater clarity and unambiguity, it is best to avoid following \ by - a digit greater than zero. Instead, use \o{} or \x{} to specify charac- - ter numbers, and \g{} to specify back references. The following para- - graphs describe the old, ambiguous syntax. - - The handling of a backslash followed by a digit other than 0 is compli- - cated, and Perl has changed in recent releases, causing PCRE also to - change. Outside a character class, PCRE reads the digit and any follow- - ing digits as a decimal number. If the number is less than 8, or if - there have been at least that many previous capturing left parentheses - in the expression, the entire sequence is taken as a back reference. A - description of how this works is given later, following the discussion - of parenthesized subpatterns. - - Inside a character class, or if the decimal number following \ is - greater than 7 and there have not been that many capturing subpatterns, - PCRE handles \8 and \9 as the literal characters "8" and "9", and oth- - erwise re-reads up to three octal digits following the backslash, using - them to generate a data character. Any subsequent digits stand for - themselves. For example: - - \040 is another way of writing an ASCII space - \40 is the same, provided there are fewer than 40 - previous capturing subpatterns - \7 is always a back reference - \11 might be a back reference, or another way of - writing a tab - \011 is always a tab - \0113 is a tab followed by the character "3" - \113 might be a back reference, otherwise the - character with octal code 113 - \377 might be a back reference, otherwise - the value 255 (decimal) - \81 is either a back reference, or the two - characters "8" and "1" - - Note that octal values of 100 or greater that are specified using this - syntax must not be introduced by a leading zero, because no more than - three octal digits are ever read. - - By default, after \x that is not followed by {, from zero to two hexa- - decimal digits are read (letters can be in upper or lower case). Any - number of hexadecimal digits may appear between \x{ and }. If a charac- - ter other than a hexadecimal digit appears between \x{ and }, or if - there is no terminating }, an error occurs. - - If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x - is as just described only when it is followed by two hexadecimal dig- - its. Otherwise, it matches a literal "x" character. In JavaScript - mode, support for code points greater than 256 is provided by \u, which - must be followed by four hexadecimal digits; otherwise it matches a - literal "u" character. - - Characters whose value is less than 256 can be defined by either of the - two syntaxes for \x (or by \u in JavaScript mode). There is no differ- - ence in the way they are handled. For example, \xdc is exactly the same - as \x{dc} (or \u00dc in JavaScript mode). - - Constraints on character values - - Characters that are specified using octal or hexadecimal numbers are - limited to certain values, as follows: - - 8-bit non-UTF mode less than 0x100 - 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint - 16-bit non-UTF mode less than 0x10000 - 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint - 32-bit non-UTF mode less than 0x100000000 - 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint - - Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so- - called "surrogate" codepoints), and 0xffef. - - Escape sequences in character classes - - All the sequences that define a single character value can be used both - inside and outside character classes. In addition, inside a character - class, \b is interpreted as the backspace character (hex 08). - - \N is not allowed in a character class. \B, \R, and \X are not special - inside a character class. Like other unrecognized escape sequences, - they are treated as the literal characters "B", "R", and "X" by - default, but cause an error if the PCRE_EXTRA option is set. Outside a - character class, these sequences have different meanings. - - Unsupported escape sequences - - In Perl, the sequences \l, \L, \u, and \U are recognized by its string - handler and used to modify the case of following characters. By - default, PCRE does not support these escape sequences. However, if the - PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and - \u can be used to define a character by code point, as described in the - previous section. - - Absolute and relative back references - - The sequence \g followed by an unsigned or a negative number, option- - ally enclosed in braces, is an absolute or relative back reference. A - named back reference can be coded as \g{name}. Back references are dis- - cussed later, following the discussion of parenthesized subpatterns. - - Absolute and relative subroutine calls - - For compatibility with Oniguruma, the non-Perl syntax \g followed by a - name or a number enclosed either in angle brackets or single quotes, is - an alternative syntax for referencing a subpattern as a "subroutine". - Details are discussed later. Note that \g{...} (Perl syntax) and - \g<...> (Oniguruma syntax) are not synonymous. The former is a back - reference; the latter is a subroutine call. - - Generic character types - - Another use of backslash is for specifying generic character types: - - \d any decimal digit - \D any character that is not a decimal digit - \h any horizontal white space character - \H any character that is not a horizontal white space character - \s any white space character - \S any character that is not a white space character - \v any vertical white space character - \V any character that is not a vertical white space character - \w any "word" character - \W any "non-word" character - - There is also the single sequence \N, which matches a non-newline char- - acter. This is the same as the "." metacharacter when PCRE_DOTALL is - not set. Perl also uses \N to match characters by name; PCRE does not - support this. - - Each pair of lower and upper case escape sequences partitions the com- - plete set of characters into two disjoint sets. Any given character - matches one, and only one, of each pair. The sequences can appear both - inside and outside character classes. They each match one character of - the appropriate type. If the current matching point is at the end of - the subject string, all of them fail, because there is no character to - match. - - For compatibility with Perl, \s did not used to match the VT character - (code 11), which made it different from the the POSIX "space" class. - However, Perl added VT at release 5.18, and PCRE followed suit at - release 8.34. The default \s characters are now HT (9), LF (10), VT - (11), FF (12), CR (13), and space (32), which are defined as white - space in the "C" locale. This list may vary if locale-specific matching - is taking place. For example, in some locales the "non-breaking space" - character (\xA0) is recognized as white space, and in others the VT - character is not. - - A "word" character is an underscore or any character that is a letter - or digit. By default, the definition of letters and digits is con- - trolled by PCRE's low-valued character tables, and may vary if locale- - specific matching is taking place (see "Locale support" in the pcreapi - page). For example, in a French locale such as "fr_FR" in Unix-like - systems, or "french" in Windows, some character codes greater than 127 - are used for accented letters, and these are then matched by \w. The - use of locales with Unicode is discouraged. - - By default, characters whose code points are greater than 127 never - match \d, \s, or \w, and always match \D, \S, and \W, although this may - vary for characters in the range 128-255 when locale-specific matching - is happening. These escape sequences retain their original meanings - from before Unicode support was available, mainly for efficiency rea- - sons. If PCRE is compiled with Unicode property support, and the - PCRE_UCP option is set, the behaviour is changed so that Unicode prop- - erties are used to determine character types, as follows: - - \d any character that matches \p{Nd} (decimal digit) - \s any character that matches \p{Z} or \h or \v - \w any character that matches \p{L} or \p{N}, plus underscore - - The upper case escapes match the inverse sets of characters. Note that - \d matches only decimal digits, whereas \w matches any Unicode digit, - as well as any Unicode letter, and underscore. Note also that PCRE_UCP - affects \b, and \B because they are defined in terms of \w and \W. - Matching these sequences is noticeably slower when PCRE_UCP is set. - - The sequences \h, \H, \v, and \V are features that were added to Perl - at release 5.10. In contrast to the other sequences, which match only - ASCII characters by default, these always match certain high-valued - code points, whether or not PCRE_UCP is set. The horizontal space char- - acters are: - - U+0009 Horizontal tab (HT) - U+0020 Space - U+00A0 Non-break space - U+1680 Ogham space mark - U+180E Mongolian vowel separator - U+2000 En quad - U+2001 Em quad - U+2002 En space - U+2003 Em space - U+2004 Three-per-em space - U+2005 Four-per-em space - U+2006 Six-per-em space - U+2007 Figure space - U+2008 Punctuation space - U+2009 Thin space - U+200A Hair space - U+202F Narrow no-break space - U+205F Medium mathematical space - U+3000 Ideographic space - - The vertical space characters are: - - U+000A Linefeed (LF) - U+000B Vertical tab (VT) - U+000C Form feed (FF) - U+000D Carriage return (CR) - U+0085 Next line (NEL) - U+2028 Line separator - U+2029 Paragraph separator - - In 8-bit, non-UTF-8 mode, only the characters with codepoints less than - 256 are relevant. - - Newline sequences - - Outside a character class, by default, the escape sequence \R matches - any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent - to the following: - - (?>\r\n|\n|\x0b|\f|\r|\x85) - - This is an example of an "atomic group", details of which are given - below. This particular group matches either the two-character sequence - CR followed by LF, or one of the single characters LF (linefeed, - U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- - riage return, U+000D), or NEL (next line, U+0085). The two-character - sequence is treated as a single unit that cannot be split. - - In other modes, two additional characters whose codepoints are greater - than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- - rator, U+2029). Unicode character property support is not needed for - these characters to be recognized. - - It is possible to restrict \R to match only CR, LF, or CRLF (instead of - the complete set of Unicode line endings) by setting the option - PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. - (BSR is an abbrevation for "backslash R".) This can be made the default - when PCRE is built; if this is the case, the other behaviour can be - requested via the PCRE_BSR_UNICODE option. It is also possible to - specify these settings by starting a pattern string with one of the - following sequences: - - (*BSR_ANYCRLF) CR, LF, or CRLF only - (*BSR_UNICODE) any Unicode newline sequence - - These override the default and the options given to the compiling func- - tion, but they can themselves be overridden by options given to a - matching function. Note that these special settings, which are not - Perl-compatible, are recognized only at the very start of a pattern, - and that they must be in upper case. If more than one of them is - present, the last one is used. They can be combined with a change of - newline convention; for example, a pattern can start with: - - (*ANY)(*BSR_ANYCRLF) - - They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) - or (*UCP) special sequences. Inside a character class, \R is treated as - an unrecognized escape sequence, and so matches the letter "R" by - default, but causes an error if PCRE_EXTRA is set. - - Unicode character properties - - When PCRE is built with Unicode character property support, three addi- - tional escape sequences that match characters with specific properties - are available. When in 8-bit non-UTF-8 mode, these sequences are of - course limited to testing characters whose codepoints are less than - 256, but they do work in this mode. The extra escape sequences are: - - \p{xx} a character with the xx property - \P{xx} a character without the xx property - \X a Unicode extended grapheme cluster - - The property names represented by xx above are limited to the Unicode - script names, the general category properties, "Any", which matches any - character (including newline), and some special PCRE properties - (described in the next section). Other Perl properties such as "InMu- - sicalSymbols" are not currently supported by PCRE. Note that \P{Any} - does not match any characters, so always causes a match failure. - - Sets of Unicode characters are defined as belonging to certain scripts. - A character from one of these sets can be matched using a script name. - For example: - - \p{Greek} - \P{Han} - - Those that are not part of an identified script are lumped together as - "Common". The current list of scripts is: - - Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo, - Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma, - Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, - Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, - Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira- - gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- - tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, - Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, - Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive, - Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko, - Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, - Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari- - tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese, - Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, - Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, - Yi. - - Each character has exactly one Unicode general category property, spec- - ified by a two-letter abbreviation. For compatibility with Perl, nega- - tion can be specified by including a circumflex between the opening - brace and the property name. For example, \p{^Lu} is the same as - \P{Lu}. - - If only one letter is specified with \p or \P, it includes all the gen- - eral category properties that start with that letter. In this case, in - the absence of negation, the curly brackets in the escape sequence are - optional; these two examples have the same effect: - - \p{L} - \pL - - The following general category property codes are supported: - - C Other - Cc Control - Cf Format - Cn Unassigned - Co Private use - Cs Surrogate - - L Letter - Ll Lower case letter - Lm Modifier letter - Lo Other letter - Lt Title case letter - Lu Upper case letter - - M Mark - Mc Spacing mark - Me Enclosing mark - Mn Non-spacing mark - - N Number - Nd Decimal number - Nl Letter number - No Other number - - P Punctuation - Pc Connector punctuation - Pd Dash punctuation - Pe Close punctuation - Pf Final punctuation - Pi Initial punctuation - Po Other punctuation - Ps Open punctuation - - S Symbol - Sc Currency symbol - Sk Modifier symbol - Sm Mathematical symbol - So Other symbol - - Z Separator - Zl Line separator - Zp Paragraph separator - Zs Space separator - - The special property L& is also supported: it matches a character that - has the Lu, Ll, or Lt property, in other words, a letter that is not - classified as a modifier or "other". - - The Cs (Surrogate) property applies only to characters in the range - U+D800 to U+DFFF. Such characters are not valid in Unicode strings and - so cannot be tested by PCRE, unless UTF validity checking has been - turned off (see the discussion of PCRE_NO_UTF8_CHECK, - PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl - does not support the Cs property. - - The long synonyms for property names that Perl supports (such as - \p{Letter}) are not supported by PCRE, nor is it permitted to prefix - any of these properties with "Is". - - No character that is in the Unicode table has the Cn (unassigned) prop- - erty. Instead, this property is assumed for any code point that is not - in the Unicode table. - - Specifying caseless matching does not affect these escape sequences. - For example, \p{Lu} always matches only upper case letters. This is - different from the behaviour of current versions of Perl. - - Matching characters by Unicode property is not fast, because PCRE has - to do a multistage table lookup in order to find a character's prop- - erty. That is why the traditional escape sequences such as \d and \w do - not use Unicode properties in PCRE by default, though you can make them - do so by setting the PCRE_UCP option or by starting the pattern with - (*UCP). - - Extended grapheme clusters - - The \X escape matches any number of Unicode characters that form an - "extended grapheme cluster", and treats the sequence as an atomic group - (see below). Up to and including release 8.31, PCRE matched an ear- - lier, simpler definition that was equivalent to - - (?>\PM\pM*) - - That is, it matched a character without the "mark" property, followed - by zero or more characters with the "mark" property. Characters with - the "mark" property are typically non-spacing accents that affect the - preceding character. - - This simple definition was extended in Unicode to include more compli- - cated kinds of composite character by giving each character a grapheme - breaking property, and creating rules that use these properties to - define the boundaries of extended grapheme clusters. In releases of - PCRE later than 8.31, \X matches one of these clusters. - - \X always matches at least one character. Then it decides whether to - add additional characters according to the following rules for ending a - cluster: - - 1. End at the end of the subject string. - - 2. Do not end between CR and LF; otherwise end after any control char- - acter. - - 3. Do not break Hangul (a Korean script) syllable sequences. Hangul - characters are of five types: L, V, T, LV, and LVT. An L character may - be followed by an L, V, LV, or LVT character; an LV or V character may - be followed by a V or T character; an LVT or T character may be follwed - only by a T character. - - 4. Do not end before extending characters or spacing marks. Characters - with the "mark" property always have the "extend" grapheme breaking - property. - - 5. Do not end after prepend characters. - - 6. Otherwise, end the cluster. - - PCRE's additional properties - - As well as the standard Unicode properties described above, PCRE sup- - ports four more that make it possible to convert traditional escape - sequences such as \w and \s to use Unicode properties. PCRE uses these - non-standard, non-Perl properties internally when PCRE_UCP is set. How- - ever, they may also be used explicitly. These properties are: - - Xan Any alphanumeric character - Xps Any POSIX space character - Xsp Any Perl space character - Xwd Any Perl "word" character - - Xan matches characters that have either the L (letter) or the N (num- - ber) property. Xps matches the characters tab, linefeed, vertical tab, - form feed, or carriage return, and any other character that has the Z - (separator) property. Xsp is the same as Xps; it used to exclude ver- - tical tab, for Perl compatibility, but Perl changed, and so PCRE fol- - lowed at release 8.34. Xwd matches the same characters as Xan, plus - underscore. - - There is another non-standard property, Xuc, which matches any charac- - ter that can be represented by a Universal Character Name in C++ and - other programming languages. These are the characters $, @, ` (grave - accent), and all characters with Unicode code points greater than or - equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that - most base (ASCII) characters are excluded. (Universal Character Names - are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. - Note that the Xuc property does not match these sequences but the char- - acters that they represent.) - - Resetting the match start - - The escape sequence \K causes any previously matched characters not to - be included in the final matched sequence. For example, the pattern: - - foo\Kbar - - matches "foobar", but reports that it has matched "bar". This feature - is similar to a lookbehind assertion (described below). However, in - this case, the part of the subject before the real match does not have - to be of fixed length, as lookbehind assertions do. The use of \K does - not interfere with the setting of captured substrings. For example, - when the pattern - - (foo)\Kbar - - matches "foobar", the first substring is still set to "foo". - - Perl documents that the use of \K within assertions is "not well - defined". In PCRE, \K is acted upon when it occurs inside positive - assertions, but is ignored in negative assertions. Note that when a - pattern such as (?=ab\K) matches, the reported start of the match can - be greater than the end of the match. - - Simple assertions - - The final use of backslash is for certain simple assertions. An asser- - tion specifies a condition that has to be met at a particular point in - a match, without consuming any characters from the subject string. The - use of subpatterns for more complicated assertions is described below. - The backslashed assertions are: - - \b matches at a word boundary - \B matches when not at a word boundary - \A matches at the start of the subject - \Z matches at the end of the subject - also matches before a newline at the end of the subject - \z matches only at the end of the subject - \G matches at the first matching position in the subject - - Inside a character class, \b has a different meaning; it matches the - backspace character. If any other of these assertions appears in a - character class, by default it matches the corresponding literal char- - acter (for example, \B matches the letter B). However, if the - PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- - ated instead. - - A word boundary is a position in the subject string where the current - character and the previous character do not both match \w or \W (i.e. - one matches \w and the other matches \W), or the start or end of the - string if the first or last character matches \w, respectively. In a - UTF mode, the meanings of \w and \W can be changed by setting the - PCRE_UCP option. When this is done, it also affects \b and \B. Neither - PCRE nor Perl has a separate "start of word" or "end of word" metase- - quence. However, whatever follows \b normally determines which it is. - For example, the fragment \ba matches "a" at the start of a word. - - The \A, \Z, and \z assertions differ from the traditional circumflex - and dollar (described in the next section) in that they only ever match - at the very start and end of the subject string, whatever options are - set. Thus, they are independent of multiline mode. These three asser- - tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which - affect only the behaviour of the circumflex and dollar metacharacters. - However, if the startoffset argument of pcre_exec() is non-zero, indi- - cating that matching is to start at a point other than the beginning of - the subject, \A can never match. The difference between \Z and \z is - that \Z matches before a newline at the end of the string as well as at - the very end, whereas \z matches only at the end. - - The \G assertion is true only when the current matching position is at - the start point of the match, as specified by the startoffset argument - of pcre_exec(). It differs from \A when the value of startoffset is - non-zero. By calling pcre_exec() multiple times with appropriate argu- - ments, you can mimic Perl's /g option, and it is in this kind of imple- - mentation where \G can be useful. - - Note, however, that PCRE's interpretation of \G, as the start of the - current match, is subtly different from Perl's, which defines it as the - end of the previous match. In Perl, these can be different when the - previously matched string was empty. Because PCRE does just one match - at a time, it cannot reproduce this behaviour. - - If all the alternatives of a pattern begin with \G, the expression is - anchored to the starting match position, and the "anchored" flag is set - in the compiled regular expression. - - -CIRCUMFLEX AND DOLLAR - - The circumflex and dollar metacharacters are zero-width assertions. - That is, they test for a particular condition being true without con- - suming any characters from the subject string. - - Outside a character class, in the default matching mode, the circumflex - character is an assertion that is true only if the current matching - point is at the start of the subject string. If the startoffset argu- - ment of pcre_exec() is non-zero, circumflex can never match if the - PCRE_MULTILINE option is unset. Inside a character class, circumflex - has an entirely different meaning (see below). - - Circumflex need not be the first character of the pattern if a number - of alternatives are involved, but it should be the first thing in each - alternative in which it appears if the pattern is ever to match that - branch. If all possible alternatives start with a circumflex, that is, - if the pattern is constrained to match only at the start of the sub- - ject, it is said to be an "anchored" pattern. (There are also other - constructs that can cause a pattern to be anchored.) - - The dollar character is an assertion that is true only if the current - matching point is at the end of the subject string, or immediately - before a newline at the end of the string (by default). Note, however, - that it does not actually match the newline. Dollar need not be the - last character of the pattern if a number of alternatives are involved, - but it should be the last item in any branch in which it appears. Dol- - lar has no special meaning in a character class. - - The meaning of dollar can be changed so that it matches only at the - very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at - compile time. This does not affect the \Z assertion. - - The meanings of the circumflex and dollar characters are changed if the - PCRE_MULTILINE option is set. When this is the case, a circumflex - matches immediately after internal newlines as well as at the start of - the subject string. It does not match after a newline that ends the - string. A dollar matches before any newlines in the string, as well as - at the very end, when PCRE_MULTILINE is set. When newline is specified - as the two-character sequence CRLF, isolated CR and LF characters do - not indicate newlines. - - For example, the pattern /^abc$/ matches the subject string "def\nabc" - (where \n represents a newline) in multiline mode, but not otherwise. - Consequently, patterns that are anchored in single line mode because - all branches start with ^ are not anchored in multiline mode, and a - match for circumflex is possible when the startoffset argument of - pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if - PCRE_MULTILINE is set. - - Note that the sequences \A, \Z, and \z can be used to match the start - and end of the subject in both modes, and if all branches of a pattern - start with \A it is always anchored, whether or not PCRE_MULTILINE is - set. - - -FULL STOP (PERIOD, DOT) AND \N - - Outside a character class, a dot in the pattern matches any one charac- - ter in the subject string except (by default) a character that signi- - fies the end of a line. - - When a line ending is defined as a single character, dot never matches - that character; when the two-character sequence CRLF is used, dot does - not match CR if it is immediately followed by LF, but otherwise it - matches all characters (including isolated CRs and LFs). When any Uni- - code line endings are being recognized, dot does not match CR or LF or - any of the other line ending characters. - - The behaviour of dot with regard to newlines can be changed. If the - PCRE_DOTALL option is set, a dot matches any one character, without - exception. If the two-character sequence CRLF is present in the subject - string, it takes two dots to match it. - - The handling of dot is entirely independent of the handling of circum- - flex and dollar, the only relationship being that they both involve - newlines. Dot has no special meaning in a character class. - - The escape sequence \N behaves like a dot, except that it is not - affected by the PCRE_DOTALL option. In other words, it matches any - character except one that signifies the end of a line. Perl also uses - \N to match characters by name; PCRE does not support this. - - -MATCHING A SINGLE DATA UNIT - - Outside a character class, the escape sequence \C matches any one data - unit, whether or not a UTF mode is set. In the 8-bit library, one data - unit is one byte; in the 16-bit library it is a 16-bit unit; in the - 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches - line-ending characters. The feature is provided in Perl in order to - match individual bytes in UTF-8 mode, but it is unclear how it can use- - fully be used. Because \C breaks up characters into individual data - units, matching one unit with \C in a UTF mode means that the rest of - the string may start with a malformed UTF character. This has undefined - results, because PCRE assumes that it is dealing with valid UTF strings - (and by default it checks this at the start of processing unless the - PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option - is used). - - PCRE does not allow \C to appear in lookbehind assertions (described - below) in a UTF mode, because this would make it impossible to calcu- - late the length of the lookbehind. - - In general, the \C escape sequence is best avoided. However, one way of - using it that avoids the problem of malformed UTF characters is to use - a lookahead to check the length of the next character, as in this pat- - tern, which could be used with a UTF-8 string (ignore white space and - line breaks): - - (?| (?=[\x00-\x7f])(\C) | - (?=[\x80-\x{7ff}])(\C)(\C) | - (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | - (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) - - A group that starts with (?| resets the capturing parentheses numbers - in each alternative (see "Duplicate Subpattern Numbers" below). The - assertions at the start of each branch check the next UTF-8 character - for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The - character's individual bytes are then captured by the appropriate num- - ber of groups. - - -SQUARE BRACKETS AND CHARACTER CLASSES - - An opening square bracket introduces a character class, terminated by a - closing square bracket. A closing square bracket on its own is not spe- - cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, - a lone closing square bracket causes a compile-time error. If a closing - square bracket is required as a member of the class, it should be the - first data character in the class (after an initial circumflex, if - present) or escaped with a backslash. - - A character class matches a single character in the subject. In a UTF - mode, the character may be more than one data unit long. A matched - character must be in the set of characters defined by the class, unless - the first character in the class definition is a circumflex, in which - case the subject character must not be in the set defined by the class. - If a circumflex is actually required as a member of the class, ensure - it is not the first character, or escape it with a backslash. - - For example, the character class [aeiou] matches any lower case vowel, - while [^aeiou] matches any character that is not a lower case vowel. - Note that a circumflex is just a convenient notation for specifying the - characters that are in the class by enumerating those that are not. A - class that starts with a circumflex is not an assertion; it still con- - sumes a character from the subject string, and therefore it fails if - the current pointer is at the end of the string. - - In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 - (0xffff) can be included in a class as a literal string of data units, - or by using the \x{ escaping mechanism. - - When caseless matching is set, any letters in a class represent both - their upper case and lower case versions, so for example, a caseless - [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not - match "A", whereas a caseful version would. In a UTF mode, PCRE always - understands the concept of case for characters whose values are less - than 128, so caseless matching is always possible. For characters with - higher values, the concept of case is supported if PCRE is compiled - with Unicode property support, but not otherwise. If you want to use - caseless matching in a UTF mode for characters 128 and above, you must - ensure that PCRE is compiled with Unicode property support as well as - with UTF support. - - Characters that might indicate line breaks are never treated in any - special way when matching character classes, whatever line-ending - sequence is in use, and whatever setting of the PCRE_DOTALL and - PCRE_MULTILINE options is used. A class such as [^a] always matches one - of these characters. - - The minus (hyphen) character can be used to specify a range of charac- - ters in a character class. For example, [d-m] matches any letter - between d and m, inclusive. If a minus character is required in a - class, it must be escaped with a backslash or appear in a position - where it cannot be interpreted as indicating a range, typically as the - first or last character in the class, or immediately after a range. For - example, [b-d-z] matches letters in the range b to d, a hyphen charac- - ter, or z. - - It is not possible to have the literal character "]" as the end charac- - ter of a range. A pattern such as [W-]46] is interpreted as a class of - two characters ("W" and "-") followed by a literal string "46]", so it - would match "W46]" or "-46]". However, if the "]" is escaped with a - backslash it is interpreted as the end of range, so [W-\]46] is inter- - preted as a class containing a range followed by two other characters. - The octal or hexadecimal representation of "]" can also be used to end - a range. - - An error is generated if a POSIX character class (see below) or an - escape sequence other than one that defines a single character appears - at a point where a range ending character is expected. For example, - [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not. - - Ranges operate in the collating sequence of character values. They can - also be used for characters specified numerically, for example - [\000-\037]. Ranges can include any characters that are valid for the - current mode. - - If a range that includes letters is used when caseless matching is set, - it matches the letters in either case. For example, [W-c] is equivalent - to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if - character tables for a French locale are in use, [\xc8-\xcb] matches - accented E characters in both cases. In UTF modes, PCRE supports the - concept of case for characters with values greater than 128 only when - it is compiled with Unicode property support. - - The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, - \w, and \W may appear in a character class, and add the characters that - they match to the class. For example, [\dABCDEF] matches any hexadeci- - mal digit. In UTF modes, the PCRE_UCP option affects the meanings of - \d, \s, \w and their upper case partners, just as it does when they - appear outside a character class, as described in the section entitled - "Generic character types" above. The escape sequence \b has a different - meaning inside a character class; it matches the backspace character. - The sequences \B, \N, \R, and \X are not special inside a character - class. Like any other unrecognized escape sequences, they are treated - as the literal characters "B", "N", "R", and "X" by default, but cause - an error if the PCRE_EXTRA option is set. - - A circumflex can conveniently be used with the upper case character - types to specify a more restricted set of characters than the matching - lower case type. For example, the class [^\W_] matches any letter or - digit, but not underscore, whereas [\w] includes underscore. A positive - character class should be read as "something OR something OR ..." and a - negative class as "NOT something AND NOT something AND NOT ...". - - The only metacharacters that are recognized in character classes are - backslash, hyphen (only where it can be interpreted as specifying a - range), circumflex (only at the start), opening square bracket (only - when it can be interpreted as introducing a POSIX class name, or for a - special compatibility feature - see the next two sections), and the - terminating closing square bracket. However, escaping other non- - alphanumeric characters does no harm. - - -POSIX CHARACTER CLASSES - - Perl supports the POSIX notation for character classes. This uses names - enclosed by [: and :] within the enclosing square brackets. PCRE also - supports this notation. For example, - - [01[:alpha:]%] - - matches "0", "1", any alphabetic character, or "%". The supported class - names are: - - alnum letters and digits - alpha letters - ascii character codes 0 - 127 - blank space or tab only - cntrl control characters - digit decimal digits (same as \d) - graph printing characters, excluding space - lower lower case letters - print printing characters, including space - punct printing characters, excluding letters and digits and space - space white space (the same as \s from PCRE 8.34) - upper upper case letters - word "word" characters (same as \w) - xdigit hexadecimal digits - - The default "space" characters are HT (9), LF (10), VT (11), FF (12), - CR (13), and space (32). If locale-specific matching is taking place, - the list of space characters may be different; there may be fewer or - more of them. "Space" used to be different to \s, which did not include - VT, for Perl compatibility. However, Perl changed at release 5.18, and - PCRE followed at release 8.34. "Space" and \s now match the same set - of characters. - - The name "word" is a Perl extension, and "blank" is a GNU extension - from Perl 5.8. Another Perl extension is negation, which is indicated - by a ^ character after the colon. For example, - - [12[:^digit:]] - - matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the - POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but - these are not supported, and an error is given if they are encountered. - - By default, characters with values greater than 128 do not match any of - the POSIX character classes. However, if the PCRE_UCP option is passed - to pcre_compile(), some of the classes are changed so that Unicode - character properties are used. This is achieved by replacing certain - POSIX classes by other sequences, as follows: - - [:alnum:] becomes \p{Xan} - [:alpha:] becomes \p{L} - [:blank:] becomes \h - [:digit:] becomes \p{Nd} - [:lower:] becomes \p{Ll} - [:space:] becomes \p{Xps} - [:upper:] becomes \p{Lu} - [:word:] becomes \p{Xwd} - - Negated versions, such as [:^alpha:] use \P instead of \p. Three other - POSIX classes are handled specially in UCP mode: - - [:graph:] This matches characters that have glyphs that mark the page - when printed. In Unicode property terms, it matches all char- - acters with the L, M, N, P, S, or Cf properties, except for: - - U+061C Arabic Letter Mark - U+180E Mongolian Vowel Separator - U+2066 - U+2069 Various "isolate"s - - - [:print:] This matches the same characters as [:graph:] plus space - characters that are not controls, that is, characters with - the Zs property. - - [:punct:] This matches all characters that have the Unicode P (punctua- - tion) property, plus those characters whose code points are - less than 128 that have the S (Symbol) property. - - The other POSIX classes are unchanged, and match only characters with - code points less than 128. - - -COMPATIBILITY FEATURE FOR WORD BOUNDARIES - - In the POSIX.2 compliant library that was included in 4.4BSD Unix, the - ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" - and "end of word". PCRE treats these items as follows: - - [[:<:]] is converted to \b(?=\w) - [[:>:]] is converted to \b(?<=\w) - - Only these exact character sequences are recognized. A sequence such as - [a[:<:]b] provokes error for an unrecognized POSIX class name. This - support is not compatible with Perl. It is provided to help migrations - from other environments, and is best not used in any new patterns. Note - that \b matches at the start and the end of a word (see "Simple asser- - tions" above), and in a Perl-style pattern the preceding or following - character normally shows which is wanted, without the need for the - assertions that are used above in order to give exactly the POSIX be- - haviour. - - -VERTICAL BAR - - Vertical bar characters are used to separate alternative patterns. For - example, the pattern - - gilbert|sullivan - - matches either "gilbert" or "sullivan". Any number of alternatives may - appear, and an empty alternative is permitted (matching the empty - string). The matching process tries each alternative in turn, from left - to right, and the first one that succeeds is used. If the alternatives - are within a subpattern (defined below), "succeeds" means matching the - rest of the main pattern as well as the alternative in the subpattern. - - -INTERNAL OPTION SETTING - - The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and - PCRE_EXTENDED options (which are Perl-compatible) can be changed from - within the pattern by a sequence of Perl option letters enclosed - between "(?" and ")". The option letters are - - i for PCRE_CASELESS - m for PCRE_MULTILINE - s for PCRE_DOTALL - x for PCRE_EXTENDED - - For example, (?im) sets caseless, multiline matching. It is also possi- - ble to unset these options by preceding the letter with a hyphen, and a - combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- - LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, - is also permitted. If a letter appears both before and after the - hyphen, the option is unset. - - The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA - can be changed in the same way as the Perl-compatible options by using - the characters J, U and X respectively. - - When one of these option changes occurs at top level (that is, not - inside subpattern parentheses), the change applies to the remainder of - the pattern that follows. If the change is placed right at the start of - a pattern, PCRE extracts it into the global options (and it will there- - fore show up in data extracted by the pcre_fullinfo() function). - - An option change within a subpattern (see below for a description of - subpatterns) affects only that part of the subpattern that follows it, - so - - (a(?i)b)c - - matches abc and aBc and no other strings (assuming PCRE_CASELESS is not - used). By this means, options can be made to have different settings - in different parts of the pattern. Any changes made in one alternative - do carry on into subsequent branches within the same subpattern. For - example, - - (a(?i)b|c) - - matches "ab", "aB", "c", and "C", even though when matching "C" the - first branch is abandoned before the option setting. This is because - the effects of option settings happen at compile time. There would be - some very weird behaviour otherwise. - - Note: There are other PCRE-specific options that can be set by the - application when the compiling or matching functions are called. In - some cases the pattern can contain special leading sequences such as - (*CRLF) to override what the application has set or what has been - defaulted. Details are given in the section entitled "Newline - sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and - (*UCP) leading sequences that can be used to set UTF and Unicode prop- - erty modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, - PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF) sequence - is a generic version that can be used with any of the libraries. How- - ever, the application can set the PCRE_NEVER_UTF option, which locks - out the use of the (*UTF) sequences. - - -SUBPATTERNS - - Subpatterns are delimited by parentheses (round brackets), which can be - nested. Turning part of a pattern into a subpattern does two things: - - 1. It localizes a set of alternatives. For example, the pattern - - cat(aract|erpillar|) - - matches "cataract", "caterpillar", or "cat". Without the parentheses, - it would match "cataract", "erpillar" or an empty string. - - 2. It sets up the subpattern as a capturing subpattern. This means - that, when the whole pattern matches, that portion of the subject - string that matched the subpattern is passed back to the caller via the - ovector argument of the matching function. (This applies only to the - traditional matching functions; the DFA matching functions do not sup- - port capturing.) - - Opening parentheses are counted from left to right (starting from 1) to - obtain numbers for the capturing subpatterns. For example, if the - string "the red king" is matched against the pattern - - the ((red|white) (king|queen)) - - the captured substrings are "red king", "red", and "king", and are num- - bered 1, 2, and 3, respectively. - - The fact that plain parentheses fulfil two functions is not always - helpful. There are often times when a grouping subpattern is required - without a capturing requirement. If an opening parenthesis is followed - by a question mark and a colon, the subpattern does not do any captur- - ing, and is not counted when computing the number of any subsequent - capturing subpatterns. For example, if the string "the white queen" is - matched against the pattern - - the ((?:red|white) (king|queen)) - - the captured substrings are "white queen" and "queen", and are numbered - 1 and 2. The maximum number of capturing subpatterns is 65535. - - As a convenient shorthand, if any option settings are required at the - start of a non-capturing subpattern, the option letters may appear - between the "?" and the ":". Thus the two patterns - - (?i:saturday|sunday) - (?:(?i)saturday|sunday) - - match exactly the same set of strings. Because alternative branches are - tried from left to right, and options are not reset until the end of - the subpattern is reached, an option setting in one branch does affect - subsequent branches, so the above patterns match "SUNDAY" as well as - "Saturday". - - -DUPLICATE SUBPATTERN NUMBERS - - Perl 5.10 introduced a feature whereby each alternative in a subpattern - uses the same numbers for its capturing parentheses. Such a subpattern - starts with (?| and is itself a non-capturing subpattern. For example, - consider this pattern: - - (?|(Sat)ur|(Sun))day - - Because the two alternatives are inside a (?| group, both sets of cap- - turing parentheses are numbered one. Thus, when the pattern matches, - you can look at captured substring number one, whichever alternative - matched. This construct is useful when you want to capture part, but - not all, of one of a number of alternatives. Inside a (?| group, paren- - theses are numbered as usual, but the number is reset at the start of - each branch. The numbers of any capturing parentheses that follow the - subpattern start after the highest number used in any branch. The fol- - lowing example is taken from the Perl documentation. The numbers under- - neath show in which buffer the captured content will be stored. - - # before ---------------branch-reset----------- after - / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x - # 1 2 2 3 2 3 4 - - A back reference to a numbered subpattern uses the most recent value - that is set for that number by any subpattern. The following pattern - matches "abcabc" or "defdef": - - /(?|(abc)|(def))\1/ - - In contrast, a subroutine call to a numbered subpattern always refers - to the first one in the pattern with the given number. The following - pattern matches "abcabc" or "defabc": - - /(?|(abc)|(def))(?1)/ - - If a condition test for a subpattern's having matched refers to a non- - unique number, the test is true if any of the subpatterns of that num- - ber have matched. - - An alternative approach to using this "branch reset" feature is to use - duplicate named subpatterns, as described in the next section. - - -NAMED SUBPATTERNS - - Identifying capturing parentheses by number is simple, but it can be - very hard to keep track of the numbers in complicated regular expres- - sions. Furthermore, if an expression is modified, the numbers may - change. To help with this difficulty, PCRE supports the naming of sub- - patterns. This feature was not added to Perl until release 5.10. Python - had the feature earlier, and PCRE introduced it at release 4.0, using - the Python syntax. PCRE now supports both the Perl and the Python syn- - tax. Perl allows identically numbered subpatterns to have different - names, but PCRE does not. - - In PCRE, a subpattern can be named in one of three ways: (?<name>...) - or (?'name'...) as in Perl, or (?P<name>...) as in Python. References - to capturing parentheses from other parts of the pattern, such as back - references, recursion, and conditions, can be made by name as well as - by number. - - Names consist of up to 32 alphanumeric characters and underscores, but - must start with a non-digit. Named capturing parentheses are still - allocated numbers as well as names, exactly as if the names were not - present. The PCRE API provides function calls for extracting the name- - to-number translation table from a compiled pattern. There is also a - convenience function for extracting a captured substring by name. - - By default, a name must be unique within a pattern, but it is possible - to relax this constraint by setting the PCRE_DUPNAMES option at compile - time. (Duplicate names are also always permitted for subpatterns with - the same number, set up as described in the previous section.) Dupli- - cate names can be useful for patterns where only one instance of the - named parentheses can match. Suppose you want to match the name of a - weekday, either as a 3-letter abbreviation or as the full name, and in - both cases you want to extract the abbreviation. This pattern (ignoring - the line breaks) does the job: - - (?<DN>Mon|Fri|Sun)(?:day)?| - (?<DN>Tue)(?:sday)?| - (?<DN>Wed)(?:nesday)?| - (?<DN>Thu)(?:rsday)?| - (?<DN>Sat)(?:urday)? - - There are five capturing substrings, but only one is ever set after a - match. (An alternative way of solving this problem is to use a "branch - reset" subpattern, as described in the previous section.) - - The convenience function for extracting the data by name returns the - substring for the first (and in this example, the only) subpattern of - that name that matched. This saves searching to find which numbered - subpattern it was. - - If you make a back reference to a non-unique named subpattern from - elsewhere in the pattern, the subpatterns to which the name refers are - checked in the order in which they appear in the overall pattern. The - first one that is set is used for the reference. For example, this pat- - tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo": - - (?:(?<n>foo)|(?<n>bar))\k<n> - - - If you make a subroutine call to a non-unique named subpattern, the one - that corresponds to the first occurrence of the name is used. In the - absence of duplicate numbers (see the previous section) this is the one - with the lowest number. - - If you use a named reference in a condition test (see the section about - conditions below), either to check whether a subpattern has matched, or - to check for recursion, all subpatterns with the same name are tested. - If the condition is true for any one of them, the overall condition is - true. This is the same behaviour as testing by number. For further - details of the interfaces for handling named subpatterns, see the - pcreapi documentation. - - Warning: You cannot use different names to distinguish between two sub- - patterns with the same number because PCRE uses only the numbers when - matching. For this reason, an error is given at compile time if differ- - ent names are given to subpatterns with the same number. However, you - can always give the same name to subpatterns with the same number, even - when PCRE_DUPNAMES is not set. - - -REPETITION - - Repetition is specified by quantifiers, which can follow any of the - following items: - - a literal data character - the dot metacharacter - the \C escape sequence - the \X escape sequence - the \R escape sequence - an escape such as \d or \pL that matches a single character - a character class - a back reference (see next section) - a parenthesized subpattern (including assertions) - a subroutine call to a subpattern (recursive or otherwise) - - The general repetition quantifier specifies a minimum and maximum num- - ber of permitted matches, by giving the two numbers in curly brackets - (braces), separated by a comma. The numbers must be less than 65536, - and the first must be less than or equal to the second. For example: - - z{2,4} - - matches "zz", "zzz", or "zzzz". A closing brace on its own is not a - special character. If the second number is omitted, but the comma is - present, there is no upper limit; if the second number and the comma - are both omitted, the quantifier specifies an exact number of required - matches. Thus - - [aeiou]{3,} - - matches at least 3 successive vowels, but may match many more, while - - \d{8} - - matches exactly 8 digits. An opening curly bracket that appears in a - position where a quantifier is not allowed, or one that does not match - the syntax of a quantifier, is taken as a literal character. For exam- - ple, {,6} is not a quantifier, but a literal string of four characters. - - In UTF modes, quantifiers apply to characters rather than to individual - data units. Thus, for example, \x{100}{2} matches two characters, each - of which is represented by a two-byte sequence in a UTF-8 string. Simi- - larly, \X{3} matches three Unicode extended grapheme clusters, each of - which may be several data units long (and they may be of different - lengths). - - The quantifier {0} is permitted, causing the expression to behave as if - the previous item and the quantifier were not present. This may be use- - ful for subpatterns that are referenced as subroutines from elsewhere - in the pattern (but see also the section entitled "Defining subpatterns - for use by reference only" below). Items other than subpatterns that - have a {0} quantifier are omitted from the compiled pattern. - - For convenience, the three most common quantifiers have single-charac- - ter abbreviations: - - * is equivalent to {0,} - + is equivalent to {1,} - ? is equivalent to {0,1} - - It is possible to construct infinite loops by following a subpattern - that can match no characters with a quantifier that has no upper limit, - for example: - - (a?)* - - Earlier versions of Perl and PCRE used to give an error at compile time - for such patterns. However, because there are cases where this can be - useful, such patterns are now accepted, but if any repetition of the - subpattern does in fact match no characters, the loop is forcibly bro- - ken. - - By default, the quantifiers are "greedy", that is, they match as much - as possible (up to the maximum number of permitted times), without - causing the rest of the pattern to fail. The classic example of where - this gives problems is in trying to match comments in C programs. These - appear between /* and */ and within the comment, individual * and / - characters may appear. An attempt to match C comments by applying the - pattern - - /\*.*\*/ - - to the string - - /* first comment */ not comment /* second comment */ - - fails, because it matches the entire string owing to the greediness of - the .* item. - - However, if a quantifier is followed by a question mark, it ceases to - be greedy, and instead matches the minimum number of times possible, so - the pattern - - /\*.*?\*/ - - does the right thing with the C comments. The meaning of the various - quantifiers is not otherwise changed, just the preferred number of - matches. Do not confuse this use of question mark with its use as a - quantifier in its own right. Because it has two uses, it can sometimes - appear doubled, as in - - \d??\d - - which matches one digit by preference, but can match two if that is the - only way the rest of the pattern matches. - - If the PCRE_UNGREEDY option is set (an option that is not available in - Perl), the quantifiers are not greedy by default, but individual ones - can be made greedy by following them with a question mark. In other - words, it inverts the default behaviour. - - When a parenthesized subpattern is quantified with a minimum repeat - count that is greater than 1 or with a limited maximum, more memory is - required for the compiled pattern, in proportion to the size of the - minimum or maximum. - - If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- - alent to Perl's /s) is set, thus allowing the dot to match newlines, - the pattern is implicitly anchored, because whatever follows will be - tried against every character position in the subject string, so there - is no point in retrying the overall match at any position after the - first. PCRE normally treats such a pattern as though it were preceded - by \A. - - In cases where it is known that the subject string contains no new- - lines, it is worth setting PCRE_DOTALL in order to obtain this opti- - mization, or alternatively using ^ to indicate anchoring explicitly. - - However, there are some cases where the optimization cannot be used. - When .* is inside capturing parentheses that are the subject of a back - reference elsewhere in the pattern, a match at the start may fail where - a later one succeeds. Consider, for example: - - (.*)abc\1 - - If the subject is "xyz123abc123" the match point is the fourth charac- - ter. For this reason, such a pattern is not implicitly anchored. - - Another case where implicit anchoring is not applied is when the lead- - ing .* is inside an atomic group. Once again, a match at the start may - fail where a later one succeeds. Consider this pattern: - - (?>.*?a)b - - It matches "ab" in the subject "aab". The use of the backtracking con- - trol verbs (*PRUNE) and (*SKIP) also disable this optimization. - - When a capturing subpattern is repeated, the value captured is the sub- - string that matched the final iteration. For example, after - - (tweedle[dume]{3}\s*)+ - - has matched "tweedledum tweedledee" the value of the captured substring - is "tweedledee". However, if there are nested capturing subpatterns, - the corresponding captured values may have been set in previous itera- - tions. For example, after - - /(a|(b))+/ - - matches "aba" the value of the second captured substring is "b". - - -ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS - - With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") - repetition, failure of what follows normally causes the repeated item - to be re-evaluated to see if a different number of repeats allows the - rest of the pattern to match. Sometimes it is useful to prevent this, - either to change the nature of the match, or to cause it fail earlier - than it otherwise might, when the author of the pattern knows there is - no point in carrying on. - - Consider, for example, the pattern \d+foo when applied to the subject - line - - 123456bar - - After matching all 6 digits and then failing to match "foo", the normal - action of the matcher is to try again with only 5 digits matching the - \d+ item, and then with 4, and so on, before ultimately failing. - "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides - the means for specifying that once a subpattern has matched, it is not - to be re-evaluated in this way. - - If we use atomic grouping for the previous example, the matcher gives - up immediately on failing to match "foo" the first time. The notation - is a kind of special parenthesis, starting with (?> as in this example: - - (?>\d+)foo - - This kind of parenthesis "locks up" the part of the pattern it con- - tains once it has matched, and a failure further into the pattern is - prevented from backtracking into it. Backtracking past it to previous - items, however, works as normal. - - An alternative description is that a subpattern of this type matches - the string of characters that an identical standalone pattern would - match, if anchored at the current point in the subject string. - - Atomic grouping subpatterns are not capturing subpatterns. Simple cases - such as the above example can be thought of as a maximizing repeat that - must swallow everything it can. So, while both \d+ and \d+? are pre- - pared to adjust the number of digits they match in order to make the - rest of the pattern match, (?>\d+) can only match an entire sequence of - digits. - - Atomic groups in general can of course contain arbitrarily complicated - subpatterns, and can be nested. However, when the subpattern for an - atomic group is just a single repeated item, as in the example above, a - simpler notation, called a "possessive quantifier" can be used. This - consists of an additional + character following a quantifier. Using - this notation, the previous example can be rewritten as - - \d++foo - - Note that a possessive quantifier can be used with an entire group, for - example: - - (abc|xyz){2,3}+ - - Possessive quantifiers are always greedy; the setting of the - PCRE_UNGREEDY option is ignored. They are a convenient notation for the - simpler forms of atomic group. However, there is no difference in the - meaning of a possessive quantifier and the equivalent atomic group, - though there may be a performance difference; possessive quantifiers - should be slightly faster. - - The possessive quantifier syntax is an extension to the Perl 5.8 syn- - tax. Jeffrey Friedl originated the idea (and the name) in the first - edition of his book. Mike McCloskey liked it, so implemented it when he - built Sun's Java package, and PCRE copied it from there. It ultimately - found its way into Perl at release 5.10. - - PCRE has an optimization that automatically "possessifies" certain sim- - ple pattern constructs. For example, the sequence A+B is treated as - A++B because there is no point in backtracking into a sequence of A's - when B must follow. - - When a pattern contains an unlimited repeat inside a subpattern that - can itself be repeated an unlimited number of times, the use of an - atomic group is the only way to avoid some failing matches taking a - very long time indeed. The pattern - - (\D+|<\d+>)*[!?] - - matches an unlimited number of substrings that either consist of non- - digits, or digits enclosed in <>, followed by either ! or ?. When it - matches, it runs quickly. However, if it is applied to - - aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa - - it takes a long time before reporting failure. This is because the - string can be divided between the internal \D+ repeat and the external - * repeat in a large number of ways, and all have to be tried. (The - example uses [!?] rather than a single character at the end, because - both PCRE and Perl have an optimization that allows for fast failure - when a single character is used. They remember the last single charac- - ter that is required for a match, and fail early if it is not present - in the string.) If the pattern is changed so that it uses an atomic - group, like this: - - ((?>\D+)|<\d+>)*[!?] - - sequences of non-digits cannot be broken, and failure happens quickly. - - -BACK REFERENCES - - Outside a character class, a backslash followed by a digit greater than - 0 (and possibly further digits) is a back reference to a capturing sub- - pattern earlier (that is, to its left) in the pattern, provided there - have been that many previous capturing left parentheses. - - However, if the decimal number following the backslash is less than 10, - it is always taken as a back reference, and causes an error only if - there are not that many capturing left parentheses in the entire pat- - tern. In other words, the parentheses that are referenced need not be - to the left of the reference for numbers less than 10. A "forward back - reference" of this type can make sense when a repetition is involved - and the subpattern to the right has participated in an earlier itera- - tion. - - It is not possible to have a numerical "forward back reference" to a - subpattern whose number is 10 or more using this syntax because a - sequence such as \50 is interpreted as a character defined in octal. - See the subsection entitled "Non-printing characters" above for further - details of the handling of digits following a backslash. There is no - such problem when named parentheses are used. A back reference to any - subpattern is possible using named parentheses (see below). - - Another way of avoiding the ambiguity inherent in the use of digits - following a backslash is to use the \g escape sequence. This escape - must be followed by an unsigned number or a negative number, optionally - enclosed in braces. These examples are all identical: - - (ring), \1 - (ring), \g1 - (ring), \g{1} - - An unsigned number specifies an absolute reference without the ambigu- - ity that is present in the older syntax. It is also useful when literal - digits follow the reference. A negative number is a relative reference. - Consider this example: - - (abc(def)ghi)\g{-1} - - The sequence \g{-1} is a reference to the most recently started captur- - ing subpattern before \g, that is, is it equivalent to \2 in this exam- - ple. Similarly, \g{-2} would be equivalent to \1. The use of relative - references can be helpful in long patterns, and also in patterns that - are created by joining together fragments that contain references - within themselves. - - A back reference matches whatever actually matched the capturing sub- - pattern in the current subject string, rather than anything matching - the subpattern itself (see "Subpatterns as subroutines" below for a way - of doing that). So the pattern - - (sens|respons)e and \1ibility - - matches "sense and sensibility" and "response and responsibility", but - not "sense and responsibility". If caseful matching is in force at the - time of the back reference, the case of letters is relevant. For exam- - ple, - - ((?i)rah)\s+\1 - - matches "rah rah" and "RAH RAH", but not "RAH rah", even though the - original capturing subpattern is matched caselessly. - - There are several different ways of writing back references to named - subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or - \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's - unified back reference syntax, in which \g can be used for both numeric - and named references, is also supported. We could rewrite the above - example in any of the following ways: - - (?<p1>(?i)rah)\s+\k<p1> - (?'p1'(?i)rah)\s+\k{p1} - (?P<p1>(?i)rah)\s+(?P=p1) - (?<p1>(?i)rah)\s+\g{p1} - - A subpattern that is referenced by name may appear in the pattern - before or after the reference. - - There may be more than one back reference to the same subpattern. If a - subpattern has not actually been used in a particular match, any back - references to it always fail by default. For example, the pattern - - (a|(bc))\2 - - always fails if it starts to match "a" rather than "bc". However, if - the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- - ence to an unset value matches an empty string. - - Because there may be many capturing parentheses in a pattern, all dig- - its following a backslash are taken as part of a potential back refer- - ence number. If the pattern continues with a digit character, some - delimiter must be used to terminate the back reference. If the - PCRE_EXTENDED option is set, this can be white space. Otherwise, the - \g{ syntax or an empty comment (see "Comments" below) can be used. - - Recursive back references - - A back reference that occurs inside the parentheses to which it refers - fails when the subpattern is first used, so, for example, (a\1) never - matches. However, such references can be useful inside repeated sub- - patterns. For example, the pattern - - (a|b\1)+ - - matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- - ation of the subpattern, the back reference matches the character - string corresponding to the previous iteration. In order for this to - work, the pattern must be such that the first iteration does not need - to match the back reference. This can be done using alternation, as in - the example above, or by a quantifier with a minimum of zero. - - Back references of this type cause the group that they reference to be - treated as an atomic group. Once the whole group has been matched, a - subsequent matching failure cannot cause backtracking into the middle - of the group. - - -ASSERTIONS - - An assertion is a test on the characters following or preceding the - current matching point that does not actually consume any characters. - The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are - described above. - - More complicated assertions are coded as subpatterns. There are two - kinds: those that look ahead of the current position in the subject - string, and those that look behind it. An assertion subpattern is - matched in the normal way, except that it does not cause the current - matching position to be changed. - - Assertion subpatterns are not capturing subpatterns. If such an asser- - tion contains capturing subpatterns within it, these are counted for - the purposes of numbering the capturing subpatterns in the whole pat- - tern. However, substring capturing is carried out only for positive - assertions. (Perl sometimes, but not always, does do capturing in nega- - tive assertions.) - - For compatibility with Perl, assertion subpatterns may be repeated; - though it makes no sense to assert the same thing several times, the - side effect of capturing parentheses may occasionally be useful. In - practice, there only three cases: - - (1) If the quantifier is {0}, the assertion is never obeyed during - matching. However, it may contain internal capturing parenthesized - groups that are called from elsewhere via the subroutine mechanism. - - (2) If quantifier is {0,n} where n is greater than zero, it is treated - as if it were {0,1}. At run time, the rest of the pattern match is - tried with and without the assertion, the order depending on the greed- - iness of the quantifier. - - (3) If the minimum repetition is greater than zero, the quantifier is - ignored. The assertion is obeyed just once when encountered during - matching. - - Lookahead assertions - - Lookahead assertions start with (?= for positive assertions and (?! for - negative assertions. For example, - - \w+(?=;) - - matches a word followed by a semicolon, but does not include the semi- - colon in the match, and - - foo(?!bar) - - matches any occurrence of "foo" that is not followed by "bar". Note - that the apparently similar pattern - - (?!foo)bar - - does not find an occurrence of "bar" that is preceded by something - other than "foo"; it finds any occurrence of "bar" whatsoever, because - the assertion (?!foo) is always true when the next three characters are - "bar". A lookbehind assertion is needed to achieve the other effect. - - If you want to force a matching failure at some point in a pattern, the - most convenient way to do it is with (?!) because an empty string - always matches, so an assertion that requires there not to be an empty - string must always fail. The backtracking control verb (*FAIL) or (*F) - is a synonym for (?!). - - Lookbehind assertions - - Lookbehind assertions start with (?<= for positive assertions and (?<! - for negative assertions. For example, - - (?<!foo)bar - - does find an occurrence of "bar" that is not preceded by "foo". The - contents of a lookbehind assertion are restricted such that all the - strings it matches must have a fixed length. However, if there are sev- - eral top-level alternatives, they do not all have to have the same - fixed length. Thus - - (?<=bullock|donkey) - - is permitted, but - - (?<!dogs?|cats?) - - causes an error at compile time. Branches that match different length - strings are permitted only at the top level of a lookbehind assertion. - This is an extension compared with Perl, which requires all branches to - match the same length of string. An assertion such as - - (?<=ab(c|de)) - - is not permitted, because its single top-level branch can match two - different lengths, but it is acceptable to PCRE if rewritten to use two - top-level branches: - - (?<=abc|abde) - - In some cases, the escape sequence \K (see above) can be used instead - of a lookbehind assertion to get round the fixed-length restriction. - - The implementation of lookbehind assertions is, for each alternative, - to temporarily move the current position back by the fixed length and - then try to match. If there are insufficient characters before the cur- - rent position, the assertion fails. - - In a UTF mode, PCRE does not allow the \C escape (which matches a sin- - gle data unit even in a UTF mode) to appear in lookbehind assertions, - because it makes it impossible to calculate the length of the lookbe- - hind. The \X and \R escapes, which can match different numbers of data - units, are also not permitted. - - "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in - lookbehinds, as long as the subpattern matches a fixed-length string. - Recursion, however, is not supported. - - Possessive quantifiers can be used in conjunction with lookbehind - assertions to specify efficient matching of fixed-length strings at the - end of subject strings. Consider a simple pattern such as - - abcd$ - - when applied to a long string that does not match. Because matching - proceeds from left to right, PCRE will look for each "a" in the subject - and then see if what follows matches the rest of the pattern. If the - pattern is specified as - - ^.*abcd$ - - the initial .* matches the entire string at first, but when this fails - (because there is no following "a"), it backtracks to match all but the - last character, then all but the last two characters, and so on. Once - again the search for "a" covers the entire string, from right to left, - so we are no better off. However, if the pattern is written as - - ^.*+(?<=abcd) - - there can be no backtracking for the .*+ item; it can match only the - entire string. The subsequent lookbehind assertion does a single test - on the last four characters. If it fails, the match fails immediately. - For long strings, this approach makes a significant difference to the - processing time. - - Using multiple assertions - - Several assertions (of any sort) may occur in succession. For example, - - (?<=\d{3})(?<!999)foo - - matches "foo" preceded by three digits that are not "999". Notice that - each of the assertions is applied independently at the same point in - the subject string. First there is a check that the previous three - characters are all digits, and then there is a check that the same - three characters are not "999". This pattern does not match "foo" pre- - ceded by six characters, the first of which are digits and the last - three of which are not "999". For example, it doesn't match "123abc- - foo". A pattern to do that is - - (?<=\d{3}...)(?<!999)foo - - This time the first assertion looks at the preceding six characters, - checking that the first three are digits, and then the second assertion - checks that the preceding three characters are not "999". - - Assertions can be nested in any combination. For example, - - (?<=(?<!foo)bar)baz - - matches an occurrence of "baz" that is preceded by "bar" which in turn - is not preceded by "foo", while - - (?<=\d{3}(?!999)...)foo - - is another pattern that matches "foo" preceded by three digits and any - three characters that are not "999". - - -CONDITIONAL SUBPATTERNS - - It is possible to cause the matching process to obey a subpattern con- - ditionally or to choose between two alternative subpatterns, depending - on the result of an assertion, or whether a specific capturing subpat- - tern has already been matched. The two possible forms of conditional - subpattern are: - - (?(condition)yes-pattern) - (?(condition)yes-pattern|no-pattern) - - If the condition is satisfied, the yes-pattern is used; otherwise the - no-pattern (if present) is used. If there are more than two alterna- - tives in the subpattern, a compile-time error occurs. Each of the two - alternatives may itself contain nested subpatterns of any form, includ- - ing conditional subpatterns; the restriction to two alternatives - applies only at the level of the condition. This pattern fragment is an - example where the alternatives are complex: - - (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) - - - There are four kinds of condition: references to subpatterns, refer- - ences to recursion, a pseudo-condition called DEFINE, and assertions. - - Checking for a used subpattern by number - - If the text between the parentheses consists of a sequence of digits, - the condition is true if a capturing subpattern of that number has pre- - viously matched. If there is more than one capturing subpattern with - the same number (see the earlier section about duplicate subpattern - numbers), the condition is true if any of them have matched. An alter- - native notation is to precede the digits with a plus or minus sign. In - this case, the subpattern number is relative rather than absolute. The - most recently opened parentheses can be referenced by (?(-1), the next - most recent by (?(-2), and so on. Inside loops it can also make sense - to refer to subsequent groups. The next parentheses to be opened can be - referenced as (?(+1), and so on. (The value zero in any of these forms - is not used; it provokes a compile-time error.) - - Consider the following pattern, which contains non-significant white - space to make it more readable (assume the PCRE_EXTENDED option) and to - divide it into three parts for ease of discussion: - - ( \( )? [^()]+ (?(1) \) ) - - The first part matches an optional opening parenthesis, and if that - character is present, sets it as the first captured substring. The sec- - ond part matches one or more characters that are not parentheses. The - third part is a conditional subpattern that tests whether or not the - first set of parentheses matched. If they did, that is, if subject - started with an opening parenthesis, the condition is true, and so the - yes-pattern is executed and a closing parenthesis is required. Other- - wise, since no-pattern is not present, the subpattern matches nothing. - In other words, this pattern matches a sequence of non-parentheses, - optionally enclosed in parentheses. - - If you were embedding this pattern in a larger one, you could use a - relative reference: - - ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... - - This makes the fragment independent of the parentheses in the larger - pattern. - - Checking for a used subpattern by name - - Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a - used subpattern by name. For compatibility with earlier versions of - PCRE, which had this facility before Perl, the syntax (?(name)...) is - also recognized. - - Rewriting the above example to use a named subpattern gives this: - - (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) - - If the name used in a condition of this kind is a duplicate, the test - is applied to all subpatterns of the same name, and is true if any one - of them has matched. - - Checking for pattern recursion - - If the condition is the string (R), and there is no subpattern with the - name R, the condition is true if a recursive call to the whole pattern - or any subpattern has been made. If digits or a name preceded by amper- - sand follow the letter R, for example: - - (?(R3)...) or (?(R&name)...) - - the condition is true if the most recent recursion is into a subpattern - whose number or name is given. This condition does not check the entire - recursion stack. If the name used in a condition of this kind is a - duplicate, the test is applied to all subpatterns of the same name, and - is true if any one of them is the most recent recursion. - - At "top level", all these recursion test conditions are false. The - syntax for recursive patterns is described below. - - Defining subpatterns for use by reference only - - If the condition is the string (DEFINE), and there is no subpattern - with the name DEFINE, the condition is always false. In this case, - there may be only one alternative in the subpattern. It is always - skipped if control reaches this point in the pattern; the idea of - DEFINE is that it can be used to define subroutines that can be refer- - enced from elsewhere. (The use of subroutines is described below.) For - example, a pattern to match an IPv4 address such as "192.168.23.245" - could be written like this (ignore white space and line breaks): - - (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) - \b (?&byte) (\.(?&byte)){3} \b - - The first part of the pattern is a DEFINE group inside which a another - group named "byte" is defined. This matches an individual component of - an IPv4 address (a number less than 256). When matching takes place, - this part of the pattern is skipped because DEFINE acts like a false - condition. The rest of the pattern uses references to the named group - to match the four dot-separated components of an IPv4 address, insist- - ing on a word boundary at each end. - - Assertion conditions - - If the condition is not in any of the above formats, it must be an - assertion. This may be a positive or negative lookahead or lookbehind - assertion. Consider this pattern, again containing non-significant - white space, and with the two alternatives on the second line: - - (?(?=[^a-z]*[a-z]) - \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) - - The condition is a positive lookahead assertion that matches an - optional sequence of non-letters followed by a letter. In other words, - it tests for the presence of at least one letter in the subject. If a - letter is found, the subject is matched against the first alternative; - otherwise it is matched against the second. This pattern matches - strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are - letters and dd are digits. - - -COMMENTS - - There are two ways of including comments in patterns that are processed - by PCRE. In both cases, the start of the comment must not be in a char- - acter class, nor in the middle of any other sequence of related charac- - ters such as (?: or a subpattern name or number. The characters that - make up a comment play no part in the pattern matching. - - The sequence (?# marks the start of a comment that continues up to the - next closing parenthesis. Nested parentheses are not permitted. If the - PCRE_EXTENDED option is set, an unescaped # character also introduces a - comment, which in this case continues to immediately after the next - newline character or character sequence in the pattern. Which charac- - ters are interpreted as newlines is controlled by the options passed to - a compiling function or by a special sequence at the start of the pat- - tern, as described in the section entitled "Newline conventions" above. - Note that the end of this type of comment is a literal newline sequence - in the pattern; escape sequences that happen to represent a newline do - not count. For example, consider this pattern when PCRE_EXTENDED is - set, and the default newline convention is in force: - - abc #comment \n still comment - - On encountering the # character, pcre_compile() skips along, looking - for a newline in the pattern. The sequence \n is still literal at this - stage, so it does not terminate the comment. Only an actual character - with the code value 0x0a (the default newline) does so. - - -RECURSIVE PATTERNS - - Consider the problem of matching a string in parentheses, allowing for - unlimited nested parentheses. Without the use of recursion, the best - that can be done is to use a pattern that matches up to some fixed - depth of nesting. It is not possible to handle an arbitrary nesting - depth. - - For some time, Perl has provided a facility that allows regular expres- - sions to recurse (amongst other things). It does this by interpolating - Perl code in the expression at run time, and the code can refer to the - expression itself. A Perl pattern using code interpolation to solve the - parentheses problem can be created like this: - - $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; - - The (?p{...}) item interpolates Perl code at run time, and in this case - refers recursively to the pattern in which it appears. - - Obviously, PCRE cannot support the interpolation of Perl code. Instead, - it supports special syntax for recursion of the entire pattern, and - also for individual subpattern recursion. After its introduction in - PCRE and Python, this kind of recursion was subsequently introduced - into Perl at release 5.10. - - A special item that consists of (? followed by a number greater than - zero and a closing parenthesis is a recursive subroutine call of the - subpattern of the given number, provided that it occurs inside that - subpattern. (If not, it is a non-recursive subroutine call, which is - described in the next section.) The special item (?R) or (?0) is a - recursive call of the entire regular expression. - - This PCRE pattern solves the nested parentheses problem (assume the - PCRE_EXTENDED option is set so that white space is ignored): - - \( ( [^()]++ | (?R) )* \) - - First it matches an opening parenthesis. Then it matches any number of - substrings which can either be a sequence of non-parentheses, or a - recursive match of the pattern itself (that is, a correctly parenthe- - sized substring). Finally there is a closing parenthesis. Note the use - of a possessive quantifier to avoid backtracking into sequences of non- - parentheses. - - If this were part of a larger pattern, you would not want to recurse - the entire pattern, so instead you could use this: - - ( \( ( [^()]++ | (?1) )* \) ) - - We have put the pattern into parentheses, and caused the recursion to - refer to them instead of the whole pattern. - - In a larger pattern, keeping track of parenthesis numbers can be - tricky. This is made easier by the use of relative references. Instead - of (?1) in the pattern above you can write (?-2) to refer to the second - most recently opened parentheses preceding the recursion. In other - words, a negative number counts capturing parentheses leftwards from - the point at which it is encountered. - - It is also possible to refer to subsequently opened parentheses, by - writing references such as (?+2). However, these cannot be recursive - because the reference is not inside the parentheses that are refer- - enced. They are always non-recursive subroutine calls, as described in - the next section. - - An alternative approach is to use named parentheses instead. The Perl - syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also - supported. We could rewrite the above example as follows: - - (?<pn> \( ( [^()]++ | (?&pn) )* \) ) - - If there is more than one subpattern with the same name, the earliest - one is used. - - This particular example pattern that we have been looking at contains - nested unlimited repeats, and so the use of a possessive quantifier for - matching strings of non-parentheses is important when applying the pat- - tern to strings that do not match. For example, when this pattern is - applied to - - (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() - - it yields "no match" quickly. However, if a possessive quantifier is - not used, the match runs for a very long time indeed because there are - so many different ways the + and * repeats can carve up the subject, - and all have to be tested before failure can be reported. - - At the end of a match, the values of capturing parentheses are those - from the outermost level. If you want to obtain intermediate values, a - callout function can be used (see below and the pcrecallout documenta- - tion). If the pattern above is matched against - - (ab(cd)ef) - - the value for the inner capturing parentheses (numbered 2) is "ef", - which is the last value taken on at the top level. If a capturing sub- - pattern is not matched at the top level, its final captured value is - unset, even if it was (temporarily) set at a deeper level during the - matching process. - - If there are more than 15 capturing parentheses in a pattern, PCRE has - to obtain extra memory to store data during a recursion, which it does - by using pcre_malloc, freeing it via pcre_free afterwards. If no memory - can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. - - Do not confuse the (?R) item with the condition (R), which tests for - recursion. Consider this pattern, which matches text in angle brack- - ets, allowing for arbitrary nesting. Only digits are allowed in nested - brackets (that is, when recursing), whereas any characters are permit- - ted at the outer level. - - < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > - - In this pattern, (?(R) is the start of a conditional subpattern, with - two different alternatives for the recursive and non-recursive cases. - The (?R) item is the actual recursive call. - - Differences in recursion processing between PCRE and Perl - - Recursion processing in PCRE differs from Perl in two important ways. - In PCRE (like Python, but unlike Perl), a recursive subpattern call is - always treated as an atomic group. That is, once it has matched some of - the subject string, it is never re-entered, even if it contains untried - alternatives and there is a subsequent matching failure. This can be - illustrated by the following pattern, which purports to match a palin- - dromic string that contains an odd number of characters (for example, - "a", "aba", "abcba", "abcdcba"): - - ^(.|(.)(?1)\2)$ - - The idea is that it either matches a single character, or two identical - characters surrounding a sub-palindrome. In Perl, this pattern works; - in PCRE it does not if the pattern is longer than three characters. - Consider the subject string "abcba": - - At the top level, the first character is matched, but as it is not at - the end of the string, the first alternative fails; the second alterna- - tive is taken and the recursion kicks in. The recursive call to subpat- - tern 1 successfully matches the next character ("b"). (Note that the - beginning and end of line tests are not part of the recursion). - - Back at the top level, the next character ("c") is compared with what - subpattern 2 matched, which was "a". This fails. Because the recursion - is treated as an atomic group, there are now no backtracking points, - and so the entire match fails. (Perl is able, at this point, to re- - enter the recursion and try the second alternative.) However, if the - pattern is written with the alternatives in the other order, things are - different: - - ^((.)(?1)\2|.)$ - - This time, the recursing alternative is tried first, and continues to - recurse until it runs out of characters, at which point the recursion - fails. But this time we do have another alternative to try at the - higher level. That is the big difference: in the previous case the - remaining alternative is at a deeper recursion level, which PCRE cannot - use. - - To change the pattern so that it matches all palindromic strings, not - just those with an odd number of characters, it is tempting to change - the pattern to this: - - ^((.)(?1)\2|.?)$ - - Again, this works in Perl, but not in PCRE, and for the same reason. - When a deeper recursion has matched a single character, it cannot be - entered again in order to match an empty string. The solution is to - separate the two cases, and write out the odd and even cases as alter- - natives at the higher level: - - ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) - - If you want to match typical palindromic phrases, the pattern has to - ignore all non-word characters, which can be done like this: - - ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ - - If run with the PCRE_CASELESS option, this pattern matches phrases such - as "A man, a plan, a canal: Panama!" and it works well in both PCRE and - Perl. Note the use of the possessive quantifier *+ to avoid backtrack- - ing into sequences of non-word characters. Without this, PCRE takes a - great deal longer (ten times or more) to match typical phrases, and - Perl takes so long that you think it has gone into a loop. - - WARNING: The palindrome-matching patterns above work only if the sub- - ject string does not start with a palindrome that is shorter than the - entire string. For example, although "abcba" is correctly matched, if - the subject is "ababa", PCRE finds the palindrome "aba" at the start, - then fails at top level because the end of the string does not follow. - Once again, it cannot jump back into the recursion to try other alter- - natives, so the entire match fails. - - The second way in which PCRE and Perl differ in their recursion pro- - cessing is in the handling of captured values. In Perl, when a subpat- - tern is called recursively or as a subpattern (see the next section), - it has no access to any values that were captured outside the recur- - sion, whereas in PCRE these values can be referenced. Consider this - pattern: - - ^(.)(\1|a(?2)) - - In PCRE, this pattern matches "bab". The first capturing parentheses - match "b", then in the second group, when the back reference \1 fails - to match "b", the second alternative matches "a" and then recurses. In - the recursion, \1 does now match "b" and so the whole match succeeds. - In Perl, the pattern fails to match because inside the recursive call - \1 cannot access the externally set value. - - -SUBPATTERNS AS SUBROUTINES - - If the syntax for a recursive subpattern call (either by number or by - name) is used outside the parentheses to which it refers, it operates - like a subroutine in a programming language. The called subpattern may - be defined before or after the reference. A numbered reference can be - absolute or relative, as in these examples: - - (...(absolute)...)...(?2)... - (...(relative)...)...(?-1)... - (...(?+1)...(relative)... - - An earlier example pointed out that the pattern - - (sens|respons)e and \1ibility - - matches "sense and sensibility" and "response and responsibility", but - not "sense and responsibility". If instead the pattern - - (sens|respons)e and (?1)ibility - - is used, it does match "sense and responsibility" as well as the other - two strings. Another example is given in the discussion of DEFINE - above. - - All subroutine calls, whether recursive or not, are always treated as - atomic groups. That is, once a subroutine has matched some of the sub- - ject string, it is never re-entered, even if it contains untried alter- - natives and there is a subsequent matching failure. Any capturing - parentheses that are set during the subroutine call revert to their - previous values afterwards. - - Processing options such as case-independence are fixed when a subpat- - tern is defined, so if it is used as a subroutine, such options cannot - be changed for different calls. For example, consider this pattern: - - (abc)(?i:(?-1)) - - It matches "abcabc". It does not match "abcABC" because the change of - processing option does not affect the called subpattern. - - -ONIGURUMA SUBROUTINE SYNTAX - - For compatibility with Oniguruma, the non-Perl syntax \g followed by a - name or a number enclosed either in angle brackets or single quotes, is - an alternative syntax for referencing a subpattern as a subroutine, - possibly recursively. Here are two of the examples used above, rewrit- - ten using this syntax: - - (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) - (sens|respons)e and \g'1'ibility - - PCRE supports an extension to Oniguruma: if a number is preceded by a - plus or a minus sign it is taken as a relative reference. For example: - - (abc)(?i:\g<-1>) - - Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not - synonymous. The former is a back reference; the latter is a subroutine - call. - - -CALLOUTS - - Perl has a feature whereby using the sequence (?{...}) causes arbitrary - Perl code to be obeyed in the middle of matching a regular expression. - This makes it possible, amongst other things, to extract different sub- - strings that match the same pair of parentheses when there is a repeti- - tion. - - PCRE provides a similar feature, but of course it cannot obey arbitrary - Perl code. The feature is called "callout". The caller of PCRE provides - an external function by putting its entry point in the global variable - pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit - library). By default, this variable contains NULL, which disables all - calling out. - - Within a regular expression, (?C) indicates the points at which the - external function is to be called. If you want to identify different - callout points, you can put a number less than 256 after the letter C. - The default value is zero. For example, this pattern has two callout - points: - - (?C1)abc(?C2)def - - If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call- - outs are automatically installed before each item in the pattern. They - are all numbered 255. If there is a conditional group in the pattern - whose condition is an assertion, an additional callout is inserted just - before the condition. An explicit callout may also be set at this posi- - tion, as in this example: - - (?(?C9)(?=a)abc|def) - - Note that this applies only to assertion conditions, not to other types - of condition. - - During matching, when PCRE reaches a callout point, the external func- - tion is called. It is provided with the number of the callout, the - position in the pattern, and, optionally, one item of data originally - supplied by the caller of the matching function. The callout function - may cause matching to proceed, to backtrack, or to fail altogether. - - By default, PCRE implements a number of optimizations at compile time - and matching time, and one side-effect is that sometimes callouts are - skipped. If you need all possible callouts to happen, you need to set - options that disable the relevant optimizations. More details, and a - complete description of the interface to the callout function, are - given in the pcrecallout documentation. - - -BACKTRACKING CONTROL - - Perl 5.10 introduced a number of "Special Backtracking Control Verbs", - which are still described in the Perl documentation as "experimental - and subject to change or removal in a future version of Perl". It goes - on to say: "Their usage in production code should be noted to avoid - problems during upgrades." The same remarks apply to the PCRE features - described in this section. - - The new verbs make use of what was previously invalid syntax: an open- - ing parenthesis followed by an asterisk. They are generally of the form - (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving - differently depending on whether or not a name is present. A name is - any sequence of characters that does not include a closing parenthesis. - The maximum length of name is 255 in the 8-bit library and 65535 in the - 16-bit and 32-bit libraries. If the name is empty, that is, if the - closing parenthesis immediately follows the colon, the effect is as if - the colon were not there. Any number of these verbs may occur in a - pattern. - - Since these verbs are specifically related to backtracking, most of - them can be used only when the pattern is to be matched using one of - the traditional matching functions, because these use a backtracking - algorithm. With the exception of (*FAIL), which behaves like a failing - negative assertion, the backtracking control verbs cause an error if - encountered by a DFA matching function. - - The behaviour of these verbs in repeated groups, assertions, and in - subpatterns called as subroutines (whether or not recursively) is docu- - mented below. - - Optimizations that affect backtracking verbs - - PCRE contains some optimizations that are used to speed up matching by - running some checks at the start of each match attempt. For example, it - may know the minimum length of matching subject, or that a particular - character must be present. When one of these optimizations bypasses the - running of a match, any included backtracking verbs will not, of - course, be processed. You can suppress the start-of-match optimizations - by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- - pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). - There is more discussion of this option in the section entitled "Option - bits for pcre_exec()" in the pcreapi documentation. - - Experiments with Perl suggest that it too has similar optimizations, - sometimes leading to anomalous results. - - Verbs that act immediately - - The following verbs act as soon as they are encountered. They may not - be followed by a name. - - (*ACCEPT) - - This verb causes the match to end successfully, skipping the remainder - of the pattern. However, when it is inside a subpattern that is called - as a subroutine, only that subpattern is ended successfully. Matching - then continues at the outer level. If (*ACCEPT) in triggered in a posi- - tive assertion, the assertion succeeds; in a negative assertion, the - assertion fails. - - If (*ACCEPT) is inside capturing parentheses, the data so far is cap- - tured. For example: - - A((?:A|B(*ACCEPT)|C)D) - - This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- - tured by the outer parentheses. - - (*FAIL) or (*F) - - This verb causes a matching failure, forcing backtracking to occur. It - is equivalent to (?!) but easier to read. The Perl documentation notes - that it is probably useful only when combined with (?{}) or (??{}). - Those are, of course, Perl features that are not present in PCRE. The - nearest equivalent is the callout feature, as for example in this pat- - tern: - - a+(?C)(*FAIL) - - A match with the string "aaaa" always fails, but the callout is taken - before each backtrack happens (in this example, 10 times). - - Recording which path was taken - - There is one verb whose main purpose is to track how a match was - arrived at, though it also has a secondary use in conjunction with - advancing the match starting point (see (*SKIP) below). - - (*MARK:NAME) or (*:NAME) - - A name is always required with this verb. There may be as many - instances of (*MARK) as you like in a pattern, and their names do not - have to be unique. - - When a match succeeds, the name of the last-encountered (*MARK:NAME), - (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to - the caller as described in the section entitled "Extra data for - pcre_exec()" in the pcreapi documentation. Here is an example of - pcretest output, where the /K modifier requests the retrieval and out- - putting of (*MARK) data: - - re> /X(*MARK:A)Y|X(*MARK:B)Z/K - data> XY - 0: XY - MK: A - XZ - 0: XZ - MK: B - - The (*MARK) name is tagged with "MK:" in this output, and in this exam- - ple it indicates which of the two alternatives matched. This is a more - efficient way of obtaining this information than putting each alterna- - tive in its own capturing parentheses. - - If a verb with a name is encountered in a positive assertion that is - true, the name is recorded and passed back if it is the last-encoun- - tered. This does not happen for negative assertions or failing positive - assertions. - - After a partial match or a failed match, the last encountered name in - the entire match process is returned. For example: - - re> /X(*MARK:A)Y|X(*MARK:B)Z/K - data> XP - No match, mark = B - - Note that in this unanchored example the mark is retained from the - match attempt that started at the letter "X" in the subject. Subsequent - match attempts starting at "P" and then with an empty string do not get - as far as the (*MARK) item, but nevertheless do not reset it. - - If you are interested in (*MARK) values after failed matches, you - should probably set the PCRE_NO_START_OPTIMIZE option (see above) to - ensure that the match is always attempted. - - Verbs that act after backtracking - - The following verbs do nothing when they are encountered. Matching con- - tinues with what follows, but if there is no subsequent match, causing - a backtrack to the verb, a failure is forced. That is, backtracking - cannot pass to the left of the verb. However, when one of these verbs - appears inside an atomic group or an assertion that is true, its effect - is confined to that group, because once the group has been matched, - there is never any backtracking into it. In this situation, backtrack- - ing can "jump back" to the left of the entire atomic group or asser- - tion. (Remember also, as stated above, that this localization also - applies in subroutine calls.) - - These verbs differ in exactly what kind of failure occurs when back- - tracking reaches them. The behaviour described below is what happens - when the verb is not in a subroutine or an assertion. Subsequent sec- - tions cover these special cases. - - (*COMMIT) - - This verb, which may not be followed by a name, causes the whole match - to fail outright if there is a later matching failure that causes back- - tracking to reach it. Even if the pattern is unanchored, no further - attempts to find a match by advancing the starting point take place. If - (*COMMIT) is the only backtracking verb that is encountered, once it - has been passed pcre_exec() is committed to finding a match at the cur- - rent starting point, or not at all. For example: - - a+(*COMMIT)b - - This matches "xxaab" but not "aacaab". It can be thought of as a kind - of dynamic anchor, or "I've started, so I must finish." The name of the - most recently passed (*MARK) in the path is passed back when (*COMMIT) - forces a match failure. - - If there is more than one backtracking verb in a pattern, a different - one that follows (*COMMIT) may be triggered first, so merely passing - (*COMMIT) during a match does not always guarantee that a match must be - at this starting point. - - Note that (*COMMIT) at the start of a pattern is not the same as an - anchor, unless PCRE's start-of-match optimizations are turned off, as - shown in this output from pcretest: - - re> /(*COMMIT)abc/ - data> xyzabc - 0: abc - data> xyzabc\Y - No match - - For this pattern, PCRE knows that any match must start with "a", so the - optimization skips along the subject to "a" before applying the pattern - to the first set of data. The match attempt then succeeds. In the sec- - ond set of data, the escape sequence \Y is interpreted by the pcretest - program. It causes the PCRE_NO_START_OPTIMIZE option to be set when - pcre_exec() is called. This disables the optimization that skips along - to the first character. The pattern is now applied starting at "x", and - so the (*COMMIT) causes the match to fail without trying any other - starting points. - - (*PRUNE) or (*PRUNE:NAME) - - This verb causes the match to fail at the current starting position in - the subject if there is a later matching failure that causes backtrack- - ing to reach it. If the pattern is unanchored, the normal "bumpalong" - advance to the next starting character then happens. Backtracking can - occur as usual to the left of (*PRUNE), before it is reached, or when - matching to the right of (*PRUNE), but if there is no match to the - right, backtracking cannot cross (*PRUNE). In simple cases, the use of - (*PRUNE) is just an alternative to an atomic group or possessive quan- - tifier, but there are some uses of (*PRUNE) that cannot be expressed in - any other way. In an anchored pattern (*PRUNE) has the same effect as - (*COMMIT). - - The behaviour of (*PRUNE:NAME) is the not the same as - (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is - remembered for passing back to the caller. However, (*SKIP:NAME) - searches only for names set with (*MARK). - - (*SKIP) - - This verb, when given without a name, is like (*PRUNE), except that if - the pattern is unanchored, the "bumpalong" advance is not to the next - character, but to the position in the subject where (*SKIP) was encoun- - tered. (*SKIP) signifies that whatever text was matched leading up to - it cannot be part of a successful match. Consider: - - a+(*SKIP)b - - If the subject is "aaaac...", after the first match attempt fails - (starting at the first character in the string), the starting point - skips on to start the next attempt at "c". Note that a possessive quan- - tifer does not have the same effect as this example; although it would - suppress backtracking during the first match attempt, the second - attempt would start at the second character instead of skipping on to - "c". - - (*SKIP:NAME) - - When (*SKIP) has an associated name, its behaviour is modified. When it - is triggered, the previous path through the pattern is searched for the - most recent (*MARK) that has the same name. If one is found, the - "bumpalong" advance is to the subject position that corresponds to that - (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with - a matching name is found, the (*SKIP) is ignored. - - Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It - ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME). - - (*THEN) or (*THEN:NAME) - - This verb causes a skip to the next innermost alternative when back- - tracking reaches it. That is, it cancels any further backtracking - within the current alternative. Its name comes from the observation - that it can be used for a pattern-based if-then-else block: - - ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... - - If the COND1 pattern matches, FOO is tried (and possibly further items - after the end of the group if FOO succeeds); on failure, the matcher - skips to the second alternative and tries COND2, without backtracking - into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- - quently BAZ fails, there are no more alternatives, so there is a back- - track to whatever came before the entire group. If (*THEN) is not - inside an alternation, it acts like (*PRUNE). - - The behaviour of (*THEN:NAME) is the not the same as - (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is - remembered for passing back to the caller. However, (*SKIP:NAME) - searches only for names set with (*MARK). - - A subpattern that does not contain a | character is just a part of the - enclosing alternative; it is not a nested alternation with only one - alternative. The effect of (*THEN) extends beyond such a subpattern to - the enclosing alternative. Consider this pattern, where A, B, etc. are - complex pattern fragments that do not contain any | characters at this - level: - - A (B(*THEN)C) | D - - If A and B are matched, but there is a failure in C, matching does not - backtrack into A; instead it moves to the next alternative, that is, D. - However, if the subpattern containing (*THEN) is given an alternative, - it behaves differently: - - A (B(*THEN)C | (*FAIL)) | D - - The effect of (*THEN) is now confined to the inner subpattern. After a - failure in C, matching moves to (*FAIL), which causes the whole subpat- - tern to fail because there are no more alternatives to try. In this - case, matching does now backtrack into A. - - Note that a conditional subpattern is not considered as having two - alternatives, because only one is ever used. In other words, the | - character in a conditional subpattern has a different meaning. Ignoring - white space, consider: - - ^.*? (?(?=a) a | b(*THEN)c ) - - If the subject is "ba", this pattern does not match. Because .*? is - ungreedy, it initially matches zero characters. The condition (?=a) - then fails, the character "b" is matched, but "c" is not. At this - point, matching does not backtrack to .*? as might perhaps be expected - from the presence of the | character. The conditional subpattern is - part of the single alternative that comprises the whole pattern, and so - the match fails. (If there was a backtrack into .*?, allowing it to - match "b", the match would succeed.) - - The verbs just described provide four different "strengths" of control - when subsequent matching fails. (*THEN) is the weakest, carrying on the - match at the next alternative. (*PRUNE) comes next, failing the match - at the current starting position, but allowing an advance to the next - character (for an unanchored pattern). (*SKIP) is similar, except that - the advance may be more than one character. (*COMMIT) is the strongest, - causing the entire match to fail. - - More than one backtracking verb - - If more than one backtracking verb is present in a pattern, the one - that is backtracked onto first acts. For example, consider this pat- - tern, where A, B, etc. are complex pattern fragments: - - (A(*COMMIT)B(*THEN)C|ABD) - - If A matches but B fails, the backtrack to (*COMMIT) causes the entire - match to fail. However, if A and B match, but C fails, the backtrack to - (*THEN) causes the next alternative (ABD) to be tried. This behaviour - is consistent, but is not always the same as Perl's. It means that if - two or more backtracking verbs appear in succession, all the the last - of them has no effect. Consider this example: - - ...(*COMMIT)(*PRUNE)... - - If there is a matching failure to the right, backtracking onto (*PRUNE) - causes it to be triggered, and its action is taken. There can never be - a backtrack onto (*COMMIT). - - Backtracking verbs in repeated groups - - PCRE differs from Perl in its handling of backtracking verbs in - repeated groups. For example, consider: - - /(a(*COMMIT)b)+ac/ - - If the subject is "abac", Perl matches, but PCRE fails because the - (*COMMIT) in the second repeat of the group acts. - - Backtracking verbs in assertions - - (*FAIL) in an assertion has its normal effect: it forces an immediate - backtrack. - - (*ACCEPT) in a positive assertion causes the assertion to succeed with- - out any further processing. In a negative assertion, (*ACCEPT) causes - the assertion to fail without any further processing. - - The other backtracking verbs are not treated specially if they appear - in a positive assertion. In particular, (*THEN) skips to the next - alternative in the innermost enclosing group that has alternations, - whether or not this is within the assertion. - - Negative assertions are, however, different, in order to ensure that - changing a positive assertion into a negative assertion changes its - result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg- - ative assertion to be true, without considering any further alternative - branches in the assertion. Backtracking into (*THEN) causes it to skip - to the next enclosing alternative within the assertion (the normal be- - haviour), but if the assertion does not have such an alternative, - (*THEN) behaves like (*PRUNE). - - Backtracking verbs in subroutines - - These behaviours occur whether or not the subpattern is called recur- - sively. Perl's treatment of subroutines is different in some cases. - - (*FAIL) in a subpattern called as a subroutine has its normal effect: - it forces an immediate backtrack. - - (*ACCEPT) in a subpattern called as a subroutine causes the subroutine - match to succeed without any further processing. Matching then contin- - ues after the subroutine call. - - (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine - cause the subroutine match to fail. - - (*THEN) skips to the next alternative in the innermost enclosing group - within the subpattern that has alternatives. If there is no such group - within the subpattern, (*THEN) causes the subroutine match to fail. - - -SEE ALSO - - pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3), - pcre16(3), pcre32(3). - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 08 January 2014 - Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRESYNTAX(3) Library Functions Manual PCRESYNTAX(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE REGULAR EXPRESSION SYNTAX SUMMARY - - The full syntax and semantics of the regular expressions that are sup- - ported by PCRE are described in the pcrepattern documentation. This - document contains a quick-reference summary of the syntax. - - -QUOTING - - \x where x is non-alphanumeric is a literal x - \Q...\E treat enclosed characters as literal - - -CHARACTERS - - \a alarm, that is, the BEL character (hex 07) - \cx "control-x", where x is any ASCII character - \e escape (hex 1B) - \f form feed (hex 0C) - \n newline (hex 0A) - \r carriage return (hex 0D) - \t tab (hex 09) - \0dd character with octal code 0dd - \ddd character with octal code ddd, or backreference - \o{ddd..} character with octal code ddd.. - \xhh character with hex code hh - \x{hhh..} character with hex code hhh.. - - Note that \0dd is always an octal code, and that \8 and \9 are the lit- - eral characters "8" and "9". - - -CHARACTER TYPES - - . any character except newline; - in dotall mode, any character whatsoever - \C one data unit, even in UTF mode (best avoided) - \d a decimal digit - \D a character that is not a decimal digit - \h a horizontal white space character - \H a character that is not a horizontal white space character - \N a character that is not a newline - \p{xx} a character with the xx property - \P{xx} a character without the xx property - \R a newline sequence - \s a white space character - \S a character that is not a white space character - \v a vertical white space character - \V a character that is not a vertical white space character - \w a "word" character - \W a "non-word" character - \X a Unicode extended grapheme cluster - - By default, \d, \s, and \w match only ASCII characters, even in UTF-8 - mode or in the 16- bit and 32-bit libraries. However, if locale-spe- - cific matching is happening, \s and \w may also match characters with - code points in the range 128-255. If the PCRE_UCP option is set, the - behaviour of these escape sequences is changed to use Unicode proper- - ties and they match many more characters. - - -GENERAL CATEGORY PROPERTIES FOR \p and \P - - C Other - Cc Control - Cf Format - Cn Unassigned - Co Private use - Cs Surrogate - - L Letter - Ll Lower case letter - Lm Modifier letter - Lo Other letter - Lt Title case letter - Lu Upper case letter - L& Ll, Lu, or Lt - - M Mark - Mc Spacing mark - Me Enclosing mark - Mn Non-spacing mark - - N Number - Nd Decimal number - Nl Letter number - No Other number - - P Punctuation - Pc Connector punctuation - Pd Dash punctuation - Pe Close punctuation - Pf Final punctuation - Pi Initial punctuation - Po Other punctuation - Ps Open punctuation - - S Symbol - Sc Currency symbol - Sk Modifier symbol - Sm Mathematical symbol - So Other symbol - - Z Separator - Zl Line separator - Zp Paragraph separator - Zs Space separator - - -PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P - - Xan Alphanumeric: union of properties L and N - Xps POSIX space: property Z or tab, NL, VT, FF, CR - Xsp Perl space: property Z or tab, NL, VT, FF, CR - Xuc Univerally-named character: one that can be - represented by a Universal Character Name - Xwd Perl word: property Xan or underscore - - Perl and POSIX space are now the same. Perl added VT to its space char- - acter set at release 5.18 and PCRE changed at release 8.34. - - -SCRIPT NAMES FOR \p AND \P - - Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo, - Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma, - Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, - Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, - Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira- - gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- - tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, - Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, - Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive, - Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko, - Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, - Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari- - tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese, - Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, - Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, - Yi. - - -CHARACTER CLASSES - - [...] positive character class - [^...] negative character class - [x-y] range (can be used for hex characters) - [[:xxx:]] positive POSIX named set - [[:^xxx:]] negative POSIX named set - - alnum alphanumeric - alpha alphabetic - ascii 0-127 - blank space or tab - cntrl control character - digit decimal digit - graph printing, excluding space - lower lower case letter - print printing, including space - punct printing, excluding alphanumeric - space white space - upper upper case letter - word same as \w - xdigit hexadecimal digit - - In PCRE, POSIX character set names recognize only ASCII characters by - default, but some of them use Unicode properties if PCRE_UCP is set. - You can use \Q...\E inside a character class. - - -QUANTIFIERS - - ? 0 or 1, greedy - ?+ 0 or 1, possessive - ?? 0 or 1, lazy - * 0 or more, greedy - *+ 0 or more, possessive - *? 0 or more, lazy - + 1 or more, greedy - ++ 1 or more, possessive - +? 1 or more, lazy - {n} exactly n - {n,m} at least n, no more than m, greedy - {n,m}+ at least n, no more than m, possessive - {n,m}? at least n, no more than m, lazy - {n,} n or more, greedy - {n,}+ n or more, possessive - {n,}? n or more, lazy - - -ANCHORS AND SIMPLE ASSERTIONS - - \b word boundary - \B not a word boundary - ^ start of subject - also after internal newline in multiline mode - \A start of subject - $ end of subject - also before newline at end of subject - also before internal newline in multiline mode - \Z end of subject - also before newline at end of subject - \z end of subject - \G first matching position in subject - - -MATCH POINT RESET - - \K reset start of match - - \K is honoured in positive assertions, but ignored in negative ones. - - -ALTERNATION - - expr|expr|expr... - - -CAPTURING - - (...) capturing group - (?<name>...) named capturing group (Perl) - (?'name'...) named capturing group (Perl) - (?P<name>...) named capturing group (Python) - (?:...) non-capturing group - (?|...) non-capturing group; reset group numbers for - capturing groups in each alternative - - -ATOMIC GROUPS - - (?>...) atomic, non-capturing group - - -COMMENT - - (?#....) comment (not nestable) - - -OPTION SETTING - - (?i) caseless - (?J) allow duplicate names - (?m) multiline - (?s) single line (dotall) - (?U) default ungreedy (lazy) - (?x) extended (ignore white space) - (?-...) unset option(s) - - The following are recognized only at the very start of a pattern or - after one of the newline or \R options with similar syntax. More than - one of them may appear. - - (*LIMIT_MATCH=d) set the match limit to d (decimal number) - (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number) - (*NO_AUTO_POSSESS) no auto-possessification (PCRE_NO_AUTO_POSSESS) - (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) - (*UTF8) set UTF-8 mode: 8-bit library (PCRE_UTF8) - (*UTF16) set UTF-16 mode: 16-bit library (PCRE_UTF16) - (*UTF32) set UTF-32 mode: 32-bit library (PCRE_UTF32) - (*UTF) set appropriate UTF mode for the library in use - (*UCP) set PCRE_UCP (use Unicode properties for \d etc) - - Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of - the limits set by the caller of pcre_exec(), not increase them. - - -NEWLINE CONVENTION - - These are recognized only at the very start of the pattern or after - option settings with a similar syntax. - - (*CR) carriage return only - (*LF) linefeed only - (*CRLF) carriage return followed by linefeed - (*ANYCRLF) all three of the above - (*ANY) any Unicode newline sequence - - -WHAT \R MATCHES - - These are recognized only at the very start of the pattern or after - option setting with a similar syntax. - - (*BSR_ANYCRLF) CR, LF, or CRLF - (*BSR_UNICODE) any Unicode newline sequence - - -LOOKAHEAD AND LOOKBEHIND ASSERTIONS - - (?=...) positive look ahead - (?!...) negative look ahead - (?<=...) positive look behind - (?<!...) negative look behind - - Each top-level branch of a look behind must be of a fixed length. - - -BACKREFERENCES - - \n reference by number (can be ambiguous) - \gn reference by number - \g{n} reference by number - \g{-n} relative reference by number - \k<name> reference by name (Perl) - \k'name' reference by name (Perl) - \g{name} reference by name (Perl) - \k{name} reference by name (.NET) - (?P=name) reference by name (Python) - - -SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) - - (?R) recurse whole pattern - (?n) call subpattern by absolute number - (?+n) call subpattern by relative number - (?-n) call subpattern by relative number - (?&name) call subpattern by name (Perl) - (?P>name) call subpattern by name (Python) - \g<name> call subpattern by name (Oniguruma) - \g'name' call subpattern by name (Oniguruma) - \g<n> call subpattern by absolute number (Oniguruma) - \g'n' call subpattern by absolute number (Oniguruma) - \g<+n> call subpattern by relative number (PCRE extension) - \g'+n' call subpattern by relative number (PCRE extension) - \g<-n> call subpattern by relative number (PCRE extension) - \g'-n' call subpattern by relative number (PCRE extension) - - -CONDITIONAL PATTERNS - - (?(condition)yes-pattern) - (?(condition)yes-pattern|no-pattern) - - (?(n)... absolute reference condition - (?(+n)... relative reference condition - (?(-n)... relative reference condition - (?(<name>)... named reference condition (Perl) - (?('name')... named reference condition (Perl) - (?(name)... named reference condition (PCRE) - (?(R)... overall recursion condition - (?(Rn)... specific group recursion condition - (?(R&name)... specific recursion condition - (?(DEFINE)... define subpattern for reference - (?(assert)... assertion condition - - -BACKTRACKING CONTROL - - The following act immediately they are reached: - - (*ACCEPT) force successful match - (*FAIL) force backtrack; synonym (*F) - (*MARK:NAME) set name to be passed back; synonym (*:NAME) - - The following act only when a subsequent match failure causes a back- - track to reach them. They all force a match failure, but they differ in - what happens afterwards. Those that advance the start-of-match point do - so only if the pattern is not anchored. - - (*COMMIT) overall failure, no advance of starting point - (*PRUNE) advance to next starting character - (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE) - (*SKIP) advance to current matching position - (*SKIP:NAME) advance to position corresponding to an earlier - (*MARK:NAME); if not found, the (*SKIP) is ignored - (*THEN) local failure, backtrack to next alternation - (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN) - - -CALLOUTS - - (?C) callout - (?Cn) callout with data n - - -SEE ALSO - - pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 08 January 2014 - Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREUNICODE(3) Library Functions Manual PCREUNICODE(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT - - As well as UTF-8 support, PCRE also supports UTF-16 (from release 8.30) - and UTF-32 (from release 8.32), by means of two additional libraries. - They can be built as well as, or instead of, the 8-bit library. - - -UTF-8 SUPPORT - - In order process UTF-8 strings, you must build PCRE's 8-bit library - with UTF support, and, in addition, you must call pcre_compile() with - the PCRE_UTF8 option flag, or the pattern must start with the sequence - (*UTF8) or (*UTF). When either of these is the case, both the pattern - and any subject strings that are matched against it are treated as - UTF-8 strings instead of strings of individual 1-byte characters. - - -UTF-16 AND UTF-32 SUPPORT - - In order process UTF-16 or UTF-32 strings, you must build PCRE's 16-bit - or 32-bit library with UTF support, and, in addition, you must call - pcre16_compile() or pcre32_compile() with the PCRE_UTF16 or PCRE_UTF32 - option flag, as appropriate. Alternatively, the pattern must start with - the sequence (*UTF16), (*UTF32), as appropriate, or (*UTF), which can - be used with either library. When UTF mode is set, both the pattern and - any subject strings that are matched against it are treated as UTF-16 - or UTF-32 strings instead of strings of individual 16-bit or 32-bit - characters. - - -UTF SUPPORT OVERHEAD - - If you compile PCRE with UTF support, but do not use it at run time, - the library will be a bit bigger, but the additional run time overhead - is limited to testing the PCRE_UTF[8|16|32] flag occasionally, so - should not be very big. - - -UNICODE PROPERTY SUPPORT - - If PCRE is built with Unicode character property support (which implies - UTF support), the escape sequences \p{..}, \P{..}, and \X can be used. - The available properties that can be tested are limited to the general - category properties such as Lu for an upper case letter or Nd for a - decimal number, the Unicode script names such as Arabic or Han, and the - derived properties Any and L&. Full lists is given in the pcrepattern - and pcresyntax documentation. Only the short names for properties are - supported. For example, \p{L} matches a letter. Its Perl synonym, - \p{Letter}, is not supported. Furthermore, in Perl, many properties - may optionally be prefixed by "Is", for compatibility with Perl 5.6. - PCRE does not support this. - - Validity of UTF-8 strings - - When you set the PCRE_UTF8 flag, the byte strings passed as patterns - and subjects are (by default) checked for validity on entry to the rel- - evant functions. The entire string is checked before any other process- - ing takes place. From release 7.3 of PCRE, the check is according the - rules of RFC 3629, which are themselves derived from the Unicode speci- - fication. Earlier releases of PCRE followed the rules of RFC 2279, - which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The - current check allows only values in the range U+0 to U+10FFFF, exclud- - ing the surrogate area. (From release 8.33 the so-called "non-charac- - ter" code points are no longer excluded because Unicode corrigendum #9 - makes it clear that they should not be.) - - Characters in the "Surrogate Area" of Unicode are reserved for use by - UTF-16, where they are used in pairs to encode codepoints with values - greater than 0xFFFF. The code points that are encoded by UTF-16 pairs - are available independently in the UTF-8 and UTF-32 encodings. (In - other words, the whole surrogate thing is a fudge for UTF-16 which - unfortunately messes up UTF-8 and UTF-32.) - - If an invalid UTF-8 string is passed to PCRE, an error return is given. - At compile time, the only additional information is the offset to the - first byte of the failing character. The run-time functions pcre_exec() - and pcre_dfa_exec() also pass back this information, as well as a more - detailed reason code if the caller has provided memory in which to do - this. - - In some situations, you may already know that your strings are valid, - and therefore want to skip these checks in order to improve perfor- - mance, for example in the case of a long subject string that is being - scanned repeatedly. If you set the PCRE_NO_UTF8_CHECK flag at compile - time or at run time, PCRE assumes that the pattern or subject it is - given (respectively) contains only valid UTF-8 codes. In this case, it - does not diagnose an invalid UTF-8 string. - - Note that passing PCRE_NO_UTF8_CHECK to pcre_compile() just disables - the check for the pattern; it does not also apply to subject strings. - If you want to disable the check for a subject string you must pass - this option to pcre_exec() or pcre_dfa_exec(). - - If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, the - result is undefined and your program may crash. - - Validity of UTF-16 strings - - When you set the PCRE_UTF16 flag, the strings of 16-bit data units that - are passed as patterns and subjects are (by default) checked for valid- - ity on entry to the relevant functions. Values other than those in the - surrogate range U+D800 to U+DFFF are independent code points. Values in - the surrogate range must be used in pairs in the correct manner. - - If an invalid UTF-16 string is passed to PCRE, an error return is - given. At compile time, the only additional information is the offset - to the first data unit of the failing character. The run-time functions - pcre16_exec() and pcre16_dfa_exec() also pass back this information, as - well as a more detailed reason code if the caller has provided memory - in which to do this. - - In some situations, you may already know that your strings are valid, - and therefore want to skip these checks in order to improve perfor- - mance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at - run time, PCRE assumes that the pattern or subject it is given (respec- - tively) contains only valid UTF-16 sequences. In this case, it does not - diagnose an invalid UTF-16 string. However, if an invalid string is - passed, the result is undefined. - - Validity of UTF-32 strings - - When you set the PCRE_UTF32 flag, the strings of 32-bit data units that - are passed as patterns and subjects are (by default) checked for valid- - ity on entry to the relevant functions. This check allows only values - in the range U+0 to U+10FFFF, excluding the surrogate area U+D800 to - U+DFFF. - - If an invalid UTF-32 string is passed to PCRE, an error return is - given. At compile time, the only additional information is the offset - to the first data unit of the failing character. The run-time functions - pcre32_exec() and pcre32_dfa_exec() also pass back this information, as - well as a more detailed reason code if the caller has provided memory - in which to do this. - - In some situations, you may already know that your strings are valid, - and therefore want to skip these checks in order to improve perfor- - mance. If you set the PCRE_NO_UTF32_CHECK flag at compile time or at - run time, PCRE assumes that the pattern or subject it is given (respec- - tively) contains only valid UTF-32 sequences. In this case, it does not - diagnose an invalid UTF-32 string. However, if an invalid string is - passed, the result is undefined. - - General comments about UTF modes - - 1. Codepoints less than 256 can be specified in patterns by either - braced or unbraced hexadecimal escape sequences (for example, \x{b3} or - \xb3). Larger values have to use braced sequences. - - 2. Octal numbers up to \777 are recognized, and in UTF-8 mode they - match two-byte characters for values greater than \177. - - 3. Repeat quantifiers apply to complete UTF characters, not to individ- - ual data units, for example: \x{100}{3}. - - 4. The dot metacharacter matches one UTF character instead of a single - data unit. - - 5. The escape sequence \C can be used to match a single byte in UTF-8 - mode, or a single 16-bit data unit in UTF-16 mode, or a single 32-bit - data unit in UTF-32 mode, but its use can lead to some strange effects - because it breaks up multi-unit characters (see the description of \C - in the pcrepattern documentation). The use of \C is not supported in - the alternative matching function pcre[16|32]_dfa_exec(), nor is it - supported in UTF mode by the JIT optimization of pcre[16|32]_exec(). If - JIT optimization is requested for a UTF pattern that contains \C, it - will not succeed, and so the matching will be carried out by the normal - interpretive function. - - 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly - test characters of any code value, but, by default, the characters that - PCRE recognizes as digits, spaces, or word characters remain the same - set as in non-UTF mode, all with values less than 256. This remains - true even when PCRE is built to include Unicode property support, - because to do otherwise would slow down PCRE in many common cases. Note - in particular that this applies to \b and \B, because they are defined - in terms of \w and \W. If you really want to test for a wider sense of, - say, "digit", you can use explicit Unicode property tests such as - \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the - character escapes work is changed so that Unicode properties are used - to determine which characters match. There are more details in the sec- - tion on generic character types in the pcrepattern documentation. - - 7. Similarly, characters that match the POSIX named character classes - are all low-valued characters, unless the PCRE_UCP option is set. - - 8. However, the horizontal and vertical white space matching escapes - (\h, \H, \v, and \V) do match all the appropriate Unicode characters, - whether or not PCRE_UCP is set. - - 9. Case-insensitive matching applies only to characters whose values - are less than 128, unless PCRE is built with Unicode property support. - A few Unicode characters such as Greek sigma have more than two code- - points that are case-equivalent. Up to and including PCRE release 8.31, - only one-to-one case mappings were supported, but later releases (with - Unicode property support) do treat as case-equivalent all versions of - characters such as Greek sigma. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 27 February 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREJIT(3) Library Functions Manual PCREJIT(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE JUST-IN-TIME COMPILER SUPPORT - - Just-in-time compiling is a heavyweight optimization that can greatly - speed up pattern matching. However, it comes at the cost of extra pro- - cessing before the match is performed. Therefore, it is of most benefit - when the same pattern is going to be matched many times. This does not - necessarily mean many calls of a matching function; if the pattern is - not anchored, matching attempts may take place many times at various - positions in the subject, even for a single call. Therefore, if the - subject string is very long, it may still pay to use JIT for one-off - matches. - - JIT support applies only to the traditional Perl-compatible matching - function. It does not apply when the DFA matching function is being - used. The code for this support was written by Zoltan Herczeg. - - -8-BIT, 16-BIT AND 32-BIT SUPPORT - - JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE - libraries. To keep this documentation simple, only the 8-bit interface - is described in what follows. If you are using the 16-bit library, sub- - stitute the 16-bit functions and 16-bit structures (for example, - pcre16_jit_stack instead of pcre_jit_stack). If you are using the - 32-bit library, substitute the 32-bit functions and 32-bit structures - (for example, pcre32_jit_stack instead of pcre_jit_stack). - - -AVAILABILITY OF JIT SUPPORT - - JIT support is an optional feature of PCRE. The "configure" option - --enable-jit (or equivalent CMake option) must be set when PCRE is - built if you want to use JIT. The support is limited to the following - hardware platforms: - - ARM v5, v7, and Thumb2 - Intel x86 32-bit and 64-bit - MIPS 32-bit - Power PC 32-bit and 64-bit - SPARC 32-bit (experimental) - - If --enable-jit is set on an unsupported platform, compilation fails. - - A program that is linked with PCRE 8.20 or later can tell if JIT sup- - port is available by calling pcre_config() with the PCRE_CONFIG_JIT - option. The result is 1 when JIT is available, and 0 otherwise. How- - ever, a simple program does not need to check this in order to use JIT. - The normal API is implemented in a way that falls back to the interpre- - tive code if JIT is not available. For programs that need the best pos- - sible performance, there is also a "fast path" API that is JIT-spe- - cific. - - If your program may sometimes be linked with versions of PCRE that are - older than 8.20, but you want to use JIT when it is available, you can - test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT - macro such as PCRE_CONFIG_JIT, for compile-time control of your code. - - -SIMPLE USE OF JIT - - You have to do two things to make use of the JIT support in the sim- - plest way: - - (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for - each compiled pattern, and pass the resulting pcre_extra block to - pcre_exec(). - - (2) Use pcre_free_study() to free the pcre_extra block when it is - no longer needed, instead of just freeing it yourself. This - ensures that - any JIT data is also freed. - - For a program that may be linked with pre-8.20 versions of PCRE, you - can insert - - #ifndef PCRE_STUDY_JIT_COMPILE - #define PCRE_STUDY_JIT_COMPILE 0 - #endif - - so that no option is passed to pcre_study(), and then use something - like this to free the study data: - - #ifdef PCRE_CONFIG_JIT - pcre_free_study(study_ptr); - #else - pcre_free(study_ptr); - #endif - - PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for - complete matches. If you want to run partial matches using the - PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you - should set one or both of the following options in addition to, or - instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study(): - - PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE - PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE - - The JIT compiler generates different optimized code for each of the - three modes (normal, soft partial, hard partial). When pcre_exec() is - called, the appropriate code is run if it is available. Otherwise, the - pattern is matched using interpretive code. - - In some circumstances you may need to call additional functions. These - are described in the section entitled "Controlling the JIT stack" - below. - - If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are - ignored, and no JIT data is created. Otherwise, the compiled pattern is - passed to the JIT compiler, which turns it into machine code that exe- - cutes much faster than the normal interpretive code. When pcre_exec() - is passed a pcre_extra block containing a pointer to JIT code of the - appropriate mode (normal or hard/soft partial), it obeys that code - instead of running the interpreter. The result is identical, but the - compiled JIT code runs much faster. - - There are some pcre_exec() options that are not supported for JIT exe- - cution. There are also some pattern items that JIT cannot handle. - Details are given below. In both cases, execution automatically falls - back to the interpretive code. If you want to know whether JIT was - actually used for a particular match, you should arrange for a JIT - callback function to be set up as described in the section entitled - "Controlling the JIT stack" below, even if you do not need to supply a - non-default JIT stack. Such a callback function is called whenever JIT - code is about to be obeyed. If the execution options are not right for - JIT execution, the callback function is not obeyed. - - If the JIT compiler finds an unsupported item, no JIT data is gener- - ated. You can find out if JIT execution is available after studying a - pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A - result of 1 means that JIT compilation was successful. A result of 0 - means that JIT support is not available, or the pattern was not studied - with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to - handle the pattern. - - Once a pattern has been studied, with or without JIT, it can be used as - many times as you like for matching different subject strings. - - -UNSUPPORTED OPTIONS AND PATTERN ITEMS - - The only pcre_exec() options that are supported for JIT execution are - PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOT- - BOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PAR- - TIAL_HARD, and PCRE_PARTIAL_SOFT. - - The only unsupported pattern items are \C (match a single data unit) - when running in a UTF mode, and a callout immediately before an asser- - tion condition in a conditional group. - - -RETURN VALUES FROM JIT EXECUTION - - When a pattern is matched using JIT execution, the return values are - the same as those given by the interpretive pcre_exec() code, with the - addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means - that the memory used for the JIT stack was insufficient. See "Control- - ling the JIT stack" below for a discussion of JIT stack usage. For com- - patibility with the interpretive pcre_exec() code, no more than two- - thirds of the ovector argument is used for passing back captured sub- - strings. - - The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if - searching a very large pattern tree goes on for too long, as it is in - the same circumstance when JIT is not used, but the details of exactly - what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error - code is never returned by JIT execution. - - -SAVING AND RESTORING COMPILED PATTERNS - - The code that is generated by the JIT compiler is architecture-spe- - cific, and is also position dependent. For those reasons it cannot be - saved (in a file or database) and restored later like the bytecode and - other data of a compiled pattern. Saving and restoring compiled pat- - terns is not something many people do. More detail about this facility - is given in the pcreprecompile documentation. It should be possible to - run pcre_study() on a saved and restored pattern, and thereby recreate - the JIT data, but because JIT compilation uses significant resources, - it is probably not worth doing this; you might as well recompile the - original pattern. - - -CONTROLLING THE JIT STACK - - When the compiled JIT code runs, it needs a block of memory to use as a - stack. By default, it uses 32K on the machine stack. However, some - large or complicated patterns need more than this. The error - PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack. - Three functions are provided for managing blocks of memory for use as - JIT stacks. There is further discussion about the use of JIT stacks in - the section entitled "JIT stack FAQ" below. - - The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments - are a starting size and a maximum size, and it returns a pointer to an - opaque structure of type pcre_jit_stack, or NULL if there is an error. - The pcre_jit_stack_free() function can be used to free a stack that is - no longer needed. (For the technically minded: the address space is - allocated by mmap or VirtualAlloc.) - - JIT uses far less memory for recursion than the interpretive code, and - a maximum stack size of 512K to 1M should be more than enough for any - pattern. - - The pcre_assign_jit_stack() function specifies which stack JIT code - should use. Its arguments are as follows: - - pcre_extra *extra - pcre_jit_callback callback - void *data - - The extra argument must be the result of studying a pattern with - PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the - other two options: - - (1) If callback is NULL and data is NULL, an internal 32K block - on the machine stack is used. - - (2) If callback is NULL and data is not NULL, data must be - a valid JIT stack, the result of calling pcre_jit_stack_alloc(). - - (3) If callback is not NULL, it must point to a function that is - called with data as an argument at the start of matching, in - order to set up a JIT stack. If the return from the callback - function is NULL, the internal 32K stack is used; otherwise the - return value must be a valid JIT stack, the result of calling - pcre_jit_stack_alloc(). - - A callback function is obeyed whenever JIT code is about to be run; it - is not obeyed when pcre_exec() is called with options that are incom- - patible for JIT execution. A callback function can therefore be used to - determine whether a match operation was executed by JIT or by the - interpreter. - - You may safely use the same JIT stack for more than one pattern (either - by assigning directly or by callback), as long as the patterns are all - matched sequentially in the same thread. In a multithread application, - if you do not specify a JIT stack, or if you assign or pass back NULL - from a callback, that is thread-safe, because each thread has its own - machine stack. However, if you assign or pass back a non-NULL JIT - stack, this must be a different stack for each thread so that the - application is thread-safe. - - Strictly speaking, even more is allowed. You can assign the same non- - NULL stack to any number of patterns as long as they are not used for - matching by multiple threads at the same time. For example, you can - assign the same stack to all compiled patterns, and use a global mutex - in the callback to wait until the stack is available for use. However, - this is an inefficient solution, and not recommended. - - This is a suggestion for how a multithreaded program that needs to set - up non-default JIT stacks might operate: - - During thread initalization - thread_local_var = pcre_jit_stack_alloc(...) - - During thread exit - pcre_jit_stack_free(thread_local_var) - - Use a one-line callback function - return thread_local_var - - All the functions described in this section do nothing if JIT is not - available, and pcre_assign_jit_stack() does nothing unless the extra - argument is non-NULL and points to a pcre_extra block that is the - result of a successful study with PCRE_STUDY_JIT_COMPILE etc. - - -JIT STACK FAQ - - (1) Why do we need JIT stacks? - - PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack - where the local data of the current node is pushed before checking its - child nodes. Allocating real machine stack on some platforms is diffi- - cult. For example, the stack chain needs to be updated every time if we - extend the stack on PowerPC. Although it is possible, its updating - time overhead decreases performance. So we do the recursion in memory. - - (2) Why don't we simply allocate blocks of memory with malloc()? - - Modern operating systems have a nice feature: they can reserve an - address space instead of allocating memory. We can safely allocate mem- - ory pages inside this address space, so the stack could grow without - moving memory data (this is important because of pointers). Thus we can - allocate 1M address space, and use only a single memory page (usually - 4K) if that is enough. However, we can still grow up to 1M anytime if - needed. - - (3) Who "owns" a JIT stack? - - The owner of the stack is the user program, not the JIT studied pattern - or anything else. The user program must ensure that if a stack is used - by pcre_exec(), (that is, it is assigned to the pattern currently run- - ning), that stack must not be used by any other threads (to avoid over- - writing the same memory area). The best practice for multithreaded pro- - grams is to allocate a stack for each thread, and return this stack - through the JIT callback function. - - (4) When should a JIT stack be freed? - - You can free a JIT stack at any time, as long as it will not be used by - pcre_exec() again. When you assign the stack to a pattern, only a - pointer is set. There is no reference counting or any other magic. You - can free the patterns and stacks in any order, anytime. Just do not - call pcre_exec() with a pattern pointing to an already freed stack, as - that will cause SEGFAULT. (Also, do not free a stack currently used by - pcre_exec() in another thread). You can also replace the stack for a - pattern at any time. You can even free the previous stack before - assigning a replacement. - - (5) Should I allocate/free a stack every time before/after calling - pcre_exec()? - - No, because this is too costly in terms of resources. However, you - could implement some clever idea which release the stack if it is not - used in let's say two minutes. The JIT callback can help to achieve - this without keeping a list of the currently JIT studied patterns. - - (6) OK, the stack is for long term memory allocation. But what happens - if a pattern causes stack overflow with a stack of 1M? Is that 1M kept - until the stack is freed? - - Especially on embedded sytems, it might be a good idea to release mem- - ory sometimes without freeing the stack. There is no API for this at - the moment. Probably a function call which returns with the currently - allocated memory for any stack and another which allows releasing mem- - ory (shrinking the stack) would be a good idea if someone needs this. - - (7) This is too much of a headache. Isn't there any better solution for - JIT stack handling? - - No, thanks to Windows. If POSIX threads were used everywhere, we could - throw out this complicated API. - - -EXAMPLE CODE - - This is a single-threaded example that specifies a JIT stack without - using a callback. - - int rc; - int ovector[30]; - pcre *re; - pcre_extra *extra; - pcre_jit_stack *jit_stack; - - re = pcre_compile(pattern, 0, &error, &erroffset, NULL); - /* Check for errors */ - extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); - jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); - /* Check for error (NULL) */ - pcre_assign_jit_stack(extra, NULL, jit_stack); - rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); - /* Check results */ - pcre_free(re); - pcre_free_study(extra); - pcre_jit_stack_free(jit_stack); - - -JIT FAST PATH API - - Because the API described above falls back to interpreted execution - when JIT is not available, it is convenient for programs that are writ- - ten for general use in many environments. However, calling JIT via - pcre_exec() does have a performance impact. Programs that are written - for use where JIT is known to be available, and which need the best - possible performance, can instead use a "fast path" API to call JIT - execution directly instead of calling pcre_exec() (obviously only for - patterns that have been successfully studied by JIT). - - The fast path function is called pcre_jit_exec(), and it takes exactly - the same arguments as pcre_exec(), plus one additional argument that - must point to a JIT stack. The JIT stack arrangements described above - do not apply. The return values are the same as for pcre_exec(). - - When you call pcre_exec(), as well as testing for invalid options, a - number of other sanity checks are performed on the arguments. For exam- - ple, if the subject pointer is NULL, or its length is negative, an - immediate error is given. Also, unless PCRE_NO_UTF[8|16|32] is set, a - UTF subject string is tested for validity. In the interests of speed, - these checks do not happen on the JIT fast path, and if invalid data is - passed, the result is undefined. - - Bypassing the sanity checks and the pcre_exec() wrapping can give - speedups of more than 10%. - - -SEE ALSO - - pcreapi(3) - - -AUTHOR - - Philip Hazel (FAQ by Zoltan Herczeg) - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 17 March 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREPARTIAL(3) Library Functions Manual PCREPARTIAL(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PARTIAL MATCHING IN PCRE - - In normal use of PCRE, if the subject string that is passed to a match- - ing function matches as far as it goes, but is too short to match the - entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances - where it might be helpful to distinguish this case from other cases in - which there is no match. - - Consider, for example, an application where a human is required to type - in data for a field with specific formatting requirements. An example - might be a date in the form ddmmmyy, defined by this pattern: - - ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ - - If the application sees the user's keystrokes one by one, and can check - that what has been typed so far is potentially valid, it is able to - raise an error as soon as a mistake is made, by beeping and not - reflecting the character that has been typed, for example. This immedi- - ate feedback is likely to be a better user interface than a check that - is delayed until the entire string has been entered. Partial matching - can also be useful when the subject string is very long and is not all - available at once. - - PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and - PCRE_PARTIAL_HARD options, which can be set when calling any of the - matching functions. For backwards compatibility, PCRE_PARTIAL is a syn- - onym for PCRE_PARTIAL_SOFT. The essential difference between the two - options is whether or not a partial match is preferred to an alterna- - tive complete match, though the details differ between the two types of - matching function. If both options are set, PCRE_PARTIAL_HARD takes - precedence. - - If you want to use partial matching with just-in-time optimized code, - you must call pcre_study(), pcre16_study() or pcre32_study() with one - or both of these options: - - PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE - PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE - - PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non- - partial matches on the same pattern. If the appropriate JIT study mode - has not been set for a match, the interpretive matching code is used. - - Setting a partial matching option disables two of PCRE's standard opti- - mizations. PCRE remembers the last literal data unit in a pattern, and - abandons matching immediately if it is not present in the subject - string. This optimization cannot be used for a subject string that - might match only partially. If the pattern was studied, PCRE knows the - minimum length of a matching string, and does not bother to run the - matching function on shorter strings. This optimization is also dis- - abled for partial matching. - - -PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec() - - A partial match occurs during a call to pcre_exec() or - pcre[16|32]_exec() when the end of the subject string is reached suc- - cessfully, but matching cannot continue because more characters are - needed. However, at least one character in the subject must have been - inspected. This character need not form part of the final matched - string; lookbehind assertions and the \K escape sequence provide ways - of inspecting characters before the start of a matched substring. The - requirement for inspecting at least one character exists because an - empty string can always be matched; without such a restriction there - would always be a partial match of an empty string at the end of the - subject. - - If there are at least two slots in the offsets vector when a partial - match is returned, the first slot is set to the offset of the earliest - character that was inspected. For convenience, the second offset points - to the end of the subject so that a substring can easily be identified. - If there are at least three slots in the offsets vector, the third slot - is set to the offset of the character where matching started. - - For the majority of patterns, the contents of the first and third slots - will be the same. However, for patterns that contain lookbehind asser- - tions, or begin with \b or \B, characters before the one where matching - started may have been inspected while carrying out the match. For exam- - ple, consider this pattern: - - /(?<=abc)123/ - - This pattern matches "123", but only if it is preceded by "abc". If the - subject string is "xyzabc12", the first two offsets after a partial - match are for the substring "abc12", because all these characters were - inspected. However, the third offset is set to 6, because that is the - offset where matching began. - - What happens when a partial match is identified depends on which of the - two partial matching options are set. - - PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec() - - If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre[16|32]_exec() - identifies a partial match, the partial match is remembered, but match- - ing continues as normal, and other alternatives in the pattern are - tried. If no complete match can be found, PCRE_ERROR_PARTIAL is - returned instead of PCRE_ERROR_NOMATCH. - - This option is "soft" because it prefers a complete match over a par- - tial match. All the various matching items in a pattern behave as if - the subject string is potentially complete. For example, \z, \Z, and $ - match at the end of the subject, as normal, and for \b and \B the end - of the subject is treated as a non-alphanumeric. - - If there is more than one partial match, the first one that was found - provides the data that is returned. Consider this pattern: - - /123\w+X|dogY/ - - If this is matched against the subject string "abc123dog", both alter- - natives fail to match, but the end of the subject is reached during - matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 - and 9, identifying "123dog" as the first partial match that was found. - (In this example, there are two partial matches, because "dog" on its - own partially matches the second alternative.) - - PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec() - - If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre[16|32]_exec(), - PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, - without continuing to search for possible complete matches. This option - is "hard" because it prefers an earlier partial match over a later com- - plete match. For this reason, the assumption is made that the end of - the supplied subject string may not be the true end of the available - data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the - subject, the result is PCRE_ERROR_PARTIAL, provided that at least one - character in the subject has been inspected. - - Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject - strings are checked for validity. Normally, an invalid sequence causes - the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the - special case of a truncated character at the end of the subject, - PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when - PCRE_PARTIAL_HARD is set. - - Comparing hard and soft partial matching - - The difference between the two partial matching options can be illus- - trated by a pattern such as: - - /dog(sbody)?/ - - This matches either "dog" or "dogsbody", greedily (that is, it prefers - the longer string if possible). If it is matched against the string - "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". - However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. - On the other hand, if the pattern is made ungreedy the result is dif- - ferent: - - /dog(sbody)??/ - - In this case the result is always a complete match because that is - found first, and matching never continues after finding a complete - match. It might be easier to follow this explanation by thinking of the - two patterns like this: - - /dog(sbody)?/ is the same as /dogsbody|dog/ - /dog(sbody)??/ is the same as /dog|dogsbody/ - - The second pattern will never match "dogsbody", because it will always - find the shorter match first. - - -PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec() - - The DFA functions move along the subject string character by character, - without backtracking, searching for all possible matches simultane- - ously. If the end of the subject is reached before the end of the pat- - tern, there is the possibility of a partial match, again provided that - at least one character has been inspected. - - When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if - there have been no complete matches. Otherwise, the complete matches - are returned. However, if PCRE_PARTIAL_HARD is set, a partial match - takes precedence over any complete matches. The portion of the string - that was inspected when the longest partial match was found is set as - the first matching string, provided there are at least two slots in the - offsets vector. - - Because the DFA functions always search for all possible matches, and - there is no difference between greedy and ungreedy repetition, their - behaviour is different from the standard functions when PCRE_PAR- - TIAL_HARD is set. Consider the string "dog" matched against the - ungreedy pattern shown above: - - /dog(sbody)??/ - - Whereas the standard functions stop as soon as they find the complete - match for "dog", the DFA functions also find the partial match for - "dogsbody", and so return that when PCRE_PARTIAL_HARD is set. - - -PARTIAL MATCHING AND WORD BOUNDARIES - - If a pattern ends with one of sequences \b or \B, which test for word - boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- - intuitive results. Consider this pattern: - - /\bcat\b/ - - This matches "cat", provided there is a word boundary at either end. If - the subject string is "the cat", the comparison of the final "t" with a - following character cannot take place, so a partial match is found. - However, normal matching carries on, and \b matches at the end of the - subject when the last character is a letter, so a complete match is - found. The result, therefore, is not PCRE_ERROR_PARTIAL. Using - PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because - then the partial match takes precedence. - - -FORMERLY RESTRICTED PATTERNS - - For releases of PCRE prior to 8.00, because of the way certain internal - optimizations were implemented in the pcre_exec() function, the - PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be - used with all patterns. From release 8.00 onwards, the restrictions no - longer apply, and partial matching with can be requested for any pat- - tern. - - Items that were formerly restricted were repeated single characters and - repeated metasequences. If PCRE_PARTIAL was set for a pattern that did - not conform to the restrictions, pcre_exec() returned the error code - PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The - PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled - pattern can be used for partial matching now always returns 1. - - -EXAMPLE OF PARTIAL MATCHING USING PCRETEST - - If the escape sequence \P is present in a pcretest data line, the - PCRE_PARTIAL_SOFT option is used for the match. Here is a run of - pcretest that uses the date example quoted above: - - re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ - data> 25jun04\P - 0: 25jun04 - 1: jun - data> 25dec3\P - Partial match: 23dec3 - data> 3ju\P - Partial match: 3ju - data> 3juj\P - No match - data> j\P - No match - - The first data string is matched completely, so pcretest shows the - matched substrings. The remaining four strings do not match the com- - plete pattern, but the first two are partial matches. Similar output is - obtained if DFA matching is used. - - If the escape sequence \P is present more than once in a pcretest data - line, the PCRE_PARTIAL_HARD option is set for the match. - - -MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec() - - When a partial match has been found using a DFA matching function, it - is possible to continue the match by providing additional subject data - and calling the function again with the same compiled regular expres- - sion, this time setting the PCRE_DFA_RESTART option. You must pass the - same working space as before, because this is where details of the pre- - vious partial match are stored. Here is an example using pcretest, - using the \R escape sequence to set the PCRE_DFA_RESTART option (\D - specifies the use of the DFA matching function): - - re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ - data> 23ja\P\D - Partial match: 23ja - data> n05\R\D - 0: n05 - - The first call has "23ja" as the subject, and requests partial match- - ing; the second call has "n05" as the subject for the continued - (restarted) match. Notice that when the match is complete, only the - last part is shown; PCRE does not retain the previously partially- - matched string. It is up to the calling program to do that if it needs - to. - - That means that, for an unanchored pattern, if a continued match fails, - it is not possible to try again at a new starting point. All this - facility is capable of doing is continuing with the previous match - attempt. In the previous example, if the second set of data is "ug23" - the result is no match, even though there would be a match for "aug23" - if the entire string were given at once. Depending on the application, - this may or may not be what you want. The only way to allow for start- - ing again at the next character is to retain the matched part of the - subject and try a new complete match. - - You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with - PCRE_DFA_RESTART to continue partial matching over multiple segments. - This facility can be used to pass very long subject strings to the DFA - matching functions. - - -MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec() - - From release 8.00, the standard matching functions can also be used to - do multi-segment matching. Unlike the DFA functions, it is not possible - to restart the previous match with a new segment of data. Instead, new - data must be added to the previous subject string, and the entire match - re-run, starting from the point where the partial match occurred. Ear- - lier data can be discarded. - - It is best to use PCRE_PARTIAL_HARD in this situation, because it does - not treat the end of a segment as the end of the subject when matching - \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches - dates: - - re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ - data> The date is 23ja\P\P - Partial match: 23ja - - At this stage, an application could discard the text preceding "23ja", - add on text from the next segment, and call the matching function - again. Unlike the DFA matching functions, the entire matching string - must always be available, and the complete matching process occurs for - each call, so more memory and more processing time is needed. - - Note: If the pattern contains lookbehind assertions, or \K, or starts - with \b or \B, the string that is returned for a partial match includes - characters that precede the start of what would be returned for a com- - plete match, because it contains all the characters that were inspected - during the partial match. - - -ISSUES WITH MULTI-SEGMENT MATCHING - - Certain types of pattern may give problems with multi-segment matching, - whichever matching function is used. - - 1. If the pattern contains a test for the beginning of a line, you need - to pass the PCRE_NOTBOL option when the subject string for any call - does start at the beginning of a line. There is also a PCRE_NOTEOL - option, but in practice when doing multi-segment matching you should be - using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. - - 2. Lookbehind assertions that have already been obeyed are catered for - in the offsets that are returned for a partial match. However a lookbe- - hind assertion later in the pattern could require even earlier charac- - ters to be inspected. You can handle this case by using the - PCRE_INFO_MAXLOOKBEHIND option of the pcre_fullinfo() or - pcre[16|32]_fullinfo() functions to obtain the length of the longest - lookbehind in the pattern. This length is given in characters, not - bytes. If you always retain at least that many characters before the - partially matched string, all should be well. (Of course, near the - start of the subject, fewer characters may be present; in that case all - characters should be retained.) - - From release 8.33, there is a more accurate way of deciding which char- - acters to retain. Instead of subtracting the length of the longest - lookbehind from the earliest inspected character (offsets[0]), the - match start position (offsets[2]) should be used, and the next match - attempt started at the offsets[2] character by setting the startoffset - argument of pcre_exec() or pcre_dfa_exec(). - - For example, if the pattern "(?<=123)abc" is partially matched against - the string "xx123a", the three offset values returned are 2, 6, and 5. - This indicates that the matching process that gave a partial match - started at offset 5, but the characters "123a" were all inspected. The - maximum lookbehind for that pattern is 3, so taking that away from 5 - shows that we need only keep "123a", and the next match attempt can be - started at offset 3 (that is, at "a") when further characters have been - added. When the match start is not the earliest inspected character, - pcretest shows it explicitly: - - re> "(?<=123)abc" - data> xx123a\P\P - Partial match at offset 5: 123a - - 3. Because a partial match must always contain at least one character, - what might be considered a partial match of an empty string actually - gives a "no match" result. For example: - - re> /c(?<=abc)x/ - data> ab\P - No match - - If the next segment begins "cx", a match should be found, but this will - only happen if characters from the previous segment are retained. For - this reason, a "no match" result should be interpreted as "partial - match of an empty string" when the pattern contains lookbehinds. - - 4. Matching a subject string that is split into multiple segments may - not always produce exactly the same result as matching over one single - long string, especially when PCRE_PARTIAL_SOFT is used. The section - "Partial Matching and Word Boundaries" above describes an issue that - arises if the pattern ends with \b or \B. Another kind of difference - may occur when there are multiple matching possibilities, because (for - PCRE_PARTIAL_SOFT) a partial match result is given only when there are - no completed matches. This means that as soon as the shortest match has - been found, continuation to a new subject segment is no longer possi- - ble. Consider again this pcretest example: - - re> /dog(sbody)?/ - data> dogsb\P - 0: dog - data> do\P\D - Partial match: do - data> gsb\R\P\D - 0: g - data> dogsbody\D - 0: dogsbody - 1: dog - - The first data line passes the string "dogsb" to a standard matching - function, setting the PCRE_PARTIAL_SOFT option. Although the string is - a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, - because the shorter string "dog" is a complete match. Similarly, when - the subject is presented to a DFA matching function in several parts - ("do" and "gsb" being the first two) the match stops when "dog" has - been found, and it is not possible to continue. On the other hand, if - "dogsbody" is presented as a single string, a DFA matching function - finds both matches. - - Because of these problems, it is best to use PCRE_PARTIAL_HARD when - matching multi-segment data. The example above then behaves differ- - ently: - - re> /dog(sbody)?/ - data> dogsb\P\P - Partial match: dogsb - data> do\P\D - Partial match: do - data> gsb\R\P\P\D - Partial match: gsb - - 5. Patterns that contain alternatives at the top level which do not all - start with the same pattern item may not work as expected when - PCRE_DFA_RESTART is used. For example, consider this pattern: - - 1234|3789 - - If the first part of the subject is "ABC123", a partial match of the - first alternative is found at offset 3. There is no partial match for - the second alternative, because such a match does not start at the same - point in the subject string. Attempting to continue with the string - "7890" does not yield a match because only those alternatives that - match at one point in the subject are remembered. The problem arises - because the start of the second alternative matches within the first - alternative. There is no problem with anchored patterns or patterns - such as: - - 1234|ABCD - - where no string can be a partial match for both alternatives. This is - not a problem if a standard matching function is used, because the - entire match has to be rerun each time: - - re> /1234|3789/ - data> ABC123\P\P - Partial match: 123 - data> 1237890 - 0: 3789 - - Of course, instead of using PCRE_DFA_RESTART, the same technique of re- - running the entire match can also be used with the DFA matching func- - tions. Another possibility is to work with two buffers. If a partial - match at offset n in the first buffer is followed by "no match" when - PCRE_DFA_RESTART is used on the second buffer, you can then try a new - match starting at offset n+1 in the first buffer. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 02 July 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREPRECOMPILE(3) Library Functions Manual PCREPRECOMPILE(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -SAVING AND RE-USING PRECOMPILED PCRE PATTERNS - - If you are running an application that uses a large number of regular - expression patterns, it may be useful to store them in a precompiled - form instead of having to compile them every time the application is - run. If you are not using any private character tables (see the - pcre_maketables() documentation), this is relatively straightforward. - If you are using private tables, it is a little bit more complicated. - However, if you are using the just-in-time optimization feature, it is - not possible to save and reload the JIT data. - - If you save compiled patterns to a file, you can copy them to a differ- - ent host and run them there. If the two hosts have different endianness - (byte order), you should run the pcre[16|32]_pat- - tern_to_host_byte_order() function on the new host before trying to - match the pattern. The matching functions return PCRE_ERROR_BADENDIAN- - NESS if they detect a pattern with the wrong endianness. - - Compiling regular expressions with one version of PCRE for use with a - different version is not guaranteed to work and may cause crashes, and - saving and restoring a compiled pattern loses any JIT optimization - data. - - -SAVING A COMPILED PATTERN - - The value returned by pcre[16|32]_compile() points to a single block of - memory that holds the compiled pattern and associated data. You can - find the length of this block in bytes by calling - pcre[16|32]_fullinfo() with an argument of PCRE_INFO_SIZE. You can then - save the data in any appropriate manner. Here is sample code for the - 8-bit library that compiles a pattern and writes it to a file. It - assumes that the variable fd refers to a file that is open for output: - - int erroroffset, rc, size; - char *error; - pcre *re; - - re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); - if (re == NULL) { ... handle errors ... } - rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); - if (rc < 0) { ... handle errors ... } - rc = fwrite(re, 1, size, fd); - if (rc != size) { ... handle errors ... } - - In this example, the bytes that comprise the compiled pattern are - copied exactly. Note that this is binary data that may contain any of - the 256 possible byte values. On systems that make a distinction - between binary and non-binary data, be sure that the file is opened for - binary output. - - If you want to write more than one pattern to a file, you will have to - devise a way of separating them. For binary data, preceding each pat- - tern with its length is probably the most straightforward approach. - Another possibility is to write out the data in hexadecimal instead of - binary, one pattern to a line. - - Saving compiled patterns in a file is only one possible way of storing - them for later use. They could equally well be saved in a database, or - in the memory of some daemon process that passes them via sockets to - the processes that want them. - - If the pattern has been studied, it is also possible to save the normal - study data in a similar way to the compiled pattern itself. However, if - the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre- - ated cannot be saved because it is too dependent on the current envi- - ronment. When studying generates additional information, - pcre[16|32]_study() returns a pointer to a pcre[16|32]_extra data - block. Its format is defined in the section on matching a pattern in - the pcreapi documentation. The study_data field points to the binary - study data, and this is what you must save (not the pcre[16|32]_extra - block itself). The length of the study data can be obtained by calling - pcre[16|32]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remem- - ber to check that pcre[16|32]_study() did return a non-NULL value - before trying to save the study data. - - -RE-USING A PRECOMPILED PATTERN - - Re-using a precompiled pattern is straightforward. Having reloaded it - into main memory, called pcre[16|32]_pattern_to_host_byte_order() if - necessary, you pass its pointer to pcre[16|32]_exec() or - pcre[16|32]_dfa_exec() in the usual way. - - However, if you passed a pointer to custom character tables when the - pattern was compiled (the tableptr argument of pcre[16|32]_compile()), - you must now pass a similar pointer to pcre[16|32]_exec() or - pcre[16|32]_dfa_exec(), because the value saved with the compiled pat- - tern will obviously be nonsense. A field in a pcre[16|32]_extra() block - is used to pass this data, as described in the section on matching a - pattern in the pcreapi documentation. - - Warning: The tables that pcre_exec() and pcre_dfa_exec() use must be - the same as those that were used when the pattern was compiled. If this - is not the case, the behaviour is undefined. - - If you did not provide custom character tables when the pattern was - compiled, the pointer in the compiled pattern is NULL, which causes the - matching functions to use PCRE's internal tables. Thus, you do not need - to take any special action at run time in this case. - - If you saved study data with the compiled pattern, you need to create - your own pcre[16|32]_extra data block and set the study_data field to - point to the reloaded study data. You must also set the - PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study - data is present. Then pass the pcre[16|32]_extra block to the matching - function in the usual way. If the pattern was studied for just-in-time - optimization, that data cannot be saved, and so is lost by a - save/restore cycle. - - -COMPATIBILITY WITH DIFFERENT PCRE RELEASES - - In general, it is safest to recompile all saved patterns when you - update to a new PCRE release, though not all updates actually require - this. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 12 November 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREPERFORM(3) Library Functions Manual PCREPERFORM(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE PERFORMANCE - - Two aspects of performance are discussed below: memory usage and pro- - cessing time. The way you express your pattern as a regular expression - can affect both of them. - - -COMPILED PATTERN MEMORY USAGE - - Patterns are compiled by PCRE into a reasonably efficient interpretive - code, so that most simple patterns do not use much memory. However, - there is one case where the memory usage of a compiled pattern can be - unexpectedly large. If a parenthesized subpattern has a quantifier with - a minimum greater than 1 and/or a limited maximum, the whole subpattern - is repeated in the compiled code. For example, the pattern - - (abc|def){2,4} - - is compiled as if it were - - (abc|def)(abc|def)((abc|def)(abc|def)?)? - - (Technical aside: It is done this way so that backtrack points within - each of the repetitions can be independently maintained.) - - For regular expressions whose quantifiers use only small numbers, this - is not usually a problem. However, if the numbers are large, and par- - ticularly if such repetitions are nested, the memory usage can become - an embarrassment. For example, the very simple pattern - - ((ab){1,1000}c){1,3} - - uses 51K bytes when compiled using the 8-bit library. When PCRE is com- - piled with its default internal pointer size of two bytes, the size - limit on a compiled pattern is 64K data units, and this is reached with - the above pattern if the outer repetition is increased from 3 to 4. - PCRE can be compiled to use larger internal pointers and thus handle - larger compiled patterns, but it is better to try to rewrite your pat- - tern to use less memory if you can. - - One way of reducing the memory usage for such patterns is to make use - of PCRE's "subroutine" facility. Re-writing the above pattern as - - ((ab)(?2){0,999}c)(?1){0,2} - - reduces the memory requirements to 18K, and indeed it remains under 20K - even with the outer repetition increased to 100. However, this pattern - is not exactly equivalent, because the "subroutine" calls are treated - as atomic groups into which there can be no backtracking if there is a - subsequent matching failure. Therefore, PCRE cannot do this kind of - rewriting automatically. Furthermore, there is a noticeable loss of - speed when executing the modified pattern. Nevertheless, if the atomic - grouping is not a problem and the loss of speed is acceptable, this - kind of rewriting will allow you to process patterns that PCRE cannot - otherwise handle. - - -STACK USAGE AT RUN TIME - - When pcre_exec() or pcre[16|32]_exec() is used for matching, certain - kinds of pattern can cause it to use large amounts of the process - stack. In some environments the default process stack is quite small, - and if it runs out the result is often SIGSEGV. This issue is probably - the most frequently raised problem with PCRE. Rewriting your pattern - can often help. The pcrestack documentation discusses this issue in - detail. - - -PROCESSING TIME - - Certain items in regular expression patterns are processed more effi- - ciently than others. It is more efficient to use a character class like - [aeiou] than a set of single-character alternatives such as - (a|e|i|o|u). In general, the simplest construction that provides the - required behaviour is usually the most efficient. Jeffrey Friedl's book - contains a lot of useful general discussion about optimizing regular - expressions for efficient performance. This document contains a few - observations about PCRE. - - Using Unicode character properties (the \p, \P, and \X escapes) is - slow, because PCRE has to use a multi-stage table lookup whenever it - needs a character's property. If you can find an alternative pattern - that does not use character properties, it will probably be faster. - - By default, the escape sequences \b, \d, \s, and \w, and the POSIX - character classes such as [:alpha:] do not use Unicode properties, - partly for backwards compatibility, and partly for performance reasons. - However, you can set PCRE_UCP if you want Unicode character properties - to be used. This can double the matching time for items such as \d, - when matched with a traditional matching function; the performance loss - is less with a DFA matching function, and in both cases there is not - much difference for \b. - - When a pattern begins with .* not in parentheses, or in parentheses - that are not the subject of a backreference, and the PCRE_DOTALL option - is set, the pattern is implicitly anchored by PCRE, since it can match - only at the start of a subject string. However, if PCRE_DOTALL is not - set, PCRE cannot make this optimization, because the . metacharacter - does not then match a newline, and if the subject string contains new- - lines, the pattern may match from the character immediately following - one of them instead of from the very start. For example, the pattern - - .*second - - matches the subject "first\nand second" (where \n stands for a newline - character), with the match starting at the seventh character. In order - to do this, PCRE has to retry the match starting after every newline in - the subject. - - If you are using such a pattern with subject strings that do not con- - tain newlines, the best performance is obtained by setting PCRE_DOTALL, - or starting the pattern with ^.* or ^.*? to indicate explicit anchor- - ing. That saves PCRE from having to scan along the subject looking for - a newline to restart at. - - Beware of patterns that contain nested indefinite repeats. These can - take a long time to run when applied to a string that does not match. - Consider the pattern fragment - - ^(a+)* - - This can match "aaaa" in 16 different ways, and this number increases - very rapidly as the string gets longer. (The * repeat can match 0, 1, - 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + - repeats can match different numbers of times.) When the remainder of - the pattern is such that the entire match is going to fail, PCRE has in - principle to try every possible variation, and this can take an - extremely long time, even for relatively short strings. - - An optimization catches some of the more simple cases such as - - (a+)*b - - where a literal character follows. Before embarking on the standard - matching procedure, PCRE checks that there is a "b" later in the sub- - ject string, and if there is not, it fails the match immediately. How- - ever, when there is no following literal this optimization cannot be - used. You can see the difference by comparing the behaviour of - - (a+)*\d - - with the pattern above. The former gives a failure almost instantly - when applied to a whole line of "a" characters, whereas the latter - takes an appreciable time with strings longer than about 20 characters. - - In many cases, the solution to this kind of performance issue is to use - an atomic group or a possessive quantifier. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 25 August 2012 - Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------- - - -PCREPOSIX(3) Library Functions Manual PCREPOSIX(3) - - - -NAME - PCRE - Perl-compatible regular expressions. - -SYNOPSIS - - #include <pcreposix.h> - - int regcomp(regex_t *preg, const char *pattern, - int cflags); - - int regexec(regex_t *preg, const char *string, - size_t nmatch, regmatch_t pmatch[], int eflags); - size_t regerror(int errcode, const regex_t *preg, - char *errbuf, size_t errbuf_size); - - void regfree(regex_t *preg); - - -DESCRIPTION - - This set of functions provides a POSIX-style API for the PCRE regular - expression 8-bit library. See the pcreapi documentation for a descrip- - tion of PCRE's native API, which contains much additional functional- - ity. There is no POSIX-style wrapper for PCRE's 16-bit and 32-bit - library. - - The functions described here are just wrapper functions that ultimately - call the PCRE native API. Their prototypes are defined in the - pcreposix.h header file, and on Unix systems the library itself is - called pcreposix.a, so can be accessed by adding -lpcreposix to the - command for linking an application that uses them. Because the POSIX - functions call the native ones, it is also necessary to add -lpcre. - - I have implemented only those POSIX option bits that can be reasonably - mapped to PCRE native options. In addition, the option REG_EXTENDED is - defined with the value zero. This has no effect, but since programs - that are written to the POSIX interface often use it, this makes it - easier to slot in PCRE as a replacement library. Other POSIX options - are not even defined. - - There are also some other options that are not defined by POSIX. These - have been added at the request of users who want to make use of certain - PCRE-specific features via the POSIX calling interface. - - When PCRE is called via these functions, it is only the API that is - POSIX-like in style. The syntax and semantics of the regular expres- - sions themselves are still those of Perl, subject to the setting of - various PCRE options, as described below. "POSIX-like in style" means - that the API approximates to the POSIX definition; it is not fully - POSIX-compatible, and in multi-byte encoding domains it is probably - even less compatible. - - The header for these functions is supplied as pcreposix.h to avoid any - potential clash with other POSIX libraries. It can, of course, be - renamed or aliased as regex.h, which is the "correct" name. It provides - two structure types, regex_t for compiled internal forms, and reg- - match_t for returning captured substrings. It also defines some con- - stants whose names start with "REG_"; these are used for setting - options and identifying error codes. - - -COMPILING A PATTERN - - The function regcomp() is called to compile a pattern into an internal - form. The pattern is a C string terminated by a binary zero, and is - passed in the argument pattern. The preg argument is a pointer to a - regex_t structure that is used as a base for storing information about - the compiled regular expression. - - The argument cflags is either zero, or contains one or more of the bits - defined by the following macros: - - REG_DOTALL - - The PCRE_DOTALL option is set when the regular expression is passed for - compilation to the native function. Note that REG_DOTALL is not part of - the POSIX standard. - - REG_ICASE - - The PCRE_CASELESS option is set when the regular expression is passed - for compilation to the native function. - - REG_NEWLINE - - The PCRE_MULTILINE option is set when the regular expression is passed - for compilation to the native function. Note that this does not mimic - the defined POSIX behaviour for REG_NEWLINE (see the following sec- - tion). - - REG_NOSUB - - The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is - passed for compilation to the native function. In addition, when a pat- - tern that is compiled with this flag is passed to regexec() for match- - ing, the nmatch and pmatch arguments are ignored, and no captured - strings are returned. - - REG_UCP - - The PCRE_UCP option is set when the regular expression is passed for - compilation to the native function. This causes PCRE to use Unicode - properties when matchine \d, \w, etc., instead of just recognizing - ASCII values. Note that REG_UTF8 is not part of the POSIX standard. - - REG_UNGREEDY - - The PCRE_UNGREEDY option is set when the regular expression is passed - for compilation to the native function. Note that REG_UNGREEDY is not - part of the POSIX standard. - - REG_UTF8 - - The PCRE_UTF8 option is set when the regular expression is passed for - compilation to the native function. This causes the pattern itself and - all data strings used for matching it to be treated as UTF-8 strings. - Note that REG_UTF8 is not part of the POSIX standard. - - In the absence of these flags, no options are passed to the native - function. This means the the regex is compiled with PCRE default - semantics. In particular, the way it handles newline characters in the - subject string is the Perl way, not the POSIX way. Note that setting - PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. - It does not affect the way newlines are matched by . (they are not) or - by a negative class such as [^a] (they are). - - The yield of regcomp() is zero on success, and non-zero otherwise. The - preg structure is filled in on success, and one member of the structure - is public: re_nsub contains the number of capturing subpatterns in the - regular expression. Various error codes are defined in the header file. - - NOTE: If the yield of regcomp() is non-zero, you must not attempt to - use the contents of the preg structure. If, for example, you pass it to - regexec(), the result is undefined and your program is likely to crash. - - -MATCHING NEWLINE CHARACTERS - - This area is not simple, because POSIX and Perl take different views of - things. It is not possible to get PCRE to obey POSIX semantics, but - then PCRE was never intended to be a POSIX engine. The following table - lists the different possibilities for matching newline characters in - PCRE: - - Default Change with - - . matches newline no PCRE_DOTALL - newline matches [^a] yes not changeable - $ matches \n at end yes PCRE_DOLLARENDONLY - $ matches \n in middle no PCRE_MULTILINE - ^ matches \n in middle no PCRE_MULTILINE - - This is the equivalent table for POSIX: - - Default Change with - - . matches newline yes REG_NEWLINE - newline matches [^a] yes REG_NEWLINE - $ matches \n at end no REG_NEWLINE - $ matches \n in middle no REG_NEWLINE - ^ matches \n in middle no REG_NEWLINE - - PCRE's behaviour is the same as Perl's, except that there is no equiva- - lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is - no way to stop newline from matching [^a]. - - The default POSIX newline handling can be obtained by setting - PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE - behave exactly as for the REG_NEWLINE action. - - -MATCHING A PATTERN - - The function regexec() is called to match a compiled pattern preg - against a given string, which is by default terminated by a zero byte - (but see REG_STARTEND below), subject to the options in eflags. These - can be: - - REG_NOTBOL - - The PCRE_NOTBOL option is set when calling the underlying PCRE matching - function. - - REG_NOTEMPTY - - The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- - ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. - However, setting this option can give more POSIX-like behaviour in some - situations. - - REG_NOTEOL - - The PCRE_NOTEOL option is set when calling the underlying PCRE matching - function. - - REG_STARTEND - - The string is considered to start at string + pmatch[0].rm_so and to - have a terminating NUL located at string + pmatch[0].rm_eo (there need - not actually be a NUL at that location), regardless of the value of - nmatch. This is a BSD extension, compatible with but not specified by - IEEE Standard 1003.2 (POSIX.2), and should be used with caution in - software intended to be portable to other systems. Note that a non-zero - rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location - of the string, not how it is matched. - - If the pattern was compiled with the REG_NOSUB flag, no data about any - matched strings is returned. The nmatch and pmatch arguments of - regexec() are ignored. - - If the value of nmatch is zero, or if the value pmatch is NULL, no data - about any matched strings is returned. - - Otherwise,the portion of the string that was matched, and also any cap- - tured substrings, are returned via the pmatch argument, which points to - an array of nmatch structures of type regmatch_t, containing the mem- - bers rm_so and rm_eo. These contain the offset to the first character - of each substring and the offset to the first character after the end - of each substring, respectively. The 0th element of the vector relates - to the entire portion of string that was matched; subsequent elements - relate to the capturing subpatterns of the regular expression. Unused - entries in the array have both structure members set to -1. - - A successful match yields a zero return; various error codes are - defined in the header file, of which REG_NOMATCH is the "expected" - failure code. - - -ERROR MESSAGES - - The regerror() function maps a non-zero errorcode from either regcomp() - or regexec() to a printable message. If preg is not NULL, the error - should have arisen from the use of that structure. A message terminated - by a binary zero is placed in errbuf. The length of the message, - including the zero, is limited to errbuf_size. The yield of the func- - tion is the size of buffer needed to hold the whole message. - - -MEMORY USAGE - - Compiling a regular expression causes memory to be allocated and asso- - ciated with the preg structure. The function regfree() frees all such - memory, after which preg may no longer be used as a compiled expres- - sion. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 09 January 2012 - Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRECPP(3) Library Functions Manual PCRECPP(3) - - - -NAME - PCRE - Perl-compatible regular expressions. - -SYNOPSIS OF C++ WRAPPER - - #include <pcrecpp.h> - - -DESCRIPTION - - The C++ wrapper for PCRE was provided by Google Inc. Some additional - functionality was added by Giuseppe Maxia. This brief man page was con- - structed from the notes in the pcrecpp.h file, which should be con- - sulted for further details. Note that the C++ wrapper supports only the - original 8-bit PCRE library. There is no 16-bit or 32-bit support at - present. - - -MATCHING INTERFACE - - The "FullMatch" operation checks that supplied text matches a supplied - pattern exactly. If pointer arguments are supplied, it copies matched - sub-strings that match sub-patterns into them. - - Example: successful match - pcrecpp::RE re("h.*o"); - re.FullMatch("hello"); - - Example: unsuccessful match (requires full match): - pcrecpp::RE re("e"); - !re.FullMatch("hello"); - - Example: creating a temporary RE object: - pcrecpp::RE("h.*o").FullMatch("hello"); - - You can pass in a "const char*" or a "string" for "text". The examples - below tend to use a const char*. You can, as in the different examples - above, store the RE object explicitly in a variable or use a temporary - RE object. The examples below use one mode or the other arbitrarily. - Either could correctly be used for any of these examples. - - You must supply extra pointer arguments to extract matched subpieces. - - Example: extracts "ruby" into "s" and 1234 into "i" - int i; - string s; - pcrecpp::RE re("(\\w+):(\\d+)"); - re.FullMatch("ruby:1234", &s, &i); - - Example: does not try to extract any extra sub-patterns - re.FullMatch("ruby:1234", &s); - - Example: does not try to extract into NULL - re.FullMatch("ruby:1234", NULL, &i); - - Example: integer overflow causes failure - !re.FullMatch("ruby:1234567891234", NULL, &i); - - Example: fails because there aren't enough sub-patterns: - !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); - - Example: fails because string cannot be stored in integer - !pcrecpp::RE("(.*)").FullMatch("ruby", &i); - - The provided pointer arguments can be pointers to any scalar numeric - type, or one of: - - string (matched piece is copied to string) - StringPiece (StringPiece is mutated to point to matched piece) - T (where "bool T::ParseFrom(const char*, int)" exists) - NULL (the corresponding matched sub-pattern is not copied) - - The function returns true iff all of the following conditions are sat- - isfied: - - a. "text" matches "pattern" exactly; - - b. The number of matched sub-patterns is >= number of supplied - pointers; - - c. The "i"th argument has a suitable type for holding the - string captured as the "i"th sub-pattern. If you pass in - void * NULL for the "i"th argument, or a non-void * NULL - of the correct type, or pass fewer arguments than the - number of sub-patterns, "i"th captured sub-pattern is - ignored. - - CAVEAT: An optional sub-pattern that does not exist in the matched - string is assigned the empty string. Therefore, the following will - return false (because the empty string is not a valid number): - - int number; - pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); - - The matching interface supports at most 16 arguments per call. If you - need more, consider using the more general interface - pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. - - NOTE: Do not use no_arg, which is used internally to mark the end of a - list of optional arguments, as a placeholder for missing arguments, as - this can lead to segfaults. - - -QUOTING METACHARACTERS - - You can use the "QuoteMeta" operation to insert backslashes before all - potentially meaningful characters in a string. The returned string, - used as a regular expression, will exactly match the original string. - - Example: - string quoted = RE::QuoteMeta(unquoted); - - Note that it's legal to escape a character even if it has no special - meaning in a regular expression -- so this function does that. (This - also makes it identical to the perl function of the same name; see - "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes - "1\.5\-2\.0\?". - - -PARTIAL MATCHES - - You can use the "PartialMatch" operation when you want the pattern to - match any substring of the text. - - Example: simple search for a string: - pcrecpp::RE("ell").PartialMatch("hello"); - - Example: find first number in a string: - int number; - pcrecpp::RE re("(\\d+)"); - re.PartialMatch("x*100 + 20", &number); - assert(number == 100); - - -UTF-8 AND THE MATCHING INTERFACE - - By default, pattern and text are plain text, one byte per character. - The UTF8 flag, passed to the constructor, causes both pattern and - string to be treated as UTF-8 text, still a byte stream but potentially - multiple bytes per character. In practice, the text is likelier to be - UTF-8 than the pattern, but the match returned may depend on the UTF8 - flag, so always use it when matching UTF8 text. For example, "." will - match one byte normally but with UTF8 set may match up to three bytes - of a multi-byte character. - - Example: - pcrecpp::RE_Options options; - options.set_utf8(); - pcrecpp::RE re(utf8_pattern, options); - re.FullMatch(utf8_string); - - Example: using the convenience function UTF8(): - pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); - re.FullMatch(utf8_string); - - NOTE: The UTF8 flag is ignored if pcre was not configured with the - --enable-utf8 flag. - - -PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE - - PCRE defines some modifiers to change the behavior of the regular - expression engine. The C++ wrapper defines an auxiliary class, - RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- - rently, the following modifiers are supported: - - modifier description Perl corresponding - - PCRE_CASELESS case insensitive match /i - PCRE_MULTILINE multiple lines match /m - PCRE_DOTALL dot matches newlines /s - PCRE_DOLLAR_ENDONLY $ matches only at end N/A - PCRE_EXTRA strict escape parsing N/A - PCRE_EXTENDED ignore white spaces /x - PCRE_UTF8 handles UTF8 chars built-in - PCRE_UNGREEDY reverses * and *? N/A - PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) - - (*) Both Perl and PCRE allow non capturing parentheses by means of the - "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- - ture, while (ab|cd) does. - - For a full account on how each modifier works, please check the PCRE - API reference page. - - For each modifier, there are two member functions whose name is made - out of the modifier in lowercase, without the "PCRE_" prefix. For - instance, PCRE_CASELESS is handled by - - bool caseless() - - which returns true if the modifier is set, and - - RE_Options & set_caseless(bool) - - which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can - be accessed through the set_match_limit() and match_limit() member - functions. Setting match_limit to a non-zero value will limit the exe- - cution of pcre to keep it from doing bad things like blowing the stack - or taking an eternity to return a result. A value of 5000 is good - enough to stop stack blowup in a 2MB thread stack. Setting match_limit - to zero disables match limiting. Alternatively, you can call - match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to - limit how much PCRE recurses. match_limit() limits the number of - matches PCRE does; match_limit_recursion() limits the depth of internal - recursion, and therefore the amount of stack that is used. - - Normally, to pass one or more modifiers to a RE class, you declare a - RE_Options object, set the appropriate options, and pass this object to - a RE constructor. Example: - - RE_Options opt; - opt.set_caseless(true); - if (RE("HELLO", opt).PartialMatch("hello world")) ... - - RE_options has two constructors. The default constructor takes no argu- - ments and creates a set of flags that are off by default. The optional - parameter option_flags is to facilitate transfer of legacy code from C - programs. This lets you do - - RE(pattern, - RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); - - However, new code is better off doing - - RE(pattern, - RE_Options().set_caseless(true).set_multiline(true)) - .PartialMatch(str); - - If you are going to pass one of the most used modifiers, there are some - convenience functions that return a RE_Options class with the appropri- - ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), - and EXTENDED(). - - If you need to set several options at once, and you don't want to go - through the pains of declaring a RE_Options object and setting several - options, there is a parallel method that give you such ability on the - fly. You can concatenate several set_xxxxx() member functions, since - each of them returns a reference to its class object. For example, to - pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one - statement, you may write: - - RE(" ^ xyz \\s+ .* blah$", - RE_Options() - .set_caseless(true) - .set_extended(true) - .set_multiline(true)).PartialMatch(sometext); - - -SCANNING TEXT INCREMENTALLY - - The "Consume" operation may be useful if you want to repeatedly match - regular expressions at the front of a string and skip over them as they - match. This requires use of the "StringPiece" type, which represents a - sub-range of a real string. Like RE, StringPiece is defined in the - pcrecpp namespace. - - Example: read lines of the form "var = value" from a string. - string contents = ...; // Fill string somehow - pcrecpp::StringPiece input(contents); // Wrap in a StringPiece - - string var; - int value; - pcrecpp::RE re("(\\w+) = (\\d+)\n"); - while (re.Consume(&input, &var, &value)) { - ...; - } - - Each successful call to "Consume" will set "var/value", and also - advance "input" so it points past the matched text. - - The "FindAndConsume" operation is similar to "Consume" but does not - anchor your match at the beginning of the string. For example, you - could extract all words from a string by repeatedly calling - - pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) - - -PARSING HEX/OCTAL/C-RADIX NUMBERS - - By default, if you pass a pointer to a numeric value, the corresponding - text is interpreted as a base-10 number. You can instead wrap the - pointer with a call to one of the operators Hex(), Octal(), or CRadix() - to interpret the text in another base. The CRadix operator interprets - C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to - base-10. - - Example: - int a, b, c, d; - pcrecpp::RE re("(.*) (.*) (.*) (.*)"); - re.FullMatch("100 40 0100 0x40", - pcrecpp::Octal(&a), pcrecpp::Hex(&b), - pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); - - will leave 64 in a, b, c, and d. - - -REPLACING PARTS OF STRINGS - - You can replace the first match of "pattern" in "str" with "rewrite". - Within "rewrite", backslash-escaped digits (\1 to \9) can be used to - insert text matching corresponding parenthesized group from the pat- - tern. \0 in "rewrite" refers to the entire matching text. For example: - - string s = "yabba dabba doo"; - pcrecpp::RE("b+").Replace("d", &s); - - will leave "s" containing "yada dabba doo". The result is true if the - pattern matches and a replacement occurs, false otherwise. - - GlobalReplace is like Replace except that it replaces all occurrences - of the pattern in the string with the rewrite. Replacements are not - subject to re-matching. For example: - - string s = "yabba dabba doo"; - pcrecpp::RE("b+").GlobalReplace("d", &s); - - will leave "s" containing "yada dada doo". It returns the number of - replacements made. - - Extract is like Replace, except that if the pattern matches, "rewrite" - is copied into "out" (an additional argument) with substitutions. The - non-matching portions of "text" are ignored. Returns true iff a match - occurred and the extraction happened successfully; if no match occurs, - the string is left unaffected. - - -AUTHOR - - The C++ wrapper was contributed by Google Inc. - Copyright (c) 2007 Google Inc. - - -REVISION - - Last updated: 08 January 2012 ------------------------------------------------------------------------------- - - -PCRESAMPLE(3) Library Functions Manual PCRESAMPLE(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE SAMPLE PROGRAM - - A simple, complete demonstration program, to get you started with using - PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A - listing of this program is given in the pcredemo documentation. If you - do not have a copy of the PCRE distribution, you can save this listing - to re-create pcredemo.c. - - The demonstration program, which uses the original PCRE 8-bit library, - compiles the regular expression that is its first argument, and matches - it against the subject string in its second argument. No PCRE options - are set, and default character tables are used. If matching succeeds, - the program outputs the portion of the subject that matched, together - with the contents of any captured substrings. - - If the -g option is given on the command line, the program then goes on - to check for further matches of the same regular expression in the same - subject string. The logic is a little bit tricky because of the possi- - bility of matching an empty string. Comments in the code explain what - is going on. - - If PCRE is installed in the standard include and library directories - for your operating system, you should be able to compile the demonstra- - tion program using this command: - - gcc -o pcredemo pcredemo.c -lpcre - - If PCRE is installed elsewhere, you may need to add additional options - to the command line. For example, on a Unix-like system that has PCRE - installed in /usr/local, you can compile the demonstration program - using a command like this: - - gcc -o pcredemo -I/usr/local/include pcredemo.c \ - -L/usr/local/lib -lpcre - - In a Windows environment, if you want to statically link the program - against a non-dll pcre.a file, you must uncomment the line that defines - PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- - loc() and pcre_free() exported functions will be declared - __declspec(dllimport), with unwanted results. - - Once you have compiled and linked the demonstration program, you can - run simple tests like this: - - ./pcredemo 'cat|dog' 'the cat sat on the mat' - ./pcredemo -g 'cat|dog' 'the dog sat on the cat' - - Note that there is a much more comprehensive test program, called - pcretest, which supports many more facilities for testing regular - expressions and both PCRE libraries. The pcredemo program is provided - as a simple coding example. - - If you try to run pcredemo when PCRE is not installed in the standard - library directory, you may get an error like this on some operating - systems (e.g. Solaris): - - ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or - directory - - This is caused by the way shared library support works on those sys- - tems. You need to add - - -R/usr/local/lib - - (for example) to the compile command to get round this problem. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 10 January 2012 - Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------- -PCRELIMITS(3) Library Functions Manual PCRELIMITS(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -SIZE AND OTHER LIMITATIONS - - There are some size limitations in PCRE but it is hoped that they will - never in practice be relevant. - - The maximum length of a compiled pattern is approximately 64K data - units (bytes for the 8-bit library, 16-bit units for the 16-bit - library, and 32-bit units for the 32-bit library) if PCRE is compiled - with the default internal linkage size, which is 2 bytes for the 8-bit - and 16-bit libraries, and 4 bytes for the 32-bit library. If you want - to process regular expressions that are truly enormous, you can compile - PCRE with an internal linkage size of 3 or 4 (when building the 16-bit - or 32-bit library, 3 is rounded up to 4). See the README file in the - source distribution and the pcrebuild documentation for details. In - these cases the limit is substantially larger. However, the speed of - execution is slower. - - All values in repeating quantifiers must be less than 65536. - - There is no limit to the number of parenthesized subpatterns, but there - can be no more than 65535 capturing subpatterns. There is, however, a - limit to the depth of nesting of parenthesized subpatterns of all - kinds. This is imposed in order to limit the amount of system stack - used at compile time. The limit can be specified when PCRE is built; - the default is 250. - - There is a limit to the number of forward references to subsequent sub- - patterns of around 200,000. Repeated forward references with fixed - upper limits, for example, (?2){0,100} when subpattern number 2 is to - the right, are included in the count. There is no limit to the number - of backward references. - - The maximum length of name for a named subpattern is 32 characters, and - the maximum number of named subpatterns is 10000. - - The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or - (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit and - 32-bit libraries. - - The maximum length of a subject string is the largest positive number - that an integer variable can hold. However, when using the traditional - matching function, PCRE uses recursion to handle subpatterns and indef- - inite repetition. This means that the available stack space may limit - the size of a subject string that can be processed by certain patterns. - For a discussion of stack issues, see the pcrestack documentation. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 05 November 2013 - Copyright (c) 1997-2013 University of Cambridge. ------------------------------------------------------------------------------- - - -PCRESTACK(3) Library Functions Manual PCRESTACK(3) - - - -NAME - PCRE - Perl-compatible regular expressions - -PCRE DISCUSSION OF STACK USAGE - - When you call pcre[16|32]_exec(), it makes use of an internal function - called match(). This calls itself recursively at branch points in the - pattern, in order to remember the state of the match so that it can - back up and try a different alternative if the first one fails. As - matching proceeds deeper and deeper into the tree of possibilities, the - recursion depth increases. The match() function is also called in other - circumstances, for example, whenever a parenthesized sub-pattern is - entered, and in certain cases of repetition. - - Not all calls of match() increase the recursion depth; for an item such - as a* it may be called several times at the same level, after matching - different numbers of a's. Furthermore, in a number of cases where the - result of the recursive call would immediately be passed back as the - result of the current call (a "tail recursion"), the function is just - restarted instead. - - The above comments apply when pcre[16|32]_exec() is run in its normal - interpretive manner. If the pattern was studied with the - PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was success- - ful, and the options passed to pcre[16|32]_exec() were not incompati- - ble, the matching process uses the JIT-compiled code instead of the - match() function. In this case, the memory requirements are handled - entirely differently. See the pcrejit documentation for details. - - The pcre[16|32]_dfa_exec() function operates in an entirely different - way, and uses recursion only when there is a regular expression recur- - sion or subroutine call in the pattern. This includes the processing of - assertion and "once-only" subpatterns, which are handled like subrou- - tine calls. Normally, these are never very deep, and the limit on the - complexity of pcre[16|32]_dfa_exec() is controlled by the amount of - workspace it is given. However, it is possible to write patterns with - runaway infinite recursions; such patterns will cause - pcre[16|32]_dfa_exec() to run out of stack. At present, there is no - protection against this. - - The comments that follow do NOT apply to pcre[16|32]_dfa_exec(); they - are relevant only for pcre[16|32]_exec() without the JIT optimization. - - Reducing pcre[16|32]_exec()'s stack usage - - Each time that match() is actually called recursively, it uses memory - from the process stack. For certain kinds of pattern and data, very - large amounts of stack may be needed, despite the recognition of "tail - recursion". You can often reduce the amount of recursion, and there- - fore the amount of stack used, by modifying the pattern that is being - matched. Consider, for example, this pattern: - - ([^<]|<(?!inet))+ - - It matches from wherever it starts until it encounters "<inet" or the - end of the data, and is the kind of pattern that might be used when - processing an XML file. Each iteration of the outer parentheses matches - either one character that is not "<" or a "<" that is not followed by - "inet". However, each time a parenthesis is processed, a recursion - occurs, so this formulation uses a stack frame for each matched charac- - ter. For a long string, a lot of stack is required. Consider now this - rewritten pattern, which matches exactly the same strings: - - ([^<]++|<(?!inet))+ - - This uses very much less stack, because runs of characters that do not - contain "<" are "swallowed" in one item inside the parentheses. Recur- - sion happens only when a "<" character that is not followed by "inet" - is encountered (and we assume this is relatively rare). A possessive - quantifier is used to stop any backtracking into the runs of non-"<" - characters, but that is not related to stack usage. - - This example shows that one way of avoiding stack problems when match- - ing long subject strings is to write repeated parenthesized subpatterns - to match more than one character whenever possible. - - Compiling PCRE to use heap instead of stack for pcre[16|32]_exec() - - In environments where stack memory is constrained, you might want to - compile PCRE to use heap memory instead of stack for remembering back- - up points when pcre[16|32]_exec() is running. This makes it run a lot - more slowly, however. Details of how to do this are given in the pcre- - build documentation. When built in this way, instead of using the - stack, PCRE obtains and frees memory by calling the functions that are - pointed to by the pcre[16|32]_stack_malloc and pcre[16|32]_stack_free - variables. By default, these point to malloc() and free(), but you can - replace the pointers to cause PCRE to use your own functions. Since the - block sizes are always the same, and are always freed in reverse order, - it may be possible to implement customized memory handlers that are - more efficient than the standard functions. - - Limiting pcre[16|32]_exec()'s stack usage - - You can set limits on the number of times that match() is called, both - in total and recursively. If a limit is exceeded, pcre[16|32]_exec() - returns an error code. Setting suitable limits should prevent it from - running out of stack. The default values of the limits are very large, - and unlikely ever to operate. They can be changed when PCRE is built, - and they can also be set when pcre[16|32]_exec() is called. For details - of these interfaces, see the pcrebuild documentation and the section on - extra data for pcre[16|32]_exec() in the pcreapi documentation. - - As a very rough rule of thumb, you should reckon on about 500 bytes per - recursion. Thus, if you want to limit your stack usage to 8Mb, you - should set the limit at 16000 recursions. A 64Mb stack, on the other - hand, can support around 128000 recursions. - - In Unix-like environments, the pcretest test program has a command line - option (-S) that can be used to increase the size of its stack. As long - as the stack is large enough, another option (-M) can be used to find - the smallest limits that allow a particular pattern to match a given - subject string. This is done by calling pcre[16|32]_exec() repeatedly - with different limits. - - Obtaining an estimate of stack usage - - The actual amount of stack used per recursion can vary quite a lot, - depending on the compiler that was used to build PCRE and the optimiza- - tion or debugging options that were set for it. The rule of thumb value - of 500 bytes mentioned above may be larger or smaller than what is - actually needed. A better approximation can be obtained by running this - command: - - pcretest -m -C - - The -C option causes pcretest to output information about the options - with which PCRE was compiled. When -m is also given (before -C), infor- - mation about stack use is given in a line like this: - - Match recursion uses stack: approximate frame size = 640 bytes - - The value is approximate because some recursions need a bit more (up to - perhaps 16 more bytes). - - If the above command is given when PCRE is compiled to use the heap - instead of the stack for recursion, the value that is output is the - size of each block that is obtained from the heap. - - Changing stack size in Unix-like systems - - In Unix-like environments, there is not often a problem with the stack - unless very long strings are involved, though the default limit on - stack size varies from system to system. Values from 8Mb to 64Mb are - common. You can find your default limit by running the command: - - ulimit -s - - Unfortunately, the effect of running out of stack is often SIGSEGV, - though sometimes a more explicit error message is given. You can nor- - mally increase the limit on stack size by code such as this: - - struct rlimit rlim; - getrlimit(RLIMIT_STACK, &rlim); - rlim.rlim_cur = 100*1024*1024; - setrlimit(RLIMIT_STACK, &rlim); - - This reads the current limits (soft and hard) using getrlimit(), then - attempts to increase the soft limit to 100Mb using setrlimit(). You - must do this before calling pcre[16|32]_exec(). - - Changing stack size in Mac OS X - - Using setrlimit(), as described above, should also work on Mac OS X. It - is also possible to set a stack size when linking a program. There is a - discussion about stack sizes in Mac OS X at this web site: - http://developer.apple.com/qa/qa2005/qa1419.html. - - -AUTHOR - - Philip Hazel - University Computing Service - Cambridge CB2 3QH, England. - - -REVISION - - Last updated: 24 June 2012 - Copyright (c) 1997-2012 University of Cambridge. ------------------------------------------------------------------------------- - - |