From 0a55fa14f462169bbd8a8de623804f039854f95f Mon Sep 17 00:00:00 2001 From: dartraiden Date: Fri, 1 Jun 2018 18:25:57 +0300 Subject: we only needs license, contributors and version info --- libs/Pcre16/docs/doc/html/pcrepattern.html | 3276 ---------------------------- 1 file changed, 3276 deletions(-) delete mode 100644 libs/Pcre16/docs/doc/html/pcrepattern.html (limited to 'libs/Pcre16/docs/doc/html/pcrepattern.html') diff --git a/libs/Pcre16/docs/doc/html/pcrepattern.html b/libs/Pcre16/docs/doc/html/pcrepattern.html deleted file mode 100644 index 96fc72986f..0000000000 --- a/libs/Pcre16/docs/doc/html/pcrepattern.html +++ /dev/null @@ -1,3276 +0,0 @@ - - -pcrepattern specification - - -

pcrepattern man page

-

-Return to the PCRE index page. -

-

-This page is part of the PCRE HTML documentation. It was generated automatically -from the original man page. If there is any nonsense in it, please consult the -man page, in case the conversion went wrong. -
-

-
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 syntax 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 syntax) 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-compatible, but -are provided to make these options accessible to pattern writers who are not -able to change the program that processes the pattern. 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 appropriate 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 determine 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 (linefeed) -character, the two-character sequence CRLF, any of the three preceding, 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 pattern -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 function. 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 assertions are -true. It also affects the interpretation of the dot metacharacter 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 -"Newline 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 -recursive 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 setting 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 system). In the -sections below, character code values are ASCII or Unicode; 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 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 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 recognized -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 backslash, 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 characters, you -can do so by putting them between \Q and \E. This is different from Perl in -that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in -Perl, $ and @ cause variable interpolation. 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 characters -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. -In an ASCII or Unicode environment, these escapes are as follows: -

-  \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. -

-

-When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t -generate the appropriate EBCDIC code values. The \c escape is processed -as specified for Perl in the perlebcdic document. The only characters -that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any -other character provokes a compile-time error. The sequence \c@ encodes -character code 0; after \c the letters (in either case) encode characters 1-26 -(hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex -1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F). -

-

-Thus, apart from \c?, these escapes generate the same character code values as -they do in an ASCII environment, though the meanings of the values mostly -differ. For example, \cG always generates code value 7, which is BEL in ASCII -but DEL in EBCDIC. -

-

-The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but -because 127 is not a control character in EBCDIC, Perl makes it generate the -APC character. Unfortunately, there are several variants of EBCDIC. In most of -them the APC character has the value 255 (hex FF), but in the one Perl calls -POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC -values, PCRE makes \c? generate 95; otherwise it generates 255. -

-

-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\015 -specifies two binary zeros followed by a CR character (code value 13). 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 character -numbers, and \g{} to specify back references. The following paragraphs -describe the old, ambiguous syntax. -

-

-The handling of a backslash followed by a digit other than 0 is complicated, -and Perl has changed in recent releases, causing PCRE also to change. Outside a -character class, PCRE reads the digit and any following 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 otherwise 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 hexadecimal -digits are read (letters can be in upper or lower case). Any number of -hexadecimal digits may appear between \x{ and }. If a character 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 digits. -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 difference 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, optionally -enclosed in braces, is an absolute or relative back reference. A named back -reference can be coded as \g{name}. Back references are discussed -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 character. -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 complete 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 controlled 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 reasons. If PCRE is compiled with -Unicode property support, and the PCRE_UCP option is set, the behaviour is -changed so that Unicode properties 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 characters 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 (carriage 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 separator, 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 function, 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 additional -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 "InMusicalSymbols" 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, -Bassa_Vah, -Batak, -Bengali, -Bopomofo, -Brahmi, -Braille, -Buginese, -Buhid, -Canadian_Aboriginal, -Carian, -Caucasian_Albanian, -Chakma, -Cham, -Cherokee, -Common, -Coptic, -Cuneiform, -Cypriot, -Cyrillic, -Deseret, -Devanagari, -Duployan, -Egyptian_Hieroglyphs, -Elbasan, -Ethiopic, -Georgian, -Glagolitic, -Gothic, -Grantha, -Greek, -Gujarati, -Gurmukhi, -Han, -Hangul, -Hanunoo, -Hebrew, -Hiragana, -Imperial_Aramaic, -Inherited, -Inscriptional_Pahlavi, -Inscriptional_Parthian, -Javanese, -Kaithi, -Kannada, -Katakana, -Kayah_Li, -Kharoshthi, -Khmer, -Khojki, -Khudawadi, -Lao, -Latin, -Lepcha, -Limbu, -Linear_A, -Linear_B, -Lisu, -Lycian, -Lydian, -Mahajani, -Malayalam, -Mandaic, -Manichaean, -Meetei_Mayek, -Mende_Kikakui, -Meroitic_Cursive, -Meroitic_Hieroglyphs, -Miao, -Modi, -Mongolian, -Mro, -Myanmar, -Nabataean, -New_Tai_Lue, -Nko, -Ogham, -Ol_Chiki, -Old_Italic, -Old_North_Arabian, -Old_Permic, -Old_Persian, -Old_South_Arabian, -Old_Turkic, -Oriya, -Osmanya, -Pahawh_Hmong, -Palmyrene, -Pau_Cin_Hau, -Phags_Pa, -Phoenician, -Psalter_Pahlavi, -Rejang, -Runic, -Samaritan, -Saurashtra, -Sharada, -Shavian, -Siddham, -Sinhala, -Sora_Sompeng, -Sundanese, -Syloti_Nagri, -Syriac, -Tagalog, -Tagbanwa, -Tai_Le, -Tai_Tham, -Tai_Viet, -Takri, -Tamil, -Telugu, -Thaana, -Thai, -Tibetan, -Tifinagh, -Tirhuta, -Ugaritic, -Vai, -Warang_Citi, -Yi. -

-

-Each character has exactly one Unicode general category property, specified by -a two-letter abbreviation. For compatibility with Perl, negation 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 general -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) property. -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 property. 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 earlier, 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 complicated -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 character. -

-

-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 supports 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. However, 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 (number) -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 vertical tab, for Perl -compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd -matches the same characters as Xan, plus underscore. -

-

-There is another non-standard property, Xuc, which matches any character 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 characters 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 assertion -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 character (for example, \B -matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid -escape sequence" error is generated 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" metasequence. 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 assertions 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, indicating 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 -arguments, you can mimic Perl's /g option, and it is in this kind of -implementation 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 consuming 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 argument 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 subject, 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. Dollar 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 character in -the subject string except (by default) a character that signifies 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 Unicode 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 circumflex 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 usefully 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 calculate 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 pattern, 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 number 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 special 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 consumes 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 characters 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 character, or z. -

-

-It is not possible to have the literal character "]" as the end character 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 interpreted 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 hexadecimal -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 characters 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 (punctuation) 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 assertions" -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 behaviour. -

-
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 possible to -unset these options by preceding the letter with a hyphen, and a combined -setting and unsetting such as (?im-sx), which sets PCRE_CASELESS 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. 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 property 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. However, 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 support 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 numbered 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 capturing, 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 capturing -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, parentheses 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 following example is taken from the Perl documentation. The -numbers underneath 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 number 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 expressions. Furthermore, -if an expression is modified, the numbers may change. To help with this -difficulty, PCRE supports the naming of subpatterns. 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 syntax. 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.) Duplicate 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 pattern 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 -subpatterns with the same number because PCRE uses only the numbers when -matching. For this reason, an error is given at compile time if different 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 number 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 example, {,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. Similarly, -\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 useful 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-character -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 broken. -

-

-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 (equivalent -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 newlines, it is -worth setting PCRE_DOTALL in order to obtain this optimization, 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 character. For -this reason, such a pattern is not implicitly anchored. -

-

-Another case where implicit anchoring is not applied is when the leading .* 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 control verbs -(*PRUNE) and (*SKIP) also disable this optimization. -

-

-When a capturing subpattern is repeated, the value captured is the substring -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 iterations. 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 contains 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 prepared 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 syntax. -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 simple -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 character 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 subpattern 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 pattern. 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 iteration. -

-

-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 ambiguity 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 capturing -subpattern before \g, that is, is it equivalent to \2 in this example. -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 subpattern 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 example, -
-  ((?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 reference to an -unset value matches an empty string. -

-

-Because there may be many capturing parentheses in a pattern, all digits -following a backslash are taken as part of a potential back reference 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 subpatterns. For -example, the pattern -

-  (a|b\1)+
-
-matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration 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 assertion -contains capturing subpatterns within it, these are counted for the purposes of -numbering the capturing subpatterns in the whole pattern. However, substring -capturing is carried out only for positive assertions. (Perl sometimes, but not -always, does do capturing in negative assertions.) -

-

-WARNING: If a positive assertion containing one or more capturing subpatterns -succeeds, but failure to match later in the pattern causes backtracking over -this assertion, the captures within the assertion are reset only if no higher -numbered captures are already set. This is, unfortunately, a fundamental -limitation of the current implementation, and as PCRE1 is now in -maintenance-only status, it is unlikely ever to change. -

-

-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 greediness 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 semicolon 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 several 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 current position, the -assertion fails. -

-

-In a UTF mode, PCRE does not allow the \C escape (which matches a single data -unit even in a UTF mode) to appear in lookbehind assertions, because it makes -it impossible to calculate the length of the lookbehind. 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" preceded by six characters, the first -of which are digits and the last three of which are not "999". For example, it -doesn't match "123abcfoo". 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 -conditionally or to choose between two alternative subpatterns, depending on -the result of an assertion, or whether a specific capturing subpattern 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 alternatives in the -subpattern, a compile-time error occurs. Each of the two alternatives may -itself contain nested subpatterns of any form, including 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, references 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 previously -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 alternative 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 second 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. Otherwise, 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 ampersand 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 referenced 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, insisting 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 character class, -nor in the middle of any other sequence of related characters 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 characters are interpreted as newlines -is controlled by the options passed to a compiling function or by a special -sequence at the start of the pattern, 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 expressions 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 parenthesized 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 referenced. 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 pattern 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 -documentation). 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 subpattern 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 brackets, allowing for -arbitrary nesting. Only digits are allowed in nested brackets (that is, when -recursing), whereas any characters are permitted 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 palindromic 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 alternative is taken -and the recursion kicks in. The recursive call to subpattern 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 alternatives 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 backtracking 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 subject -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 alternatives, so the entire match fails. -

-

-The second way in which PCRE and Perl differ in their recursion processing is -in the handling of captured values. In Perl, when a subpattern is called -recursively or as a subpattern (see the next section), it has no access to any -values that were captured outside the recursion, 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 subject string, it -is never re-entered, even if it contains untried alternatives 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 subpattern 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, rewritten 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 substrings that match the -same pair of parentheses when there is a repetition. -

-

-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, callouts 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 position, 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 function 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 opening -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 documented 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_compile() 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 positive assertion, the -assertion succeeds; in a negative assertion, the assertion fails. -

-

-If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For -example: -

-  A((?:A|B(*ACCEPT)|C)D)
-
-This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured 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 pattern: -
-  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 outputting 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 example it -indicates which of the two alternatives matched. This is a more efficient way -of obtaining this information than putting each alternative 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-encountered. 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 continues -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, backtracking can "jump back" to the left of the entire atomic group -or assertion. (Remember also, as stated above, that this localization also -applies in subroutine calls.) -

-

-These verbs differ in exactly what kind of failure occurs when backtracking -reaches them. The behaviour described below is what happens when the verb is -not in a subroutine or an assertion. Subsequent sections 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 backtracking 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 current 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 second 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 backtracking 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 quantifier, 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 encountered. (*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 quantifer 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 backtracking -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 subsequently BAZ fails, there are no -more alternatives, so there is a backtrack 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 subpattern 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 pattern, 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 without 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 negative 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 behaviour), 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 recursively. -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 continues 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: 23 October 2016 -
-Copyright © 1997-2016 University of Cambridge. -
-

-Return to the PCRE index page. -

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