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| author | dartraiden <wowemuh@gmail.com> | 2018-06-01 18:25:57 +0300 |
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| committer | dartraiden <wowemuh@gmail.com> | 2018-06-01 18:26:31 +0300 |
| commit | 0a55fa14f462169bbd8a8de623804f039854f95f (patch) | |
| tree | 19fb2ef7ee1d7b6f3c80b3d83bc010733bc0f58f /libs/Pcre16/docs/doc/html/pcrepattern.html | |
| parent | 25f2c798a74bf6f72f2d6ba40e37a89c662204ba (diff) | |
we only needs license, contributors and version info
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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 @@ -<html> -<head> -<title>pcrepattern specification</title> -</head> -<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> -<h1>pcrepattern man page</h1> -<p> -Return to the <a href="index.html">PCRE index page</a>. -</p> -<p> -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. -<br> -<ul> -<li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a> -<li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a> -<li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a> -<li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a> -<li><a name="TOC5" href="#SEC5">BACKSLASH</a> -<li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a> -<li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a> -<li><a name="TOC8" href="#SEC8">MATCHING A SINGLE DATA UNIT</a> -<li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a> -<li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a> -<li><a name="TOC11" href="#SEC11">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a> -<li><a name="TOC12" href="#SEC12">VERTICAL BAR</a> -<li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a> -<li><a name="TOC14" href="#SEC14">SUBPATTERNS</a> -<li><a name="TOC15" href="#SEC15">DUPLICATE SUBPATTERN NUMBERS</a> -<li><a name="TOC16" href="#SEC16">NAMED SUBPATTERNS</a> -<li><a name="TOC17" href="#SEC17">REPETITION</a> -<li><a name="TOC18" href="#SEC18">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a> -<li><a name="TOC19" href="#SEC19">BACK REFERENCES</a> -<li><a name="TOC20" href="#SEC20">ASSERTIONS</a> -<li><a name="TOC21" href="#SEC21">CONDITIONAL SUBPATTERNS</a> -<li><a name="TOC22" href="#SEC22">COMMENTS</a> -<li><a name="TOC23" href="#SEC23">RECURSIVE PATTERNS</a> -<li><a name="TOC24" href="#SEC24">SUBPATTERNS AS SUBROUTINES</a> -<li><a name="TOC25" href="#SEC25">ONIGURUMA SUBROUTINE SYNTAX</a> -<li><a name="TOC26" href="#SEC26">CALLOUTS</a> -<li><a name="TOC27" href="#SEC27">BACKTRACKING CONTROL</a> -<li><a name="TOC28" href="#SEC28">SEE ALSO</a> -<li><a name="TOC29" href="#SEC29">AUTHOR</a> -<li><a name="TOC30" href="#SEC30">REVISION</a> -</ul> -<br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br> -<P> -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 -<a href="pcresyntax.html"><b>pcresyntax</b></a> -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. -</P> -<P> -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. -</P> -<P> -This document discusses the patterns that are supported by PCRE when one its -main matching functions, <b>pcre_exec()</b> (8-bit) or <b>pcre[16|32]_exec()</b> -(16- or 32-bit), is used. PCRE also has alternative matching functions, -<b>pcre_dfa_exec()</b> and <b>pcre[16|32_dfa_exec()</b>, 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 -<a href="pcrematching.html"><b>pcrematching</b></a> -page. -</P> -<br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br> -<P> -A number of options that can be passed to <b>pcre_compile()</b> 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. -</P> -<br><b> -UTF support -</b><br> -<P> -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: -<pre> - (*UTF8) - (*UTF16) - (*UTF32) - (*UTF) -</pre> -(*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 -<a href="pcreunicode.html"><b>pcreunicode</b></a> -page. -</P> -<P> -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. -</P> -<br><b> -Unicode property support -</b><br> -<P> -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. -</P> -<br><b> -Disabling auto-possessification -</b><br> -<P> -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 -<a href="pcreapi.html"><b>pcreapi</b></a> -documentation. -</P> -<br><b> -Disabling start-up optimizations -</b><br> -<P> -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 -<a href="pcreapi.html"><b>pcreapi</b></a> -documentation. -<a name="newlines"></a></P> -<br><b> -Newline conventions -</b><br> -<P> -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 -<a href="pcreapi.html"><b>pcreapi</b></a> -page has -<a href="pcreapi.html#newlines">further discussion</a> -about newlines, and shows how to set the newline convention in the -<i>options</i> arguments for the compiling and matching functions. -</P> -<P> -It is also possible to specify a newline convention by starting a pattern -string with one of the following five sequences: -<pre> - (*CR) carriage return - (*LF) linefeed - (*CRLF) carriage return, followed by linefeed - (*ANYCRLF) any of the three above - (*ANY) all Unicode newline sequences -</pre> -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 -<pre> - (*CR)a.b -</pre> -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. -</P> -<P> -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 -<a href="#newlineseq">"Newline sequences"</a> -below. A change of \R setting can be combined with a change of newline -convention. -</P> -<br><b> -Setting match and recursion limits -</b><br> -<P> -The caller of <b>pcre_exec()</b> can set a limit on the number of times the -internal <b>match()</b> 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, <b>pcre_exec()</b> -gives an error return. The limits can also be set by items at the start of the -pattern of the form -<pre> - (*LIMIT_MATCH=d) - (*LIMIT_RECURSION=d) -</pre> -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 <b>pcre_exec()</b> -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. -</P> -<br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br> -<P> -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. -</P> -<br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br> -<P> -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 -<pre> - The quick brown fox -</pre> -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. -</P> -<P> -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 -<i>metacharacters</i>, which do not stand for themselves but instead are -interpreted in some special way. -</P> -<P> -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: -<pre> - \ 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 -</pre> -Part of a pattern that is in square brackets is called a "character class". In -a character class the only metacharacters are: -<pre> - \ 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 -</pre> -The following sections describe the use of each of the metacharacters. -</P> -<br><a name="SEC5" href="#TOC1">BACKSLASH</a><br> -<P> -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. -</P> -<P> -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 \\. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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: -<pre> - 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 -</pre> -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. -<a name="digitsafterbackslash"></a></P> -<br><b> -Non-printing characters -</b><br> -<P> -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: -<pre> - \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) -</pre> -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. -</P> -<P> -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 <b>perlebcdic</b> 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). -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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 <i>back reference</i>. A description of how this works is given -<a href="#backreferences">later,</a> -following the discussion of -<a href="#subpattern">parenthesized subpatterns.</a> -</P> -<P> -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: -<pre> - \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" -</pre> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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). -</P> -<br><b> -Constraints on character values -</b><br> -<P> -Characters that are specified using octal or hexadecimal numbers are -limited to certain values, as follows: -<pre> - 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 -</pre> -Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called -"surrogate" codepoints), and 0xffef. -</P> -<br><b> -Escape sequences in character classes -</b><br> -<P> -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). -</P> -<P> -\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. -</P> -<br><b> -Unsupported escape sequences -</b><br> -<P> -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. -</P> -<br><b> -Absolute and relative back references -</b><br> -<P> -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 -<a href="#backreferences">later,</a> -following the discussion of -<a href="#subpattern">parenthesized subpatterns.</a> -</P> -<br><b> -Absolute and relative subroutine calls -</b><br> -<P> -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 -<a href="#onigurumasubroutines">later.</a> -Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> -synonymous. The former is a back reference; the latter is a -<a href="#subpatternsassubroutines">subroutine</a> -call. -<a name="genericchartypes"></a></P> -<br><b> -Generic character types -</b><br> -<P> -Another use of backslash is for specifying generic character types: -<pre> - \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 -</pre> -There is also the single sequence \N, which matches a non-newline character. -This is the same as -<a href="#fullstopdot">the "." metacharacter</a> -when PCRE_DOTALL is not set. Perl also uses \N to match characters by name; -PCRE does not support this. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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 -<a href="pcreapi.html#localesupport">"Locale support"</a> -in the -<a href="pcreapi.html"><b>pcreapi</b></a> -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. -</P> -<P> -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: -<pre> - \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 -</pre> -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. -</P> -<P> -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: -<pre> - 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 -</pre> -The vertical space characters are: -<pre> - 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 -</pre> -In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are -relevant. -<a name="newlineseq"></a></P> -<br><b> -Newline sequences -</b><br> -<P> -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: -<pre> - (?>\r\n|\n|\x0b|\f|\r|\x85) -</pre> -This is an example of an "atomic group", details of which are given -<a href="#atomicgroup">below.</a> -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. -</P> -<P> -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. -</P> -<P> -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: -<pre> - (*BSR_ANYCRLF) CR, LF, or CRLF only - (*BSR_UNICODE) any Unicode newline sequence -</pre> -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: -<pre> - (*ANY)(*BSR_ANYCRLF) -</pre> -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. -<a name="uniextseq"></a></P> -<br><b> -Unicode character properties -</b><br> -<P> -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: -<pre> - \p{<i>xx</i>} a character with the <i>xx</i> property - \P{<i>xx</i>} a character without the <i>xx</i> property - \X a Unicode extended grapheme cluster -</pre> -The property names represented by <i>xx</i> 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 -<a href="#extraprops">next section).</a> -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. -</P> -<P> -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: -<pre> - \p{Greek} - \P{Han} -</pre> -Those that are not part of an identified script are lumped together as -"Common". The current list of scripts is: -</P> -<P> -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. -</P> -<P> -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}. -</P> -<P> -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: -<pre> - \p{L} - \pL -</pre> -The following general category property codes are supported: -<pre> - 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 -</pre> -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". -</P> -<P> -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 -<a href="pcreapi.html"><b>pcreapi</b></a> -page). Perl does not support the Cs property. -</P> -<P> -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". -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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). -</P> -<br><b> -Extended grapheme clusters -</b><br> -<P> -The \X escape matches any number of Unicode characters that form an "extended -grapheme cluster", and treats the sequence as an atomic group -<a href="#atomicgroup">(see below).</a> -Up to and including release 8.31, PCRE matched an earlier, simpler definition -that was equivalent to -<pre> - (?>\PM\pM*) -</pre> -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. -</P> -<P> -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. -</P> -<P> -\X always matches at least one character. Then it decides whether to add -additional characters according to the following rules for ending a cluster: -</P> -<P> -1. End at the end of the subject string. -</P> -<P> -2. Do not end between CR and LF; otherwise end after any control character. -</P> -<P> -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. -</P> -<P> -4. Do not end before extending characters or spacing marks. Characters with -the "mark" property always have the "extend" grapheme breaking property. -</P> -<P> -5. Do not end after prepend characters. -</P> -<P> -6. Otherwise, end the cluster. -<a name="extraprops"></a></P> -<br><b> -PCRE's additional properties -</b><br> -<P> -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: -<pre> - Xan Any alphanumeric character - Xps Any POSIX space character - Xsp Any Perl space character - Xwd Any Perl "word" character -</pre> -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. -</P> -<P> -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.) -<a name="resetmatchstart"></a></P> -<br><b> -Resetting the match start -</b><br> -<P> -The escape sequence \K causes any previously matched characters not to be -included in the final matched sequence. For example, the pattern: -<pre> - foo\Kbar -</pre> -matches "foobar", but reports that it has matched "bar". This feature is -similar to a lookbehind assertion -<a href="#lookbehind">(described below).</a> -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 -<a href="#subpattern">captured substrings.</a> -For example, when the pattern -<pre> - (foo)\Kbar -</pre> -matches "foobar", the first substring is still set to "foo". -</P> -<P> -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. -<a name="smallassertions"></a></P> -<br><b> -Simple assertions -</b><br> -<P> -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 -<a href="#bigassertions">below.</a> -The backslashed assertions are: -<pre> - \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 -</pre> -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. -</P> -<P> -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. -</P> -<P> -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 <i>startoffset</i> -argument of <b>pcre_exec()</b> 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. -</P> -<P> -The \G assertion is true only when the current matching position is at the -start point of the match, as specified by the <i>startoffset</i> argument of -<b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is -non-zero. By calling <b>pcre_exec()</b> 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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br> -<P> -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. -</P> -<P> -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 <i>startoffset</i> argument of -<b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE -option is unset. Inside a character class, circumflex has an entirely different -meaning -<a href="#characterclass">(see below).</a> -</P> -<P> -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.) -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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 <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The -PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. -</P> -<P> -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. -<a name="fullstopdot"></a></P> -<br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<br><a name="SEC8" href="#TOC1">MATCHING A SINGLE DATA UNIT</a><br> -<P> -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). -</P> -<P> -PCRE does not allow \C to appear in lookbehind assertions -<a href="#lookbehind">(described below)</a> -in a UTF mode, because this would make it impossible to calculate the length of -the lookbehind. -</P> -<P> -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): -<pre> - (?| (?=[\x00-\x7f])(\C) | - (?=[\x80-\x{7ff}])(\C)(\C) | - (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | - (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) -</pre> -A group that starts with (?| resets the capturing parentheses numbers in each -alternative (see -<a href="#dupsubpatternnumber">"Duplicate Subpattern Numbers"</a> -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. -<a name="characterclass"></a></P> -<br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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 -<a href="#genericchartypes">"Generic character types"</a> -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. -</P> -<P> -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 ...". -</P> -<P> -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. -</P> -<br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br> -<P> -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, -<pre> - [01[:alpha:]%] -</pre> -matches "0", "1", any alphabetic character, or "%". The supported class names -are: -<pre> - 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 -</pre> -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. -</P> -<P> -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, -<pre> - [12[:^digit:]] -</pre> -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. -</P> -<P> -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 -<b>pcre_compile()</b>, 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: -<pre> - [: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} -</pre> -Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX -classes are handled specially in UCP mode: -</P> -<P> -[: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: -<pre> - U+061C Arabic Letter Mark - U+180E Mongolian Vowel Separator - U+2066 - U+2069 Various "isolate"s - -</PRE> -</P> -<P> -[:print:] -This matches the same characters as [:graph:] plus space characters that are -not controls, that is, characters with the Zs property. -</P> -<P> -[: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. -</P> -<P> -The other POSIX classes are unchanged, and match only characters with code -points less than 128. -</P> -<br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br> -<P> -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: -<pre> - [[:<:]] is converted to \b(?=\w) - [[:>:]] is converted to \b(?<=\w) -</pre> -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 -<a href="#smallassertions">"Simple assertions"</a> -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. -</P> -<br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br> -<P> -Vertical bar characters are used to separate alternative patterns. For example, -the pattern -<pre> - gilbert|sullivan -</pre> -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 -<a href="#subpattern">(defined below),</a> -"succeeds" means matching the rest of the main pattern as well as the -alternative in the subpattern. -</P> -<br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br> -<P> -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 -<pre> - i for PCRE_CASELESS - m for PCRE_MULTILINE - s for PCRE_DOTALL - x for PCRE_EXTENDED -</pre> -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. -</P> -<P> -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. -</P> -<P> -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 -<pre> - (a(?i)b)c -</pre> -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, -<pre> - (a(?i)b|c) -</pre> -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. -</P> -<P> -<b>Note:</b> 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 -<a href="#newlineseq">"Newline sequences"</a> -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. -<a name="subpattern"></a></P> -<br><a name="SEC14" href="#TOC1">SUBPATTERNS</a><br> -<P> -Subpatterns are delimited by parentheses (round brackets), which can be nested. -Turning part of a pattern into a subpattern does two things: -<br> -<br> -1. It localizes a set of alternatives. For example, the pattern -<pre> - cat(aract|erpillar|) -</pre> -matches "cataract", "caterpillar", or "cat". Without the parentheses, it would -match "cataract", "erpillar" or an empty string. -<br> -<br> -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 <i>ovector</i> argument of the -matching function. (This applies only to the traditional matching functions; -the DFA matching functions do not support capturing.) -</P> -<P> -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 -<pre> - the ((red|white) (king|queen)) -</pre> -the captured substrings are "red king", "red", and "king", and are numbered 1, -2, and 3, respectively. -</P> -<P> -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 -<pre> - the ((?:red|white) (king|queen)) -</pre> -the captured substrings are "white queen" and "queen", and are numbered 1 and -2. The maximum number of capturing subpatterns is 65535. -</P> -<P> -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 -<pre> - (?i:saturday|sunday) - (?:(?i)saturday|sunday) -</pre> -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". -<a name="dupsubpatternnumber"></a></P> -<br><a name="SEC15" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br> -<P> -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: -<pre> - (?|(Sat)ur|(Sun))day -</pre> -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. -<pre> - # before ---------------branch-reset----------- after - / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x - # 1 2 2 3 2 3 4 -</pre> -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": -<pre> - /(?|(abc)|(def))\1/ -</pre> -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": -<pre> - /(?|(abc)|(def))(?1)/ -</pre> -If a -<a href="#conditions">condition test</a> -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. -</P> -<P> -An alternative approach to using this "branch reset" feature is to use -duplicate named subpatterns, as described in the next section. -</P> -<br><a name="SEC16" href="#TOC1">NAMED SUBPATTERNS</a><br> -<P> -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. -</P> -<P> -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 -<a href="#backreferences">back references,</a> -<a href="#recursion">recursion,</a> -and -<a href="#conditions">conditions,</a> -can be made by name as well as by number. -</P> -<P> -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. -</P> -<P> -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: -<pre> - (?<DN>Mon|Fri|Sun)(?:day)?| - (?<DN>Tue)(?:sday)?| - (?<DN>Wed)(?:nesday)?| - (?<DN>Thu)(?:rsday)?| - (?<DN>Sat)(?:urday)? -</pre> -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.) -</P> -<P> -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. -</P> -<P> -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": -<pre> - (?:(?<n>foo)|(?<n>bar))\k<n> - -</PRE> -</P> -<P> -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. -</P> -<P> -If you use a named reference in a condition -test (see the -<a href="#conditions">section about conditions</a> -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 -<a href="pcreapi.html"><b>pcreapi</b></a> -documentation. -</P> -<P> -<b>Warning:</b> 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. -</P> -<br><a name="SEC17" href="#TOC1">REPETITION</a><br> -<P> -Repetition is specified by quantifiers, which can follow any of the following -items: -<pre> - 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) -</pre> -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: -<pre> - z{2,4} -</pre> -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 -<pre> - [aeiou]{3,} -</pre> -matches at least 3 successive vowels, but may match many more, while -<pre> - \d{8} -</pre> -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. -</P> -<P> -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). -</P> -<P> -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 -<a href="#subpatternsassubroutines">subroutines</a> -from elsewhere in the pattern (but see also the section entitled -<a href="#subdefine">"Defining subpatterns for use by reference only"</a> -below). Items other than subpatterns that have a {0} quantifier are omitted -from the compiled pattern. -</P> -<P> -For convenience, the three most common quantifiers have single-character -abbreviations: -<pre> - * is equivalent to {0,} - + is equivalent to {1,} - ? is equivalent to {0,1} -</pre> -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: -<pre> - (a?)* -</pre> -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. -</P> -<P> -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 -<pre> - /\*.*\*/ -</pre> -to the string -<pre> - /* first comment */ not comment /* second comment */ -</pre> -fails, because it matches the entire string owing to the greediness of the .* -item. -</P> -<P> -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 -<pre> - /\*.*?\*/ -</pre> -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 -<pre> - \d??\d -</pre> -which matches one digit by preference, but can match two if that is the only -way the rest of the pattern matches. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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: -<pre> - (.*)abc\1 -</pre> -If the subject is "xyz123abc123" the match point is the fourth character. For -this reason, such a pattern is not implicitly anchored. -</P> -<P> -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: -<pre> - (?>.*?a)b -</pre> -It matches "ab" in the subject "aab". The use of the backtracking control verbs -(*PRUNE) and (*SKIP) also disable this optimization. -</P> -<P> -When a capturing subpattern is repeated, the value captured is the substring -that matched the final iteration. For example, after -<pre> - (tweedle[dume]{3}\s*)+ -</pre> -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 -<pre> - /(a|(b))+/ -</pre> -matches "aba" the value of the second captured substring is "b". -<a name="atomicgroup"></a></P> -<br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br> -<P> -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. -</P> -<P> -Consider, for example, the pattern \d+foo when applied to the subject line -<pre> - 123456bar -</pre> -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. -</P> -<P> -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: -<pre> - (?>\d+)foo -</pre> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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 -<pre> - \d++foo -</pre> -Note that a possessive quantifier can be used with an entire group, for -example: -<pre> - (abc|xyz){2,3}+ -</pre> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -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 -<pre> - (\D+|<\d+>)*[!?] -</pre> -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 -<pre> - aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -</pre> -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: -<pre> - ((?>\D+)|<\d+>)*[!?] -</pre> -sequences of non-digits cannot be broken, and failure happens quickly. -<a name="backreferences"></a></P> -<br><a name="SEC19" href="#TOC1">BACK REFERENCES</a><br> -<P> -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. -</P> -<P> -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. -</P> -<P> -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" -<a href="#digitsafterbackslash">above</a> -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). -</P> -<P> -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: -<pre> - (ring), \1 - (ring), \g1 - (ring), \g{1} -</pre> -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: -<pre> - (abc(def)ghi)\g{-1} -</pre> -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. -</P> -<P> -A back reference matches whatever actually matched the capturing subpattern in -the current subject string, rather than anything matching the subpattern -itself (see -<a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a> -below for a way of doing that). So the pattern -<pre> - (sens|respons)e and \1ibility -</pre> -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, -<pre> - ((?i)rah)\s+\1 -</pre> -matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original -capturing subpattern is matched caselessly. -</P> -<P> -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: -<pre> - (?<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} -</pre> -A subpattern that is referenced by name may appear in the pattern before or -after the reference. -</P> -<P> -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 -<pre> - (a|(bc))\2 -</pre> -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. -</P> -<P> -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 -<a href="#comments">"Comments"</a> -below) can be used. -</P> -<br><b> -Recursive back references -</b><br> -<P> -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 -<pre> - (a|b\1)+ -</pre> -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. -</P> -<P> -Back references of this type cause the group that they reference to be treated -as an -<a href="#atomicgroup">atomic group.</a> -Once the whole group has been matched, a subsequent matching failure cannot -cause backtracking into the middle of the group. -<a name="bigassertions"></a></P> -<br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br> -<P> -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 -<a href="#smallassertions">above.</a> -</P> -<P> -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. -</P> -<P> -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.) -</P> -<P> -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. -</P> -<P> -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: -<br> -<br> -(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 -<a href="#subpatternsassubroutines">subroutine mechanism.</a> -<br> -<br> -(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. -<br> -<br> -(3) If the minimum repetition is greater than zero, the quantifier is ignored. -The assertion is obeyed just once when encountered during matching. -</P> -<br><b> -Lookahead assertions -</b><br> -<P> -Lookahead assertions start with (?= for positive assertions and (?! for -negative assertions. For example, -<pre> - \w+(?=;) -</pre> -matches a word followed by a semicolon, but does not include the semicolon in -the match, and -<pre> - foo(?!bar) -</pre> -matches any occurrence of "foo" that is not followed by "bar". Note that the -apparently similar pattern -<pre> - (?!foo)bar -</pre> -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. -</P> -<P> -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 (?!). -<a name="lookbehind"></a></P> -<br><b> -Lookbehind assertions -</b><br> -<P> -Lookbehind assertions start with (?<= for positive assertions and (?<! for -negative assertions. For example, -<pre> - (?<!foo)bar -</pre> -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 -<pre> - (?<=bullock|donkey) -</pre> -is permitted, but -<pre> - (?<!dogs?|cats?) -</pre> -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 -<pre> - (?<=ab(c|de)) -</pre> -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: -<pre> - (?<=abc|abde) -</pre> -In some cases, the escape sequence \K -<a href="#resetmatchstart">(see above)</a> -can be used instead of a lookbehind assertion to get round the fixed-length -restriction. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -<a href="#subpatternsassubroutines">"Subroutine"</a> -calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long -as the subpattern matches a fixed-length string. -<a href="#recursion">Recursion,</a> -however, is not supported. -</P> -<P> -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 -<pre> - abcd$ -</pre> -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 -<pre> - ^.*abcd$ -</pre> -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 -<pre> - ^.*+(?<=abcd) -</pre> -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. -</P> -<br><b> -Using multiple assertions -</b><br> -<P> -Several assertions (of any sort) may occur in succession. For example, -<pre> - (?<=\d{3})(?<!999)foo -</pre> -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 <i>not</i> 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 -<pre> - (?<=\d{3}...)(?<!999)foo -</pre> -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". -</P> -<P> -Assertions can be nested in any combination. For example, -<pre> - (?<=(?<!foo)bar)baz -</pre> -matches an occurrence of "baz" that is preceded by "bar" which in turn is not -preceded by "foo", while -<pre> - (?<=\d{3}(?!999)...)foo -</pre> -is another pattern that matches "foo" preceded by three digits and any three -characters that are not "999". -<a name="conditions"></a></P> -<br><a name="SEC21" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br> -<P> -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: -<pre> - (?(condition)yes-pattern) - (?(condition)yes-pattern|no-pattern) -</pre> -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: -<pre> - (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) - -</PRE> -</P> -<P> -There are four kinds of condition: references to subpatterns, references to -recursion, a pseudo-condition called DEFINE, and assertions. -</P> -<br><b> -Checking for a used subpattern by number -</b><br> -<P> -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 -<a href="#recursion">section about duplicate subpattern numbers),</a> -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.) -</P> -<P> -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: -<pre> - ( \( )? [^()]+ (?(1) \) ) -</pre> -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. -</P> -<P> -If you were embedding this pattern in a larger one, you could use a relative -reference: -<pre> - ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... -</pre> -This makes the fragment independent of the parentheses in the larger pattern. -</P> -<br><b> -Checking for a used subpattern by name -</b><br> -<P> -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. -</P> -<P> -Rewriting the above example to use a named subpattern gives this: -<pre> - (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) -</pre> -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. -</P> -<br><b> -Checking for pattern recursion -</b><br> -<P> -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: -<pre> - (?(R3)...) or (?(R&name)...) -</pre> -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. -</P> -<P> -At "top level", all these recursion test conditions are false. -<a href="#recursion">The syntax for recursive patterns</a> -is described below. -<a name="subdefine"></a></P> -<br><b> -Defining subpatterns for use by reference only -</b><br> -<P> -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 -<a href="#subpatternsassubroutines">subroutines</a> -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): -<pre> - (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) - \b (?&byte) (\.(?&byte)){3} \b -</pre> -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. -</P> -<br><b> -Assertion conditions -</b><br> -<P> -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: -<pre> - (?(?=[^a-z]*[a-z]) - \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) -</pre> -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. -<a name="comments"></a></P> -<br><a name="SEC22" href="#TOC1">COMMENTS</a><br> -<P> -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. -</P> -<P> -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 -<a href="#newlines">"Newline conventions"</a> -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: -<pre> - abc #comment \n still comment -</pre> -On encountering the # character, <b>pcre_compile()</b> 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. -<a name="recursion"></a></P> -<br><a name="SEC23" href="#TOC1">RECURSIVE PATTERNS</a><br> -<P> -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. -</P> -<P> -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: -<pre> - $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; -</pre> -The (?p{...}) item interpolates Perl code at run time, and in this case refers -recursively to the pattern in which it appears. -</P> -<P> -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. -</P> -<P> -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 -<a href="#subpatternsassubroutines">non-recursive subroutine</a> -call, which is described in the next section.) The special item (?R) or (?0) is -a recursive call of the entire regular expression. -</P> -<P> -This PCRE pattern solves the nested parentheses problem (assume the -PCRE_EXTENDED option is set so that white space is ignored): -<pre> - \( ( [^()]++ | (?R) )* \) -</pre> -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. -</P> -<P> -If this were part of a larger pattern, you would not want to recurse the entire -pattern, so instead you could use this: -<pre> - ( \( ( [^()]++ | (?1) )* \) ) -</pre> -We have put the pattern into parentheses, and caused the recursion to refer to -them instead of the whole pattern. -</P> -<P> -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. -</P> -<P> -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 -<a href="#subpatternsassubroutines">non-recursive subroutine</a> -calls, as described in the next section. -</P> -<P> -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: -<pre> - (?<pn> \( ( [^()]++ | (?&pn) )* \) ) -</pre> -If there is more than one subpattern with the same name, the earliest one is -used. -</P> -<P> -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 -<pre> - (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() -</pre> -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. -</P> -<P> -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 -<a href="pcrecallout.html"><b>pcrecallout</b></a> -documentation). If the pattern above is matched against -<pre> - (ab(cd)ef) -</pre> -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. -</P> -<P> -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 -<b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no memory can -be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. -</P> -<P> -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. -<pre> - < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > -</pre> -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. -<a name="recursiondifference"></a></P> -<br><b> -Differences in recursion processing between PCRE and Perl -</b><br> -<P> -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"): -<pre> - ^(.|(.)(?1)\2)$ -</pre> -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": -</P> -<P> -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). -</P> -<P> -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: -<pre> - ^((.)(?1)\2|.)$ -</pre> -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. -</P> -<P> -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: -<pre> - ^((.)(?1)\2|.?)$ -</pre> -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: -<pre> - ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) -</pre> -If you want to match typical palindromic phrases, the pattern has to ignore all -non-word characters, which can be done like this: -<pre> - ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ -</pre> -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. -</P> -<P> -<b>WARNING</b>: 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. -</P> -<P> -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: -<pre> - ^(.)(\1|a(?2)) -</pre> -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. -<a name="subpatternsassubroutines"></a></P> -<br><a name="SEC24" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br> -<P> -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: -<pre> - (...(absolute)...)...(?2)... - (...(relative)...)...(?-1)... - (...(?+1)...(relative)... -</pre> -An earlier example pointed out that the pattern -<pre> - (sens|respons)e and \1ibility -</pre> -matches "sense and sensibility" and "response and responsibility", but not -"sense and responsibility". If instead the pattern -<pre> - (sens|respons)e and (?1)ibility -</pre> -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. -</P> -<P> -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. -</P> -<P> -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: -<pre> - (abc)(?i:(?-1)) -</pre> -It matches "abcabc". It does not match "abcABC" because the change of -processing option does not affect the called subpattern. -<a name="onigurumasubroutines"></a></P> -<br><a name="SEC25" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br> -<P> -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: -<pre> - (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) - (sens|respons)e and \g'1'ibility -</pre> -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: -<pre> - (abc)(?i:\g<-1>) -</pre> -Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i> -synonymous. The former is a back reference; the latter is a subroutine call. -</P> -<br><a name="SEC26" href="#TOC1">CALLOUTS</a><br> -<P> -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. -</P> -<P> -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 <i>pcre_callout</i> -(8-bit library) or <i>pcre[16|32]_callout</i> (16-bit or 32-bit library). -By default, this variable contains NULL, which disables all calling out. -</P> -<P> -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: -<pre> - (?C1)abc(?C2)def -</pre> -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: -<pre> - (?(?C9)(?=a)abc|def) -</pre> -Note that this applies only to assertion conditions, not to other types of -condition. -</P> -<P> -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. -</P> -<P> -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 -<a href="pcrecallout.html"><b>pcrecallout</b></a> -documentation. -<a name="backtrackcontrol"></a></P> -<br><a name="SEC27" href="#TOC1">BACKTRACKING CONTROL</a><br> -<P> -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. -</P> -<P> -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. -</P> -<P> -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. -</P> -<P> -The behaviour of these verbs in -<a href="#btrepeat">repeated groups,</a> -<a href="#btassert">assertions,</a> -and in -<a href="#btsub">subpatterns called as subroutines</a> -(whether or not recursively) is documented below. -<a name="nooptimize"></a></P> -<br><b> -Optimizations that affect backtracking verbs -</b><br> -<P> -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 <b>pcre_compile()</b> or <b>pcre_exec()</b>, or by starting the -pattern with (*NO_START_OPT). There is more discussion of this option in the -section entitled -<a href="pcreapi.html#execoptions">"Option bits for <b>pcre_exec()</b>"</a> -in the -<a href="pcreapi.html"><b>pcreapi</b></a> -documentation. -</P> -<P> -Experiments with Perl suggest that it too has similar optimizations, sometimes -leading to anomalous results. -</P> -<br><b> -Verbs that act immediately -</b><br> -<P> -The following verbs act as soon as they are encountered. They may not be -followed by a name. -<pre> - (*ACCEPT) -</pre> -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. -</P> -<P> -If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For -example: -<pre> - A((?:A|B(*ACCEPT)|C)D) -</pre> -This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by -the outer parentheses. -<pre> - (*FAIL) or (*F) -</pre> -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: -<pre> - a+(?C)(*FAIL) -</pre> -A match with the string "aaaa" always fails, but the callout is taken before -each backtrack happens (in this example, 10 times). -</P> -<br><b> -Recording which path was taken -</b><br> -<P> -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). -<pre> - (*MARK:NAME) or (*:NAME) -</pre> -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. -</P> -<P> -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 -<a href="pcreapi.html#extradata">"Extra data for <b>pcre_exec()</b>"</a> -in the -<a href="pcreapi.html"><b>pcreapi</b></a> -documentation. Here is an example of <b>pcretest</b> output, where the /K -modifier requests the retrieval and outputting of (*MARK) data: -<pre> - re> /X(*MARK:A)Y|X(*MARK:B)Z/K - data> XY - 0: XY - MK: A - XZ - 0: XZ - MK: B -</pre> -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. -</P> -<P> -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. -</P> -<P> -After a partial match or a failed match, the last encountered name in the -entire match process is returned. For example: -<pre> - re> /X(*MARK:A)Y|X(*MARK:B)Z/K - data> XP - No match, mark = B -</pre> -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. -</P> -<P> -If you are interested in (*MARK) values after failed matches, you should -probably set the PCRE_NO_START_OPTIMIZE option -<a href="#nooptimize">(see above)</a> -to ensure that the match is always attempted. -</P> -<br><b> -Verbs that act after backtracking -</b><br> -<P> -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.) -</P> -<P> -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. -<pre> - (*COMMIT) -</pre> -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 <b>pcre_exec()</b> is -committed to finding a match at the current starting point, or not at all. For -example: -<pre> - a+(*COMMIT)b -</pre> -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. -</P> -<P> -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. -</P> -<P> -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 <b>pcretest</b>: -<pre> - re> /(*COMMIT)abc/ - data> xyzabc - 0: abc - data> xyzabc\Y - No match -</pre> -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 <b>pcretest</b> program. It causes -the PCRE_NO_START_OPTIMIZE option to be set when <b>pcre_exec()</b> 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. -<pre> - (*PRUNE) or (*PRUNE:NAME) -</pre> -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). -</P> -<P> -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). -<pre> - (*SKIP) -</pre> -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: -<pre> - a+(*SKIP)b -</pre> -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". -<pre> - (*SKIP:NAME) -</pre> -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. -</P> -<P> -Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores -names that are set by (*PRUNE:NAME) or (*THEN:NAME). -<pre> - (*THEN) or (*THEN:NAME) -</pre> -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: -<pre> - ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... -</pre> -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). -</P> -<P> -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). -</P> -<P> -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: -<pre> - A (B(*THEN)C) | D -</pre> -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: -<pre> - A (B(*THEN)C | (*FAIL)) | D -</pre> -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. -</P> -<P> -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: -<pre> - ^.*? (?(?=a) a | b(*THEN)c ) -</pre> -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.) -</P> -<P> -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. -</P> -<br><b> -More than one backtracking verb -</b><br> -<P> -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: -<pre> - (A(*COMMIT)B(*THEN)C|ABD) -</pre> -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: -<pre> - ...(*COMMIT)(*PRUNE)... -</pre> -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). -<a name="btrepeat"></a></P> -<br><b> -Backtracking verbs in repeated groups -</b><br> -<P> -PCRE differs from Perl in its handling of backtracking verbs in repeated -groups. For example, consider: -<pre> - /(a(*COMMIT)b)+ac/ -</pre> -If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in -the second repeat of the group acts. -<a name="btassert"></a></P> -<br><b> -Backtracking verbs in assertions -</b><br> -<P> -(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack. -</P> -<P> -(*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. -</P> -<P> -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. -</P> -<P> -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). -<a name="btsub"></a></P> -<br><b> -Backtracking verbs in subroutines -</b><br> -<P> -These behaviours occur whether or not the subpattern is called recursively. -Perl's treatment of subroutines is different in some cases. -</P> -<P> -(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces -an immediate backtrack. -</P> -<P> -(*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. -</P> -<P> -(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause -the subroutine match to fail. -</P> -<P> -(*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. -</P> -<br><a name="SEC28" href="#TOC1">SEE ALSO</a><br> -<P> -<b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), -<b>pcresyntax</b>(3), <b>pcre</b>(3), <b>pcre16(3)</b>, <b>pcre32(3)</b>. -</P> -<br><a name="SEC29" href="#TOC1">AUTHOR</a><br> -<P> -Philip Hazel -<br> -University Computing Service -<br> -Cambridge CB2 3QH, England. -<br> -</P> -<br><a name="SEC30" href="#TOC1">REVISION</a><br> -<P> -Last updated: 23 October 2016 -<br> -Copyright © 1997-2016 University of Cambridge. -<br> -<p> -Return to the <a href="index.html">PCRE index page</a>. -</p> |