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+
+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.
+
+
+The syntax and semantics of the regular expressions that are supported by PCRE +are described in detail below. There is a quick-reference syntax summary in the +pcresyntax +page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE +also supports some alternative regular expression syntax (which does not +conflict with the Perl syntax) in order to provide some compatibility with +regular expressions in Python, .NET, and Oniguruma. +
++Perl's regular expressions are described in its own documentation, and +regular expressions in general are covered in a number of books, some of which +have copious examples. Jeffrey Friedl's "Mastering Regular Expressions", +published by O'Reilly, covers regular expressions in great detail. This +description of PCRE's regular expressions is intended as reference material. +
++The original operation of PCRE was on strings of one-byte characters. However, +there is now also support for UTF-8 character strings. To use this, you must +build PCRE to include UTF-8 support, and then call pcre_compile() with +the PCRE_UTF8 option. How this affects pattern matching is mentioned in several +places below. There is also a summary of UTF-8 features in the +section on UTF-8 support +in the main +pcre +page. +
++The remainder of this document discusses the patterns that are supported by +PCRE when its main matching function, pcre_exec(), is used. +From release 6.0, PCRE offers a second matching function, +pcre_dfa_exec(), which matches using a different algorithm that is not +Perl-compatible. Some of the features discussed below are not available when +pcre_dfa_exec() is used. The advantages and disadvantages of the +alternative function, and how it differs from the normal function, are +discussed in the +pcrematching +page. +
++PCRE supports five different conventions for indicating line breaks in +strings: a single CR (carriage return) character, a single LF (linefeed) +character, the two-character sequence CRLF, any of the three preceding, or any +Unicode newline sequence. The +pcreapi +page has +further discussion +about newlines, and shows how to set the newline convention in the +options arguments for the compiling and matching functions. +
++It is also possible to specify a newline convention by starting a pattern +string with one of the following five sequences: +
+ (*CR) carriage return + (*LF) linefeed + (*CRLF) carriage return, followed by linefeed + (*ANYCRLF) any of the three above + (*ANY) all Unicode newline sequences ++These override the default and the options given to pcre_compile(). For +example, on a Unix system where LF is the default newline sequence, the pattern +
+ (*CR)a.b ++changes the convention to CR. That pattern matches "a\nb" because LF is no +longer a newline. 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. + +
+The newline convention does not affect what the \R escape sequence matches. By +default, this is any Unicode newline sequence, for Perl compatibility. However, +this can be changed; see the description of \R in the section entitled +"Newline sequences" +below. A change of \R setting can be combined with a change of newline +convention. +
++A regular expression is a pattern that is matched against a subject string from +left to right. Most characters stand for themselves in a pattern, and match the +corresponding characters in the subject. As a trivial example, the pattern +
+ The quick brown fox ++matches a portion of a subject string that is identical to itself. When +caseless matching is specified (the PCRE_CASELESS option), letters are matched +independently of case. In UTF-8 mode, PCRE always understands the concept of +case for characters whose values are less than 128, so caseless matching is +always possible. For characters with higher values, the concept of case is +supported if PCRE is 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-8 support. + +
+The power of regular expressions comes from the ability to include alternatives +and repetitions in the pattern. These are encoded in the pattern by the use of +metacharacters, which do not stand for themselves but instead are +interpreted in some special way. +
++There are two different sets of metacharacters: those that are recognized +anywhere in the pattern except within square brackets, and those that are +recognized within square brackets. Outside square brackets, the metacharacters +are as follows: +
+ \ general escape character with several uses
+ ^ assert start of string (or line, in multiline mode)
+ $ assert end of string (or line, in multiline mode)
+ . match any character except newline (by default)
+ [ start character class definition
+ | start of alternative branch
+ ( start subpattern
+ ) end subpattern
+ ? extends the meaning of (
+ also 0 or 1 quantifier
+ also quantifier minimizer
+ * 0 or more quantifier
+ + 1 or more quantifier
+ also "possessive quantifier"
+ { start min/max quantifier
+
+Part of a pattern that is in square brackets is called a "character class". In
+a character class the only metacharacters are:
++ \ general escape character + ^ negate the class, but only if the first character + - indicates character range + [ POSIX character class (only if followed by POSIX syntax) + ] terminates the character class ++The following sections describe the use of each of the metacharacters. + +
+The backslash character has several uses. Firstly, if it is followed by a +non-alphanumeric character, it takes away any special meaning that character +may have. This use of backslash as an escape character applies both inside and +outside character classes. +
++For example, if you want to match a * character, you write \* in the pattern. +This escaping action applies whether or not the following character would +otherwise be interpreted as a metacharacter, so it is always safe to precede a +non-alphanumeric with backslash to specify that it stands for itself. In +particular, if you want to match a backslash, you write \\. +
++If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the +pattern (other than in a character class) and characters between a # outside +a character class and the next newline are ignored. An escaping backslash can +be used to include a whitespace or # character as part of the pattern. +
++If you want to remove the special meaning from a sequence of characters, you +can do so by putting them between \Q and \E. This is different from Perl in +that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in +Perl, $ and @ cause variable interpolation. Note the following examples: +
+ Pattern PCRE matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz ++The \Q...\E sequence is recognized both inside and outside character classes. + +
+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 usually easier to +use one of the following escape sequences than the binary character it +represents: +
+ \a alarm, that is, the BEL character (hex 07)
+ \cx "control-x", where x is any character
+ \e escape (hex 1B)
+ \f formfeed (hex 0C)
+ \n linefeed (hex 0A)
+ \r carriage return (hex 0D)
+ \t tab (hex 09)
+ \ddd character with octal code ddd, or backreference
+ \xhh character with hex code hh
+ \x{hhh..} character with hex code hhh..
+
+The precise effect of \cx 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 \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
+7B.
+
++After \x, 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 }, but the value of the character code must be less than 256 in non-UTF-8 +mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in +hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code +point, which is 10FFFF. +
++If characters other than hexadecimal digits appear between \x{ and }, or if +there is no terminating }, this form of escape is not recognized. Instead, the +initial \x will be interpreted as a basic hexadecimal escape, with no +following digits, giving a character whose value is zero. +
++Characters whose value is less than 256 can be defined by either of the two +syntaxes for \x. There is no difference in the way they are handled. For +example, \xdc is exactly the same as \x{dc}. +
++After \0 up to two further octal digits are read. If there are fewer than two +digits, just those that are present are used. Thus the sequence \0\x\07 +specifies two binary zeros followed by a BEL character (code value 7). Make +sure you supply two digits after the initial zero if the pattern character that +follows is itself an octal digit. +
++The handling of a backslash followed by a digit other than 0 is complicated. +Outside a character class, PCRE reads it and any following digits as a decimal +number. If the number is less than 10, or if there have been at least that many +previous capturing left parentheses in the expression, the entire sequence is +taken as a back reference. A description of how this works is given +later, +following the discussion of +parenthesized subpatterns. +
++Inside a character class, or if the decimal number is greater than 9 and there +have not been that many capturing subpatterns, PCRE re-reads up to three octal +digits following the backslash, and uses them to generate a data character. Any +subsequent digits stand for themselves. In non-UTF-8 mode, the value of a +character specified in octal must be less than \400. In UTF-8 mode, values up +to \777 are permitted. For example: +
+ \040 is another way of writing a 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 byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero followed by the two characters "8" and "1" ++Note that octal values of 100 or greater must not be introduced by a leading +zero, because no more than three octal digits are ever read. + +
+All the sequences that define a single character value can be used both inside +and outside character classes. In addition, inside a character class, the +sequence \b is interpreted as the backspace character (hex 08), and the +sequences \R and \X are interpreted as the characters "R" and "X", +respectively. Outside a character class, these sequences have different +meanings +(see below). +
++The sequence \g followed by an unsigned or a negative number, optionally +enclosed in braces, is an absolute or relative back reference. A named back +reference can be coded as \g{name}. Back references are discussed +later, +following the discussion of +parenthesized subpatterns. +
++For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or +a number enclosed either in angle brackets or single quotes, is an alternative +syntax for referencing a subpattern as a "subroutine". Details are discussed +later. +Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not +synonymous. The former is a back reference; the latter is a subroutine call. +
++Another use of backslash is for specifying generic character types. The +following are always recognized: +
+ \d any decimal digit + \D any character that is not a decimal digit + \h any horizontal whitespace character + \H any character that is not a horizontal whitespace character + \s any whitespace character + \S any character that is not a whitespace character + \v any vertical whitespace character + \V any character that is not a vertical whitespace character + \w any "word" character + \W any "non-word" character ++Each pair of escape sequences partitions the complete set of characters into +two disjoint sets. Any given character matches one, and only one, of each pair. + +
+These character type 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, since +there is no character to match. +
++For compatibility with Perl, \s does not match the VT character (code 11). +This makes it different from the the POSIX "space" class. The \s characters +are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is +included in a Perl script, \s may match the VT character. In PCRE, it never +does. +
++In UTF-8 mode, characters with values greater than 128 never match \d, \s, or +\w, and always match \D, \S, and \W. This is true even when Unicode +character property support is available. These sequences retain their original +meanings from before UTF-8 support was available, mainly for efficiency +reasons. +
++The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the +other sequences, these do match certain high-valued codepoints in UTF-8 mode. +The horizontal space characters are: +
+ U+0009 Horizontal tab + U+0020 Space + U+00A0 Non-break space + U+1680 Ogham space mark + U+180E Mongolian vowel separator + U+2000 En quad + U+2001 Em quad + U+2002 En space + U+2003 Em space + U+2004 Three-per-em space + U+2005 Four-per-em space + U+2006 Six-per-em space + U+2007 Figure space + U+2008 Punctuation space + U+2009 Thin space + U+200A Hair space + U+202F Narrow no-break space + U+205F Medium mathematical space + U+3000 Ideographic space ++The vertical space characters are: +
+ U+000A Linefeed + U+000B Vertical tab + U+000C Formfeed + U+000D Carriage return + U+0085 Next line + U+2028 Line separator + U+2029 Paragraph separator ++ +
+A "word" character is an underscore or any character less than 256 that is a +letter or digit. The definition of letters and digits is controlled by PCRE's +low-valued character tables, and may vary if locale-specific matching is taking +place (see +"Locale support" +in the +pcreapi +page). For example, in a French locale such as "fr_FR" in Unix-like systems, +or "french" in Windows, some character codes greater than 128 are used for +accented letters, and these are matched by \w. The use of locales with Unicode +is discouraged. +
++Outside a character class, by default, the escape sequence \R matches any +Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is +equivalent to the following: +
+ (?>\r\n|\n|\x0b|\f|\r|\x85) ++This is an example of an "atomic group", details of which are given +below. +This particular group matches either the two-character sequence CR followed by +LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, +U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next +line, U+0085). The two-character sequence is treated as a single unit that +cannot be split. + +
+In UTF-8 mode, two additional characters whose codepoints are greater than 255 +are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). +Unicode character property support is not needed for these characters to be +recognized. +
++It is possible to restrict \R to match only CR, LF, or CRLF (instead of the +complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF +either at compile time or when the pattern is matched. (BSR is an abbrevation +for "backslash R".) This can be made the default when PCRE is built; if this is +the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option. +It is also possible to specify these settings by starting a pattern string with +one of the following sequences: +
+ (*BSR_ANYCRLF) CR, LF, or CRLF only + (*BSR_UNICODE) any Unicode newline sequence ++These override the default and the options given to pcre_compile(), but +they can be overridden by options given to pcre_exec(). Note that these +special settings, which are not Perl-compatible, are recognized only at the +very start of a pattern, and that they must be in upper case. If more than one +of them is present, the last one is used. They can be combined with a change of +newline convention, for example, a pattern can start with: +
+ (*ANY)(*BSR_ANYCRLF) ++Inside a character class, \R matches the letter "R". + +
+When PCRE is built with Unicode character property support, three additional +escape sequences that match characters with specific properties are available. +When not in UTF-8 mode, these sequences are of course limited to testing +characters whose codepoints are less than 256, but they do work in this mode. +The extra escape sequences are: +
+ \p{xx} a character with the xx property
+ \P{xx} a character without the xx property
+ \X an extended Unicode sequence
+
+The property names represented by xx above are limited to the Unicode
+script names, the general category properties, and "Any", which matches any
+character (including newline). Other properties such as "InMusicalSymbols" are
+not currently supported by PCRE. Note that \P{Any} does not match any
+characters, so always causes a match failure.
+
++Sets of Unicode characters are defined as belonging to certain scripts. A +character from one of these sets can be matched using a script name. For +example: +
+ \p{Greek}
+ \P{Han}
+
+Those that are not part of an identified script are lumped together as
+"Common". The current list of scripts is:
+
++Arabic, +Armenian, +Balinese, +Bengali, +Bopomofo, +Braille, +Buginese, +Buhid, +Canadian_Aboriginal, +Cherokee, +Common, +Coptic, +Cuneiform, +Cypriot, +Cyrillic, +Deseret, +Devanagari, +Ethiopic, +Georgian, +Glagolitic, +Gothic, +Greek, +Gujarati, +Gurmukhi, +Han, +Hangul, +Hanunoo, +Hebrew, +Hiragana, +Inherited, +Kannada, +Katakana, +Kharoshthi, +Khmer, +Lao, +Latin, +Limbu, +Linear_B, +Malayalam, +Mongolian, +Myanmar, +New_Tai_Lue, +Nko, +Ogham, +Old_Italic, +Old_Persian, +Oriya, +Osmanya, +Phags_Pa, +Phoenician, +Runic, +Shavian, +Sinhala, +Syloti_Nagri, +Syriac, +Tagalog, +Tagbanwa, +Tai_Le, +Tamil, +Telugu, +Thaana, +Thai, +Tibetan, +Tifinagh, +Ugaritic, +Yi. +
++Each character has exactly one general category property, specified by a +two-letter abbreviation. For compatibility with Perl, negation can be specified +by including a circumflex between the opening brace and the property name. For +example, \p{^Lu} is the same as \P{Lu}. +
++If only one letter is specified with \p or \P, it includes all the general +category properties that start with that letter. In this case, in the absence +of negation, the curly brackets in the escape sequence are optional; these two +examples have the same effect: +
+ \p{L}
+ \pL
+
+The following general category property codes are supported:
++ C Other + Cc Control + Cf Format + Cn Unassigned + Co Private use + Cs Surrogate + + L Letter + Ll Lower case letter + Lm Modifier letter + Lo Other letter + Lt Title case letter + Lu Upper case letter + + M Mark + Mc Spacing mark + Me Enclosing mark + Mn Non-spacing mark + + N Number + Nd Decimal number + Nl Letter number + No Other number + + P Punctuation + Pc Connector punctuation + Pd Dash punctuation + Pe Close punctuation + Pf Final punctuation + Pi Initial punctuation + Po Other punctuation + Ps Open punctuation + + S Symbol + Sc Currency symbol + Sk Modifier symbol + Sm Mathematical symbol + So Other symbol + + Z Separator + Zl Line separator + Zp Paragraph separator + Zs Space separator ++The special property L& is also supported: it matches a character that has +the Lu, Ll, or Lt property, in other words, a letter that is not classified as +a modifier or "other". + +
+The Cs (Surrogate) property applies only to characters in the range U+D800 to +U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so +cannot be tested by PCRE, unless UTF-8 validity checking has been turned off +(see the discussion of PCRE_NO_UTF8_CHECK in the +pcreapi +page). +
++The long synonyms for these properties that Perl supports (such as \p{Letter}) +are not supported by PCRE, nor is it permitted to prefix any of these +properties with "Is". +
++No character that is in the Unicode table has the Cn (unassigned) property. +Instead, this property is assumed for any code point that is not in the +Unicode table. +
++Specifying caseless matching does not affect these escape sequences. For +example, \p{Lu} always matches only upper case letters. +
++The \X escape matches any number of Unicode characters that form an extended +Unicode sequence. \X is equivalent to +
+ (?>\PM\pM*) ++That is, it matches a character without the "mark" property, followed by zero +or more characters with the "mark" property, and treats the sequence as an +atomic group +(see below). +Characters with the "mark" property are typically accents that affect the +preceding character. None of them have codepoints less than 256, so in +non-UTF-8 mode \X matches any one character. + +
+Matching characters by Unicode property is not fast, because PCRE has to search +a structure that contains data for over fifteen thousand characters. That is +why the traditional escape sequences such as \d and \w do not use Unicode +properties in PCRE. +
++The escape sequence \K, which is a Perl 5.10 feature, causes any previously +matched characters not to be included in the final matched sequence. For +example, the pattern: +
+ foo\Kbar ++matches "foobar", but reports that it has matched "bar". This feature is +similar to a lookbehind assertion +(described below). +However, in this case, the part of the subject before the real match does not +have to be of fixed length, as lookbehind assertions do. The use of \K does +not interfere with the setting of +captured substrings. +For example, when the pattern +
+ (foo)\Kbar ++matches "foobar", the first substring is still set to "foo". + +
+The final use of backslash is for certain simple assertions. An assertion +specifies a condition that has to be met at a particular point in a match, +without consuming any characters from the subject string. The use of +subpatterns for more complicated assertions is described +below. +The backslashed assertions are: +
+ \b matches at a word boundary + \B matches when not at a word boundary + \A matches at the start of the subject + \Z matches at the end of the subject + also matches before a newline at the end of the subject + \z matches only at the end of the subject + \G matches at the first matching position in the subject ++These assertions may not appear in character classes (but note that \b has a +different meaning, namely the backspace character, inside a character class). + +
+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. +
++The \A, \Z, and \z assertions differ from the traditional circumflex and +dollar (described in the next section) in that they only ever match at the very +start and end of the subject string, whatever options are set. Thus, they are +independent of multiline mode. These three assertions are not affected by the +PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the +circumflex and dollar metacharacters. However, if the startoffset +argument of pcre_exec() is non-zero, indicating that matching is to start +at a point other than the beginning of the subject, \A can never match. The +difference between \Z and \z is that \Z matches before a newline at the end +of the string as well as at the very end, whereas \z matches only at the end. +
++The \G assertion is true only when the current matching position is at the +start point of the match, as specified by the startoffset argument of +pcre_exec(). It differs from \A when the value of startoffset is +non-zero. By calling pcre_exec() multiple times with appropriate +arguments, you can mimic Perl's /g option, and it is in this kind of +implementation where \G can be useful. +
++Note, however, that PCRE's interpretation of \G, as the start of the current +match, is subtly different from Perl's, which defines it as the end of the +previous match. In Perl, these can be different when the previously matched +string was empty. Because PCRE does just one match at a time, it cannot +reproduce this behaviour. +
++If all the alternatives of a pattern begin with \G, the expression is anchored +to the starting match position, and the "anchored" flag is set in the compiled +regular expression. +
++Outside a character class, in the default matching mode, the circumflex +character is an assertion that is true only if the current matching point is +at the start of the subject string. If the startoffset argument of +pcre_exec() is non-zero, circumflex can never match if the PCRE_MULTILINE +option is unset. Inside a character class, circumflex has an entirely different +meaning +(see below). +
++Circumflex need not be the first character of the pattern if a number of +alternatives are involved, but it should be the first thing in each alternative +in which it appears if the pattern is ever to match that branch. If all +possible alternatives start with a circumflex, that is, if the pattern is +constrained to match only at the start of the subject, it is said to be an +"anchored" pattern. (There are also other constructs that can cause a pattern +to be anchored.) +
++A 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). Dollar need not be the last character of +the pattern if a number of alternatives are involved, but it should be the last +item in any branch in which it appears. Dollar has no special meaning in a +character class. +
++The meaning of dollar can be changed so that it matches only at the very end of +the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This +does not affect the \Z assertion. +
++The meanings of the circumflex and dollar characters are changed if the +PCRE_MULTILINE option is set. When this is the case, a circumflex matches +immediately after internal newlines as well as at the start of the subject +string. It does not match after a newline that ends the string. A dollar +matches before any newlines in the string, as well as at the very end, when +PCRE_MULTILINE is set. When newline is specified as the two-character +sequence CRLF, isolated CR and LF characters do not indicate newlines. +
++For example, the pattern /^abc$/ matches the subject string "def\nabc" (where +\n represents a newline) in multiline mode, but not otherwise. Consequently, +patterns that are anchored in single line mode because all branches start with +^ are not anchored in multiline mode, and a match for circumflex is possible +when the startoffset argument of pcre_exec() is non-zero. The +PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. +
++Note that the sequences \A, \Z, and \z can be used to match the start and +end of the subject in both modes, and if all branches of a pattern start with +\A it is always anchored, whether or not PCRE_MULTILINE is set. +
++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. In UTF-8 mode, the matched character may be more than one byte long. +
++When a line ending is defined as a single character, dot never matches that +character; when the two-character sequence CRLF is used, dot does not match CR +if it is immediately followed by LF, but otherwise it matches all characters +(including isolated CRs and LFs). When any Unicode line endings are being +recognized, dot does not match CR or LF or any of the other line ending +characters. +
++The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL +option is set, a dot matches any one character, without exception. If the +two-character sequence CRLF is present in the subject string, it takes two dots +to match it. +
++The handling of dot is entirely independent of the handling of circumflex and +dollar, the only relationship being that they both involve newlines. Dot has no +special meaning in a character class. +
++Outside a character class, the escape sequence \C matches any one byte, both +in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending +characters. The feature is provided in Perl in order to match individual bytes +in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, +what remains in the string may be a malformed UTF-8 string. For this reason, +the \C escape sequence is best avoided. +
++PCRE does not allow \C to appear in lookbehind assertions +(described below), +because in UTF-8 mode this would make it impossible to calculate the length of +the lookbehind. +
++An opening square bracket introduces a character class, terminated by a closing +square bracket. A closing square bracket on its own is not special. If a +closing square bracket is required as a member of the class, it should be the +first data character in the class (after an initial circumflex, if present) or +escaped with a backslash. +
++A character class matches a single character in the subject. In UTF-8 mode, the +character may occupy more than one byte. A matched character must be in the set +of characters defined by the class, unless the first character in the class +definition is a circumflex, in which case the subject character must not be in +the set defined by the class. If a circumflex is actually required as a member +of the class, ensure it is not the first character, or escape it with a +backslash. +
++For example, the character class [aeiou] matches any lower case vowel, while +[^aeiou] matches any character that is not a lower case vowel. Note that a +circumflex is just a convenient notation for specifying the characters that +are in the class by enumerating those that are not. A class that starts with a +circumflex is not an assertion: it still consumes a character from the subject +string, and therefore it fails if the current pointer is at the end of the +string. +
++In UTF-8 mode, characters with values greater than 255 can be included in a +class as a literal string of bytes, or by using the \x{ escaping mechanism. +
++When caseless matching is set, any letters in a class represent both their +upper case and lower case versions, so for example, a caseless [aeiou] matches +"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a +caseful version would. In UTF-8 mode, PCRE always understands the concept of +case for characters whose values are less than 128, so caseless matching is +always possible. For characters with higher values, the concept of case is +supported if PCRE is 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-8 support. +
++Characters that might indicate line breaks are never treated in any special way +when matching character classes, whatever line-ending sequence is in use, and +whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class +such as [^a] always matches one of these characters. +
++The minus (hyphen) character can be used to specify a range of characters in a +character class. For example, [d-m] matches any letter between d and m, +inclusive. If a minus character is required in a class, it must be escaped with +a backslash or appear in a position where it cannot be interpreted as +indicating a range, typically as the first or last character in the class. +
++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. +
++Ranges operate in the collating sequence of character values. They can also be +used for characters specified numerically, for example [\000-\037]. In UTF-8 +mode, ranges can include characters whose values are greater than 255, for +example [\x{100}-\x{2ff}]. +
++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 non-UTF-8 mode, if character +tables for a French locale are in use, [\xc8-\xcb] matches accented E +characters in both cases. In UTF-8 mode, PCRE supports the concept of case for +characters with values greater than 128 only when it is compiled with Unicode +property support. +
++The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear +in a character class, and add the characters that they match to the class. For +example, [\dABCDEF] matches any hexadecimal digit. 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. +
++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 - see the next section), and the terminating +closing square bracket. However, escaping other non-alphanumeric characters +does no harm. +
++Perl supports the POSIX notation for character classes. This uses names +enclosed by [: and :] within the enclosing square brackets. PCRE also supports +this notation. For example, +
+ [01[:alpha:]%] ++matches "0", "1", any alphabetic character, or "%". The supported class names +are +
+ alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits + space white space (not quite the same as \s) + upper upper case letters + word "word" characters (same as \w) + xdigit hexadecimal digits ++The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and +space (32). Notice that this list includes the VT character (code 11). This +makes "space" different to \s, which does not include VT (for Perl +compatibility). + +
+The name "word" is a Perl extension, and "blank" is a GNU extension from Perl +5.8. Another Perl extension is negation, which is indicated by a ^ character +after the colon. For example, +
+ [12[:^digit:]] ++matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX +syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not +supported, and an error is given if they are encountered. + +
+In UTF-8 mode, characters with values greater than 128 do not match any of +the POSIX character classes. +
++Vertical bar characters are used to separate alternative patterns. For example, +the pattern +
+ gilbert|sullivan ++matches either "gilbert" or "sullivan". Any number of alternatives may appear, +and an empty alternative is permitted (matching the empty string). The matching +process tries each alternative in turn, from left to right, and the first one +that succeeds is used. If the alternatives are within a subpattern +(defined below), +"succeeds" means matching the rest of the main pattern as well as the +alternative in the subpattern. + +
+The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and +PCRE_EXTENDED options (which are Perl-compatible) can be changed from within +the pattern by a sequence of Perl option letters enclosed between "(?" and ")". +The option letters are +
+ i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED ++For example, (?im) sets caseless, multiline matching. It is also possible to +unset these options by preceding the letter with a hyphen, and a combined +setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and +PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also +permitted. If a letter appears both before and after the hyphen, the option is +unset. + +
+The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be +changed in the same way as the Perl-compatible options by using the characters +J, U and X respectively. +
++When an option change occurs at top level (that is, not inside subpattern +parentheses), the change applies to the remainder of the pattern that follows. +If the change is placed right at the start of a pattern, PCRE extracts it into +the global options (and it will therefore show up in data extracted by the +pcre_fullinfo() function). +
++An option change within a subpattern (see below for a description of +subpatterns) affects only that part of the current pattern that follows it, so +
+ (a(?i)b)c ++matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used). +By this means, options can be made to have different settings in different +parts of the pattern. Any changes made in one alternative do carry on +into subsequent branches within the same subpattern. For example, +
+ (a(?i)b|c) ++matches "ab", "aB", "c", and "C", even though when matching "C" the first +branch is abandoned before the option setting. This is because the effects of +option settings happen at compile time. There would be some very weird +behaviour otherwise. + +
+Note: There are other PCRE-specific options that can be set by the +application when the compile or match functions are called. In some cases the +pattern can contain special leading sequences to override what the application +has set or what has been defaulted. Details are given in the section entitled +"Newline sequences" +above. +
+
+Subpatterns are delimited by parentheses (round brackets), which can be nested.
+Turning part of a pattern into a subpattern does two things:
+
+
+1. It localizes a set of alternatives. For example, the pattern
+
+ cat(aract|erpillar|) ++matches one of the words "cat", "cataract", or "caterpillar". Without the +parentheses, it would match "cataract", "erpillar" or an empty string. +
+For example, if the string "the red king" is matched against the pattern +
+ the ((red|white) (king|queen)) ++the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3, respectively. + +
+The fact that plain parentheses fulfil two functions is not always helpful. +There are often times when a grouping subpattern is required without a +capturing requirement. If an opening parenthesis is followed by a question mark +and a colon, the subpattern does not do any capturing, and is not counted when +computing the number of any subsequent capturing subpatterns. For example, if +the string "the white queen" is matched against the pattern +
+ the ((?:red|white) (king|queen)) ++the captured substrings are "white queen" and "queen", and are numbered 1 and +2. The maximum number of capturing subpatterns is 65535. + +
+As a convenient shorthand, if any option settings are required at the start of +a non-capturing subpattern, the option letters may appear between the "?" and +the ":". Thus the two patterns +
+ (?i:saturday|sunday) + (?:(?i)saturday|sunday) ++match exactly the same set of strings. Because alternative branches are tried +from left to right, and options are not reset until the end of the subpattern +is reached, an option setting in one branch does affect subsequent branches, so +the above patterns match "SUNDAY" as well as "Saturday". + +
+Perl 5.10 introduced a feature whereby each alternative in a subpattern uses +the same numbers for its capturing parentheses. Such a subpattern starts with +(?| and is itself a non-capturing subpattern. For example, consider this +pattern: +
+ (?|(Sat)ur|(Sun))day ++Because the two alternatives are inside a (?| group, both sets of capturing +parentheses are numbered one. Thus, when the pattern matches, you can look +at captured substring number one, whichever alternative matched. This construct +is useful when you want to capture part, but not all, of one of a number of +alternatives. Inside a (?| group, parentheses are numbered as usual, but the +number is reset at the start of each branch. The numbers of any capturing +buffers that follow the subpattern start after the highest number used in any +branch. The following example is taken from the Perl documentation. +The numbers underneath show in which buffer the captured content will be +stored. +
+ # before ---------------branch-reset----------- after + / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x + # 1 2 2 3 2 3 4 ++A backreference or a recursive call to a numbered subpattern always refers to +the first one in the pattern with the given number. + +
+An alternative approach to using this "branch reset" feature is to use +duplicate named subpatterns, as described in the next section. +
++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. +
++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 +backreferences, +recursion, +and +conditions, +can be made by name as well as by number. +
++Names consist of up to 32 alphanumeric characters and underscores. Named +capturing parentheses are still allocated numbers as well as names, exactly as +if the names were not present. The PCRE API provides function calls for +extracting the name-to-number translation table from a compiled pattern. There +is also a convenience function for extracting a captured substring by name. +
++By default, a name must be unique within a pattern, but it is possible to relax +this constraint by setting the PCRE_DUPNAMES option at compile time. This can +be useful for patterns where only one instance of the named parentheses can +match. Suppose you want to match the name of a weekday, either as a 3-letter +abbreviation or as the full name, and in both cases you want to extract the +abbreviation. This pattern (ignoring the line breaks) does the job: +
+ (?<DN>Mon|Fri|Sun)(?:day)?| + (?<DN>Tue)(?:sday)?| + (?<DN>Wed)(?:nesday)?| + (?<DN>Thu)(?:rsday)?| + (?<DN>Sat)(?:urday)? ++There are five capturing substrings, but only one is ever set after a match. +(An alternative way of solving this problem is to use a "branch reset" +subpattern, as described in the previous section.) + +
+The convenience function for extracting the data by name returns the substring +for the first (and in this example, the only) subpattern of that name that +matched. This saves searching to find which numbered subpattern it was. If you +make a reference to a non-unique named subpattern from elsewhere in the +pattern, the one that corresponds to the lowest number is used. For further +details of the interfaces for handling named subpatterns, see the +pcreapi +documentation. +
++Repetition is specified by quantifiers, which can follow any of the following +items: +
+ a literal data character + the dot metacharacter + the \C escape sequence + the \X escape sequence (in UTF-8 mode with Unicode properties) + the \R escape sequence + an escape such as \d that matches a single character + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion) ++The general repetition quantifier specifies a minimum and maximum number of +permitted matches, by giving the two numbers in curly brackets (braces), +separated by a comma. The numbers must be less than 65536, and the first must +be less than or equal to the second. For example: +
+ z{2,4}
+
+matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
+character. If the second number is omitted, but the comma is present, there is
+no upper limit; if the second number and the comma are both omitted, the
+quantifier specifies an exact number of required matches. Thus
+
+ [aeiou]{3,}
+
+matches at least 3 successive vowels, but may match many more, while
+
+ \d{8}
+
+matches exactly 8 digits. An opening curly bracket that appears in a position
+where a quantifier is not allowed, or one that does not match the syntax of a
+quantifier, is taken as a literal character. For example, {,6} is not a
+quantifier, but a literal string of four characters.
+
++In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual +bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of +which is represented by a two-byte sequence. Similarly, when Unicode property +support is available, \X{3} matches three Unicode extended sequences, each of +which may be several bytes long (and they may be of different lengths). +
++The quantifier {0} is permitted, causing the expression to behave as if the +previous item and the quantifier were not present. This may be useful for +subpatterns that are referenced as +subroutines +from elsewhere in the pattern. Items other than subpatterns that have a {0} +quantifier are omitted from the compiled pattern. +
++For convenience, the three most common quantifiers have single-character +abbreviations: +
+ * is equivalent to {0,}
+ + is equivalent to {1,}
+ ? is equivalent to {0,1}
+
+It is possible to construct infinite loops by following a subpattern that can
+match no characters with a quantifier that has no upper limit, for example:
++ (a?)* ++Earlier versions of Perl and PCRE used to give an error at compile time for +such patterns. However, because there are cases where this can be useful, such +patterns are now accepted, but if any repetition of the subpattern does in fact +match no characters, the loop is forcibly broken. + +
+By default, the quantifiers are "greedy", that is, they match as much as +possible (up to the maximum number of permitted times), without causing the +rest of the pattern to fail. The classic example of where this gives problems +is in trying to match comments in C programs. These appear between /* and */ +and within the comment, individual * and / characters may appear. An attempt to +match C comments by applying the pattern +
+ /\*.*\*/ ++to the string +
+ /* first comment */ not comment /* second comment */ ++fails, because it matches the entire string owing to the greediness of the .* +item. + +
+However, if a quantifier is followed by a question mark, it ceases to be +greedy, and instead matches the minimum number of times possible, so the +pattern +
+ /\*.*?\*/ ++does the right thing with the C comments. The meaning of the various +quantifiers is not otherwise changed, just the preferred number of matches. +Do not confuse this use of question mark with its use as a quantifier in its +own right. Because it has two uses, it can sometimes appear doubled, as in +
+ \d??\d ++which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. + +
+If the PCRE_UNGREEDY option is set (an option that is not available in Perl), +the quantifiers are not greedy by default, but individual ones can be made +greedy by following them with a question mark. In other words, it inverts the +default behaviour. +
++When a parenthesized subpattern is quantified with a minimum repeat count that +is greater than 1 or with a limited maximum, more memory is required for the +compiled pattern, in proportion to the size of the minimum or maximum. +
++If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent +to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is +implicitly anchored, because whatever follows will be tried against every +character position in the subject string, so there is no point in retrying the +overall match at any position after the first. PCRE normally treats such a +pattern as though it were preceded by \A. +
++In cases where it is known that the subject string contains no newlines, it is +worth setting PCRE_DOTALL in order to obtain this optimization, or +alternatively using ^ to indicate anchoring explicitly. +
++However, there is one situation where the optimization cannot be used. When .* +is inside capturing parentheses that are the subject of a backreference +elsewhere in the pattern, a match at the start may fail where a later one +succeeds. Consider, for example: +
+ (.*)abc\1 ++If the subject is "xyz123abc123" the match point is the fourth character. For +this reason, such a pattern is not implicitly anchored. + +
+When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after +
+ (tweedle[dume]{3}\s*)+
+
+has matched "tweedledum tweedledee" the value of the captured substring is
+"tweedledee". However, if there are nested capturing subpatterns, the
+corresponding captured values may have been set in previous iterations. For
+example, after
++ /(a|(b))+/ ++matches "aba" the value of the second captured substring is "b". + +
+With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") +repetition, failure of what follows normally causes the repeated item to be +re-evaluated to see if a different number of repeats allows the rest of the +pattern to match. Sometimes it is useful to prevent this, either to change the +nature of the match, or to cause it fail earlier than it otherwise might, when +the author of the pattern knows there is no point in carrying on. +
++Consider, for example, the pattern \d+foo when applied to the subject line +
+ 123456bar ++After matching all 6 digits and then failing to match "foo", the normal +action of the matcher is to try again with only 5 digits matching the \d+ +item, and then with 4, and so on, before ultimately failing. "Atomic grouping" +(a term taken from Jeffrey Friedl's book) provides the means for specifying +that once a subpattern has matched, it is not to be re-evaluated in this way. + +
+If we use atomic grouping for the previous example, the matcher gives up +immediately on failing to match "foo" the first time. The notation is a kind of +special parenthesis, starting with (?> as in this example: +
+ (?>\d+)foo ++This kind of parenthesis "locks up" the part of the pattern it contains once +it has matched, and a failure further into the pattern is prevented from +backtracking into it. Backtracking past it to previous items, however, works as +normal. + +
+An alternative description is that a subpattern of this type matches the string +of characters that an identical standalone pattern would match, if anchored at +the current point in the subject string. +
++Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as +the above example can be thought of as a maximizing repeat that must swallow +everything it can. So, while both \d+ and \d+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\d+) can only match an entire sequence of digits. +
++Atomic groups in general can of course contain arbitrarily complicated +subpatterns, and can be nested. However, when the subpattern for an atomic +group is just a single repeated item, as in the example above, a simpler +notation, called a "possessive quantifier" can be used. This consists of an +additional + character following a quantifier. Using this notation, the +previous example can be rewritten as +
+ \d++foo ++Note that a possessive quantifier can be used with an entire group, for +example: +
+ (abc|xyz){2,3}+
+
+Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
+option is ignored. They are a convenient notation for the simpler forms of
+atomic group. However, there is no difference in the meaning of a possessive
+quantifier and the equivalent atomic group, though there may be a performance
+difference; possessive quantifiers should be slightly faster.
+
++The possessive quantifier syntax is an extension to the Perl 5.8 syntax. +Jeffrey Friedl originated the idea (and the name) in the first edition of his +book. Mike McCloskey liked it, so implemented it when he built Sun's Java +package, and PCRE copied it from there. It ultimately found its way into Perl +at release 5.10. +
++PCRE has an optimization that automatically "possessifies" certain simple +pattern constructs. For example, the sequence A+B is treated as A++B because +there is no point in backtracking into a sequence of A's when B must follow. +
++When a pattern contains an unlimited repeat inside a subpattern that can itself +be repeated an unlimited number of times, the use of an atomic group is the +only way to avoid some failing matches taking a very long time indeed. The +pattern +
+ (\D+|<\d+>)*[!?] ++matches an unlimited number of substrings that either consist of non-digits, or +digits enclosed in <>, followed by either ! or ?. When it matches, it runs +quickly. However, if it is applied to +
+ aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa ++it takes a long time before reporting failure. This is because the string can +be divided between the internal \D+ repeat and the external * repeat in a +large number of ways, and all have to be tried. (The example uses [!?] rather +than a single character at the end, because both PCRE and Perl have an +optimization that allows for fast failure when a single character is used. They +remember the last single character that is required for a match, and fail early +if it is not present in the string.) If the pattern is changed so that it uses +an atomic group, like this: +
+ ((?>\D+)|<\d+>)*[!?] ++sequences of non-digits cannot be broken, and failure happens quickly. + +
+Outside a character class, a backslash followed by a digit greater than 0 (and +possibly further digits) is a back reference to a capturing subpattern earlier +(that is, to its left) in the pattern, provided there have been that many +previous capturing left parentheses. +
++However, if the decimal number following the backslash is less than 10, it is +always taken as a back reference, and causes an error only if there are not +that many capturing left parentheses in the entire pattern. In other words, the +parentheses that are referenced need not be to the left of the reference for +numbers less than 10. A "forward back reference" of this type can make sense +when a repetition is involved and the subpattern to the right has participated +in an earlier iteration. +
++It is not possible to have a numerical "forward back reference" to a subpattern +whose number is 10 or more using this syntax because a sequence such as \50 is +interpreted as a character defined in octal. See the subsection entitled +"Non-printing characters" +above +for further details of the handling of digits following a backslash. There is +no such problem when named parentheses are used. A back reference to any +subpattern is possible using named parentheses (see below). +
++Another way of avoiding the ambiguity inherent in the use of digits following a +backslash is to use the \g escape sequence, which is a feature introduced in +Perl 5.10. This escape must be followed by an unsigned number or a negative +number, optionally enclosed in braces. These examples are all identical: +
+ (ring), \1
+ (ring), \g1
+ (ring), \g{1}
+
+An unsigned number specifies an absolute reference without the ambiguity that
+is present in the older syntax. It is also useful when literal digits follow
+the reference. A negative number is a relative reference. Consider this
+example:
+
+ (abc(def)ghi)\g{-1}
+
+The sequence \g{-1} is a reference to the most recently started capturing
+subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
+would be equivalent to \1. The use of relative references can be helpful in
+long patterns, and also in patterns that are created by joining together
+fragments that contain references within themselves.
+
++A back reference matches whatever actually matched the capturing subpattern in +the current subject string, rather than anything matching the subpattern +itself (see +"Subpatterns as subroutines" +below for a way of doing that). So the pattern +
+ (sens|respons)e and \1ibility ++matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If caseful matching is in force at the time of the +back reference, the case of letters is relevant. For example, +
+ ((?i)rah)\s+\1 ++matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. + +
+There are several different ways of writing back references to named +subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or +\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified +back reference syntax, in which \g can be used for both numeric and named +references, is also supported. We could rewrite the above example in any of +the following ways: +
+ (?<p1>(?i)rah)\s+\k<p1>
+ (?'p1'(?i)rah)\s+\k{p1}
+ (?P<p1>(?i)rah)\s+(?P=p1)
+ (?<p1>(?i)rah)\s+\g{p1}
+
+A subpattern that is referenced by name may appear in the pattern before or
+after the reference.
+
++There may be more than one back reference to the same subpattern. If a +subpattern has not actually been used in a particular match, any back +references to it always fail. For example, the pattern +
+ (a|(bc))\2 ++always fails if it starts to match "a" rather than "bc". Because there may be +many capturing parentheses in a pattern, all digits following the 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 whitespace. +Otherwise an empty comment (see +"Comments" +below) can be used. + +
+A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern +
+ (a|b\1)+ ++matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of +the subpattern, the back reference matches the character string corresponding +to the previous iteration. In order for this to work, the pattern must be such +that the first iteration does not need to match the back reference. This can be +done using alternation, as in the example above, or by a quantifier with a +minimum of zero. + +
+An assertion is a test on the characters following or preceding the current +matching point that does not actually consume any characters. The simple +assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described +above. +
++More complicated assertions are coded as subpatterns. There are two kinds: +those that look ahead of the current position in the subject string, and those +that look behind it. An assertion subpattern is matched in the normal way, +except that it does not cause the current matching position to be changed. +
++Assertion subpatterns are not capturing subpatterns, and may not be repeated, +because it makes no sense to assert the same thing several times. If any kind +of 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, +because it does not make sense for negative assertions. +
++Lookahead assertions start with (?= for positive assertions and (?! for +negative assertions. For example, +
+ \w+(?=;) ++matches a word followed by a semicolon, but does not include the semicolon in +the match, and +
+ foo(?!bar) ++matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern +
+ (?!foo)bar ++does not find an occurrence of "bar" that is preceded by something other than +"foo"; it finds any occurrence of "bar" whatsoever, because the assertion +(?!foo) is always true when the next three characters are "bar". A +lookbehind assertion is needed to achieve the other effect. + +
+If you want to force a matching failure at some point in a pattern, the most +convenient way to do it is with (?!) because an empty string always matches, so +an assertion that requires there not to be an empty string must always fail. +
++Lookbehind assertions start with (?<= for positive assertions and (?<! for +negative assertions. For example, +
+ (?<!foo)bar ++does find an occurrence of "bar" that is not preceded by "foo". The contents of +a lookbehind assertion are restricted such that all the strings it matches must +have a fixed length. However, if there are several top-level alternatives, they +do not all have to have the same fixed length. Thus +
+ (?<=bullock|donkey) ++is permitted, but +
+ (?<!dogs?|cats?) ++causes an error at compile time. Branches that match different length strings +are permitted only at the top level of a lookbehind assertion. This is an +extension compared with Perl (at least for 5.8), which requires all branches to +match the same length of string. An assertion such as +
+ (?<=ab(c|de)) ++is not permitted, because its single top-level branch can match two different +lengths, but it is acceptable if rewritten to use two top-level branches: +
+ (?<=abc|abde) ++In some cases, the Perl 5.10 escape sequence \K +(see above) +can be used instead of a lookbehind assertion; this is not restricted to a +fixed-length. + +
+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. +
++PCRE does not allow the \C escape (which matches a single byte in UTF-8 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 bytes, are also not permitted. +
++Possessive quantifiers can be used in conjunction with lookbehind assertions to +specify efficient matching at the end of the subject string. Consider a simple +pattern such as +
+ abcd$ ++when applied to a long string that does not match. Because matching proceeds +from left to right, PCRE will look for each "a" in the subject and then see if +what follows matches the rest of the pattern. If the pattern is specified as +
+ ^.*abcd$ ++the initial .* matches the entire string at first, but when this fails (because +there is no following "a"), it backtracks to match all but the last character, +then all but the last two characters, and so on. Once again the search for "a" +covers the entire string, from right to left, so we are no better off. However, +if the pattern is written as +
+ ^.*+(?<=abcd) ++there can be no backtracking for the .*+ item; it can match only the entire +string. The subsequent lookbehind assertion does a single test on the last four +characters. If it fails, the match fails immediately. For long strings, this +approach makes a significant difference to the processing time. + +
+Several assertions (of any sort) may occur in succession. For example, +
+ (?<=\d{3})(?<!999)foo
+
+matches "foo" preceded by three digits that are not "999". Notice that each of
+the assertions is applied independently at the same point in the subject
+string. First there is a check that the previous three characters are all
+digits, and then there is a check that the same three characters are not "999".
+This pattern does not match "foo" preceded by six characters, the first
+of which are digits and the last three of which are not "999". For example, it
+doesn't match "123abcfoo". A pattern to do that is
+
+ (?<=\d{3}...)(?<!999)foo
+
+This time the first assertion looks at the preceding six characters, checking
+that the first three are digits, and then the second assertion checks that the
+preceding three characters are not "999".
+
++Assertions can be nested in any combination. For example, +
+ (?<=(?<!foo)bar)baz ++matches an occurrence of "baz" that is preceded by "bar" which in turn is not +preceded by "foo", while +
+ (?<=\d{3}(?!999)...)foo
+
+is another pattern that matches "foo" preceded by three digits and any three
+characters that are not "999".
+
++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 previous capturing subpattern matched +or not. The two possible forms of conditional subpattern are +
+ (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) ++If the condition is satisfied, the yes-pattern is used; otherwise the +no-pattern (if present) is used. If there are more than two alternatives in the +subpattern, a compile-time error occurs. + +
+There are four kinds of condition: references to subpatterns, references to +recursion, a pseudo-condition called DEFINE, and assertions. +
++If the text between the parentheses consists of a sequence of digits, the +condition is true if the capturing subpattern of that number has previously +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. In looping constructs it can also make sense to +refer to subsequent groups with constructs such as (?(+2). +
++Consider the following pattern, which contains non-significant white space to +make it more readable (assume the PCRE_EXTENDED option) and to divide it into +three parts for ease of discussion: +
+ ( \( )? [^()]+ (?(1) \) ) ++The first part matches an optional opening parenthesis, and if that +character is present, sets it as the first captured substring. The second part +matches one or more characters that are not parentheses. The third part is a +conditional subpattern that tests whether the first set of parentheses matched +or not. If they did, that is, if subject started with an opening parenthesis, +the condition is true, and so the yes-pattern is executed and a closing +parenthesis is required. Otherwise, since no-pattern is not present, the +subpattern matches nothing. In other words, this pattern matches a sequence of +non-parentheses, optionally enclosed in parentheses. + +
+If you were embedding this pattern in a larger one, you could use a relative +reference: +
+ ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... ++This makes the fragment independent of the parentheses in the larger pattern. + +
+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. However, +there is a possible ambiguity with this syntax, because subpattern names may +consist entirely of digits. PCRE looks first for a named subpattern; if it +cannot find one and the name consists entirely of digits, PCRE looks for a +subpattern of that number, which must be greater than zero. Using subpattern +names that consist entirely of digits is not recommended. +
++Rewriting the above example to use a named subpattern gives this: +
+ (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) + ++ +
+If the condition is the string (R), and there is no subpattern with the name R, +the condition is true if a recursive call to the whole pattern or any +subpattern has been made. If digits or a name preceded by ampersand follow the +letter R, for example: +
+ (?(R3)...) or (?(R&name)...) ++the condition is true if the most recent recursion is into the subpattern whose +number or name is given. This condition does not check the entire recursion +stack. + +
+At "top level", all these recursion test conditions are false. Recursive +patterns are described below. +
++If the condition is the string (DEFINE), and there is no subpattern with the +name DEFINE, the condition is always false. In this case, there may be only one +alternative in the subpattern. It is always skipped if control reaches this +point in the pattern; the idea of DEFINE is that it can be used to define +"subroutines" that can be referenced from elsewhere. (The use of "subroutines" +is described below.) For example, a pattern to match an IPv4 address could be +written like this (ignore whitespace and line breaks): +
+ (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
+ \b (?&byte) (\.(?&byte)){3} \b
+
+The first part of the pattern is a DEFINE group inside which a another group
+named "byte" is defined. This matches an individual component of an IPv4
+address (a number less than 256). When matching takes place, this part of the
+pattern is skipped because DEFINE acts like a false condition.
+
++The rest of the pattern uses references to the named group to match the four +dot-separated components of an IPv4 address, insisting on a word boundary at +each end. +
++If the condition is not in any of the above formats, it must be an assertion. +This may be a positive or negative lookahead or lookbehind assertion. Consider +this pattern, again containing non-significant white space, and with the two +alternatives on the second line: +
+ (?(?=[^a-z]*[a-z])
+ \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
+
+The condition is a positive lookahead assertion that matches an optional
+sequence of non-letters followed by a letter. In other words, it tests for the
+presence of at least one letter in the subject. If a letter is found, the
+subject is matched against the first alternative; otherwise it is matched
+against the second. This pattern matches strings in one of the two forms
+dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
+
++The sequence (?# marks the start of a comment that continues up to the next +closing parenthesis. Nested parentheses are not permitted. The characters +that make up a comment play no part in the pattern matching at all. +
++If the PCRE_EXTENDED option is set, an unescaped # character outside a +character class introduces a comment that continues to immediately after the +next newline in the pattern. +
++Consider the problem of matching a string in parentheses, allowing for +unlimited nested parentheses. Without the use of recursion, the best that can +be done is to use a pattern that matches up to some fixed depth of nesting. It +is not possible to handle an arbitrary nesting depth. +
++For some time, Perl has provided a facility that allows regular expressions to +recurse (amongst other things). It does this by interpolating Perl code in the +expression at run time, and the code can refer to the expression itself. A Perl +pattern using code interpolation to solve the parentheses problem can be +created like this: +
+ $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
+
+The (?p{...}) item interpolates Perl code at run time, and in this case refers
+recursively to the pattern in which it appears.
+
++Obviously, PCRE cannot support the interpolation of Perl code. Instead, it +supports special syntax for recursion of the entire pattern, and also for +individual subpattern recursion. After its introduction in PCRE and Python, +this kind of recursion was introduced into Perl at release 5.10. +
++A special item that consists of (? followed by a number greater than zero and a +closing parenthesis is a recursive call of the subpattern of the given number, +provided that it occurs inside that subpattern. (If not, it is a "subroutine" +call, which is described in the next section.) The special item (?R) or (?0) is +a recursive call of the entire regular expression. +
++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 PCRE pattern solves the nested parentheses problem (assume the +PCRE_EXTENDED option is set so that white space is ignored): +
+ \( ( (?>[^()]+) | (?R) )* \) ++First it matches an opening parenthesis. Then it matches any number of +substrings which can either be a sequence of non-parentheses, or a recursive +match of the pattern itself (that is, a correctly parenthesized substring). +Finally there is a closing parenthesis. + +
+If this were part of a larger pattern, you would not want to recurse the entire +pattern, so instead you could use this: +
+ ( \( ( (?>[^()]+) | (?1) )* \) ) ++We have put the pattern into parentheses, and caused the recursion to refer to +them instead of the whole pattern. + +
+In a larger pattern, keeping track of parenthesis numbers can be tricky. This +is made easier by the use of relative references. (A Perl 5.10 feature.) +Instead of (?1) in the pattern above you can write (?-2) to refer to the second +most recently opened parentheses preceding the recursion. In other words, a +negative number counts capturing parentheses leftwards from the point at which +it is encountered. +
++It is also possible to refer to subsequently opened parentheses, by writing +references such as (?+2). However, these cannot be recursive because the +reference is not inside the parentheses that are referenced. They are always +"subroutine" calls, as described in the next section. +
++An alternative approach is to use named parentheses instead. The Perl syntax +for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We +could rewrite the above example as follows: +
+ (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) ) ++If there is more than one subpattern with the same name, the earliest one is +used. + +
+This particular example pattern that we have been looking at contains nested +unlimited repeats, and so the use of atomic grouping for matching strings of +non-parentheses is important when applying the pattern to strings that do not +match. For example, when this pattern is applied to +
+ (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() ++it yields "no match" quickly. However, if atomic grouping is not used, +the match runs for a very long time indeed because there are so many different +ways the + and * repeats can carve up the subject, and all have to be tested +before failure can be reported. + +
+At the end of a match, the values set for any capturing subpatterns are those +from the outermost level of the recursion at which the subpattern value is set. +If you want to obtain intermediate values, a callout function can be used (see +below and the +pcrecallout +documentation). If the pattern above is matched against +
+ (ab(cd)ef) ++the value for the capturing parentheses is "ef", which is the last value taken +on at the top level. If additional parentheses are added, giving +
+ \( ( ( (?>[^()]+) | (?R) )* ) \) + ^ ^ + ^ ^ ++the string they capture is "ab(cd)ef", the contents of the top level +parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE +has to obtain extra memory to store data during a recursion, which it does by +using pcre_malloc, freeing it via pcre_free afterwards. If no +memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. + +
+Do not confuse the (?R) item with the condition (R), which tests for recursion. +Consider this pattern, which matches text in angle brackets, allowing for +arbitrary nesting. Only digits are allowed in nested brackets (that is, when +recursing), whereas any characters are permitted at the outer level. +
+ < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > ++In this pattern, (?(R) is the start of a conditional subpattern, with two +different alternatives for the recursive and non-recursive cases. The (?R) item +is the actual recursive call. + +
+If the syntax for a recursive subpattern reference (either by number or by +name) is used outside the parentheses to which it refers, it operates like a +subroutine in a programming language. The "called" subpattern may be defined +before or after the reference. A numbered reference can be absolute or +relative, as in these examples: +
+ (...(absolute)...)...(?2)... + (...(relative)...)...(?-1)... + (...(?+1)...(relative)... ++An earlier example pointed out that the pattern +
+ (sens|respons)e and \1ibility ++matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If instead the pattern +
+ (sens|respons)e and (?1)ibility ++is used, it does match "sense and responsibility" as well as the other two +strings. Another example is given in the discussion of DEFINE above. + +
+Like recursive subpatterns, a "subroutine" 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. +
++When a subpattern is used as a subroutine, processing options such as +case-independence are fixed when the subpattern is defined. They cannot be +changed for different calls. For example, consider this pattern: +
+ (abc)(?i:(?-1)) ++It matches "abcabc". It does not match "abcABC" because the change of +processing option does not affect the called subpattern. + +
+For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or +a number enclosed either in angle brackets or single quotes, is an alternative +syntax for referencing a subpattern as a subroutine, possibly recursively. Here +are two of the examples used above, rewritten using this syntax: +
+ (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) + (sens|respons)e and \g'1'ibility ++PCRE supports an extension to Oniguruma: if a number is preceded by a +plus or a minus sign it is taken as a relative reference. For example: +
+ (abc)(?i:\g<-1>) ++Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not +synonymous. The former is a back reference; the latter is a subroutine call. + +
+Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl +code to be obeyed in the middle of matching a regular expression. This makes it +possible, amongst other things, to extract different substrings that match the +same pair of parentheses when there is a repetition. +
++PCRE provides a similar feature, but of course it cannot obey arbitrary Perl +code. The feature is called "callout". The caller of PCRE provides an external +function by putting its entry point in the global variable pcre_callout. +By default, this variable contains NULL, which disables all calling out. +
++Within a regular expression, (?C) indicates the points at which the external +function is to be called. If you want to identify different callout points, you +can put a number less than 256 after the letter C. The default value is zero. +For example, this pattern has two callout points: +
+ (?C1)abc(?C2)def ++If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are +automatically installed before each item in the pattern. They are all numbered +255. + +
+During matching, when PCRE reaches a callout point (and pcre_callout is +set), 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 pcre_exec(). The callout function +may cause matching to proceed, to backtrack, or to fail altogether. A complete +description of the interface to the callout function is given in the +pcrecallout +documentation. +
++Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which +are 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. +
++Since these verbs are specifically related to backtracking, most of them can be +used only when the pattern is to be matched using pcre_exec(), which uses +a backtracking algorithm. With the exception of (*FAIL), which behaves like a +failing negative assertion, they cause an error if encountered by +pcre_dfa_exec(). +
++The new verbs make use of what was previously invalid syntax: an opening +parenthesis followed by an asterisk. In Perl, they are generally of the form +(*VERB:ARG) but PCRE does not support the use of arguments, so its general +form is just (*VERB). Any number of these verbs may occur in a pattern. There +are two kinds: +
++The following verbs act as soon as they are encountered: +
+ (*ACCEPT) ++This verb causes the match to end successfully, skipping the remainder of the +pattern. When inside a recursion, only the innermost pattern is ended +immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside +capturing parentheses. In Perl, the data so far is captured: in PCRE no data is +captured. For example: +
+ A(A|B(*ACCEPT)|C)D ++This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is +captured. +
+ (*FAIL) or (*F) ++This verb causes the match to fail, forcing backtracking to occur. It is +equivalent to (?!) but easier to read. The Perl documentation notes that it is +probably useful only when combined with (?{}) or (??{}). Those are, of course, +Perl features that are not present in PCRE. The nearest equivalent is the +callout feature, as for example in this pattern: +
+ a+(?C)(*FAIL) ++A match with the string "aaaa" always fails, but the callout is taken before +each backtrack happens (in this example, 10 times). + +
+The following verbs do nothing when they are encountered. Matching continues +with what follows, but if there is no subsequent match, a failure is forced. +The verbs differ in exactly what kind of failure occurs. +
+ (*COMMIT) ++This verb causes the whole match to fail outright if the rest of the pattern +does not match. Even if the pattern is unanchored, no further attempts to find +a match by advancing the start point take place. Once (*COMMIT) has been +passed, pcre_exec() is committed to finding a match at the current +starting point, or not at all. For example: +
+ a+(*COMMIT)b ++This matches "xxaab" but not "aacaab". It can be thought of as a kind of +dynamic anchor, or "I've started, so I must finish." +
+ (*PRUNE) ++This verb causes the match to fail at the current position if the rest of the +pattern does not match. 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), 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. +
+ (*SKIP) ++This verb is like (*PRUNE), except that if the pattern is unanchored, the +"bumpalong" advance is not to the next character, but to the position in the +subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text +was matched leading up to it cannot be part of a successful match. Consider: +
+ a+(*SKIP)b ++If the subject is "aaaac...", after the first match attempt fails (starting at +the first character in the string), the starting point skips on to start the +next attempt at "c". Note that a possessive quantifer does not have the same +effect in 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". +
+ (*THEN) ++This verb causes a skip to the next alternation if the rest of the pattern does +not match. That is, it cancels pending backtracking, but only within the +current alternation. Its name comes from the observation that it can be used +for a pattern-based if-then-else block: +
+ ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... ++If the COND1 pattern matches, FOO is tried (and possibly further items after +the end of the group if FOO succeeds); on failure the matcher skips to the +second alternative and tries COND2, without backtracking into COND1. If (*THEN) +is used outside of any alternation, it acts exactly like (*PRUNE). + +
+pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). +
+
+Philip Hazel
+
+University Computing Service
+
+Cambridge CB2 3QH, England.
+
+
+Last updated: 19 April 2008
+
+Copyright © 1997-2008 University of Cambridge.
+
+
+Return to the PCRE index page. +