diff -r 000000000000 -r 7f656887cf89 libraries/spcre/libpcre/pcre/doc/pcrepattern.3 --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/libraries/spcre/libpcre/pcre/doc/pcrepattern.3 Wed Jun 23 15:52:26 2010 +0100 @@ -0,0 +1,2245 @@ +.TH PCREPATTERN 3 +.SH NAME +PCRE - Perl-compatible regular expressions +.SH "PCRE REGULAR EXPRESSION DETAILS" +.rs +.sp +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 +.\" HREF +\fBpcresyntax\fP +.\" +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 +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 +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 \fBpcre_compile()\fP 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 +.\" HTML +.\" +section on UTF-8 support +.\" +in the main +.\" HREF +\fBpcre\fP +.\" +page. +.P +The remainder of this document discusses the patterns that are supported by +PCRE when its main matching function, \fBpcre_exec()\fP, is used. +From release 6.0, PCRE offers a second matching function, +\fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not +Perl-compatible. Some of the features discussed below are not available when +\fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the +alternative function, and how it differs from the normal function, are +discussed in the +.\" HREF +\fBpcrematching\fP +.\" +page. +. +. +.SH "NEWLINE CONVENTIONS" +.rs +.sp +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 +.\" HREF +\fBpcreapi\fP +.\" +page has +.\" HTML +.\" +further discussion +.\" +about newlines, and shows how to set the newline convention in the +\fIoptions\fP arguments for the compiling and matching functions. +.P +It is also possible to specify a newline convention by starting a pattern +string with one of the following five sequences: +.sp + (*CR) carriage return + (*LF) linefeed + (*CRLF) carriage return, followed by linefeed + (*ANYCRLF) any of the three above + (*ANY) all Unicode newline sequences +.sp +These override the default and the options given to \fBpcre_compile()\fP. For +example, on a Unix system where LF is the default newline sequence, the pattern +.sp + (*CR)a.b +.sp +changes the convention to CR. That pattern matches "a\enb" 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. +.P +The newline convention does not affect what the \eR escape sequence matches. By +default, this is any Unicode newline sequence, for Perl compatibility. However, +this can be changed; see the description of \eR in the section entitled +.\" HTML +.\" +"Newline sequences" +.\" +below. A change of \eR setting can be combined with a change of newline +convention. +. +. +.SH "CHARACTERS AND METACHARACTERS" +.rs +.sp +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 +.sp + The quick brown fox +.sp +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. +.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 +\fImetacharacters\fP, which do not stand for themselves but instead are +interpreted in some special way. +.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: +.sp + \e 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 +.sp +Part of a pattern that is in square brackets is called a "character class". In +a character class the only metacharacters are: +.sp + \e general escape character + ^ negate the class, but only if the first character + - indicates character range +.\" JOIN + [ POSIX character class (only if followed by POSIX + syntax) + ] terminates the character class +.sp +The following sections describe the use of each of the metacharacters. +. +. +.SH BACKSLASH +.rs +.sp +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. +.P +For example, if you want to match a * character, you write \e* 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 \e\e. +.P +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. +.P +If you want to remove the special meaning from a sequence of characters, you +can do so by putting them between \eQ and \eE. This is different from Perl in +that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in +Perl, $ and @ cause variable interpolation. Note the following examples: +.sp + Pattern PCRE matches Perl matches +.sp +.\" JOIN + \eQabc$xyz\eE abc$xyz abc followed by the + contents of $xyz + \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz + \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz +.sp +The \eQ...\eE sequence is recognized both inside and outside character classes. +. +. +.\" HTML +.SS "Non-printing characters" +.rs +.sp +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: +.sp + \ea alarm, that is, the BEL character (hex 07) + \ecx "control-x", where x is any character + \ee escape (hex 1B) + \ef formfeed (hex 0C) + \en linefeed (hex 0A) + \er carriage return (hex 0D) + \et tab (hex 09) + \eddd character with octal code ddd, or backreference + \exhh character with hex code hh + \ex{hhh..} character with hex code hhh.. +.sp +The precise effect of \ecx 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 \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex +7B. +.P +After \ex, from zero to two hexadecimal digits are read (letters can be in +upper or lower case). Any number of hexadecimal digits may appear between \ex{ +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. +.P +If characters other than hexadecimal digits appear between \ex{ and }, or if +there is no terminating }, this form of escape is not recognized. Instead, the +initial \ex will be interpreted as a basic hexadecimal escape, with no +following digits, giving a character whose value is zero. +.P +Characters whose value is less than 256 can be defined by either of the two +syntaxes for \ex. There is no difference in the way they are handled. For +example, \exdc is exactly the same as \ex{dc}. +.P +After \e0 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 \e0\ex\e07 +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. +.P +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 \fIback reference\fP. A description of how this works is given +.\" HTML +.\" +later, +.\" +following the discussion of +.\" HTML +.\" +parenthesized subpatterns. +.\" +.P +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 \e400. In UTF-8 mode, values up +to \e777 are permitted. For example: +.sp + \e040 is another way of writing a space +.\" JOIN + \e40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \e7 is always a back reference +.\" JOIN + \e11 might be a back reference, or another way of + writing a tab + \e011 is always a tab + \e0113 is a tab followed by the character "3" +.\" JOIN + \e113 might be a back reference, otherwise the + character with octal code 113 +.\" JOIN + \e377 might be a back reference, otherwise + the byte consisting entirely of 1 bits +.\" JOIN + \e81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" +.sp +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. +.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, the +sequence \eb is interpreted as the backspace character (hex 08), and the +sequences \eR and \eX are interpreted as the characters "R" and "X", +respectively. Outside a character class, these sequences have different +meanings +.\" HTML +.\" +(see below). +.\" +. +. +.SS "Absolute and relative back references" +.rs +.sp +The sequence \eg 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 \eg{name}. Back references are discussed +.\" HTML +.\" +later, +.\" +following the discussion of +.\" HTML +.\" +parenthesized subpatterns. +.\" +. +. +.SS "Absolute and relative subroutine calls" +.rs +.sp +For compatibility with Oniguruma, the non-Perl syntax \eg 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 +.\" HTML +.\" +later. +.\" +Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP +synonymous. The former is a back reference; the latter is a subroutine call. +. +. +.SS "Generic character types" +.rs +.sp +Another use of backslash is for specifying generic character types. The +following are always recognized: +.sp + \ed any decimal digit + \eD any character that is not a decimal digit + \eh any horizontal whitespace character + \eH any character that is not a horizontal whitespace character + \es any whitespace character + \eS any character that is not a whitespace character + \ev any vertical whitespace character + \eV any character that is not a vertical whitespace character + \ew any "word" character + \eW any "non-word" character +.sp +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. +.P +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. +.P +For compatibility with Perl, \es does not match the VT character (code 11). +This makes it different from the the POSIX "space" class. The \es characters +are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is +included in a Perl script, \es may match the VT character. In PCRE, it never +does. +.P +In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or +\ew, and always match \eD, \eS, and \eW. 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. +.P +The sequences \eh, \eH, \ev, and \eV 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: +.sp + 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 +.sp +The vertical space characters are: +.sp + 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 +.P +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 +.\" HTML +.\" +"Locale support" +.\" +in the +.\" HREF +\fBpcreapi\fP +.\" +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 \ew. The use of locales with Unicode +is discouraged. +. +. +.\" HTML +.SS "Newline sequences" +.rs +.sp +Outside a character class, by default, the escape sequence \eR matches any +Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is +equivalent to the following: +.sp + (?>\er\en|\en|\ex0b|\ef|\er|\ex85) +.sp +This is an example of an "atomic group", details of which are given +.\" HTML +.\" +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. +.P +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. +.P +It is possible to restrict \eR 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: +.sp + (*BSR_ANYCRLF) CR, LF, or CRLF only + (*BSR_UNICODE) any Unicode newline sequence +.sp +These override the default and the options given to \fBpcre_compile()\fP, but +they can be overridden by options given to \fBpcre_exec()\fP. 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: +.sp + (*ANY)(*BSR_ANYCRLF) +.sp +Inside a character class, \eR matches the letter "R". +. +. +.\" HTML +.SS Unicode character properties +.rs +.sp +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: +.sp + \ep{\fIxx\fP} a character with the \fIxx\fP property + \eP{\fIxx\fP} a character without the \fIxx\fP property + \eX an extended Unicode sequence +.sp +The property names represented by \fIxx\fP 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 \eP{Any} does not match any +characters, so always causes a match failure. +.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: +.sp + \ep{Greek} + \eP{Han} +.sp +Those that are not part of an identified script are lumped together as +"Common". The current list of scripts is: +.P +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. +.P +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, \ep{^Lu} is the same as \eP{Lu}. +.P +If only one letter is specified with \ep or \eP, 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: +.sp + \ep{L} + \epL +.sp +The following general category property codes are supported: +.sp + C Other + Cc Control + Cf Format + Cn Unassigned + Co Private use + Cs Surrogate +.sp + L Letter + Ll Lower case letter + Lm Modifier letter + Lo Other letter + Lt Title case letter + Lu Upper case letter +.sp + M Mark + Mc Spacing mark + Me Enclosing mark + Mn Non-spacing mark +.sp + N Number + Nd Decimal number + Nl Letter number + No Other number +.sp + P Punctuation + Pc Connector punctuation + Pd Dash punctuation + Pe Close punctuation + Pf Final punctuation + Pi Initial punctuation + Po Other punctuation + Ps Open punctuation +.sp + S Symbol + Sc Currency symbol + Sk Modifier symbol + Sm Mathematical symbol + So Other symbol +.sp + Z Separator + Zl Line separator + Zp Paragraph separator + Zs Space separator +.sp +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 +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 +.\" HREF +\fBpcreapi\fP +.\" +page). +.P +The long synonyms for these properties that Perl supports (such as \ep{Letter}) +are not supported by PCRE, nor is it permitted to prefix any of these +properties with "Is". +.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 +Specifying caseless matching does not affect these escape sequences. For +example, \ep{Lu} always matches only upper case letters. +.P +The \eX escape matches any number of Unicode characters that form an extended +Unicode sequence. \eX is equivalent to +.sp + (?>\ePM\epM*) +.sp +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 +.\" HTML +.\" +(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 \eX matches any one character. +.P +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 \ed and \ew do not use Unicode +properties in PCRE. +. +. +.\" HTML +.SS "Resetting the match start" +.rs +.sp +The escape sequence \eK, 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: +.sp + foo\eKbar +.sp +matches "foobar", but reports that it has matched "bar". This feature is +similar to a lookbehind assertion +.\" HTML +.\" +(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 \eK does +not interfere with the setting of +.\" HTML +.\" +captured substrings. +.\" +For example, when the pattern +.sp + (foo)\eKbar +.sp +matches "foobar", the first substring is still set to "foo". +. +. +.\" HTML +.SS "Simple assertions" +.rs +.sp +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 +.\" HTML +.\" +below. +.\" +The backslashed assertions are: +.sp + \eb matches at a word boundary + \eB matches when not at a word boundary + \eA matches at the start of the subject + \eZ matches at the end of the subject + also matches before a newline at the end of the subject + \ez matches only at the end of the subject + \eG matches at the first matching position in the subject +.sp +These assertions may not appear in character classes (but note that \eb has a +different meaning, namely the backspace character, inside a character class). +.P +A word boundary is a position in the subject string where the current character +and the previous character do not both match \ew or \eW (i.e. one matches +\ew and the other matches \eW), or the start or end of the string if the +first or last character matches \ew, respectively. +.P +The \eA, \eZ, and \ez 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 \fIstartoffset\fP +argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start +at a point other than the beginning of the subject, \eA can never match. The +difference between \eZ and \ez is that \eZ matches before a newline at the end +of the string as well as at the very end, whereas \ez matches only at the end. +.P +The \eG assertion is true only when the current matching position is at the +start point of the match, as specified by the \fIstartoffset\fP argument of +\fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is +non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate +arguments, you can mimic Perl's /g option, and it is in this kind of +implementation where \eG can be useful. +.P +Note, however, that PCRE's interpretation of \eG, 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 +If all the alternatives of a pattern begin with \eG, the expression is anchored +to the starting match position, and the "anchored" flag is set in the compiled +regular expression. +. +. +.SH "CIRCUMFLEX AND DOLLAR" +.rs +.sp +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 \fIstartoffset\fP argument of +\fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE +option is unset. Inside a character class, circumflex has an entirely different +meaning +.\" HTML +.\" +(see below). +.\" +.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 +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. +.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 \eZ assertion. +.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 +For example, the pattern /^abc$/ matches the subject string "def\enabc" (where +\en 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 \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The +PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. +.P +Note that the sequences \eA, \eZ, and \ez can be used to match the start and +end of the subject in both modes, and if all branches of a pattern start with +\eA it is always anchored, whether or not PCRE_MULTILINE is set. +. +. +.SH "FULL STOP (PERIOD, DOT)" +.rs +.sp +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. +.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 +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 +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. +. +. +.SH "MATCHING A SINGLE BYTE" +.rs +.sp +Outside a character class, the escape sequence \eC 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 \eC escape sequence is best avoided. +.P +PCRE does not allow \eC to appear in lookbehind assertions +.\" HTML +.\" +(described below), +.\" +because in UTF-8 mode this would make it impossible to calculate the length of +the lookbehind. +. +. +.\" HTML +.SH "SQUARE BRACKETS AND CHARACTER CLASSES" +.rs +.sp +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. +.P +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. +.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 +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 \ex{ escaping mechanism. +.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 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. +.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 +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. +.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-\e]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 +Ranges operate in the collating sequence of character values. They can also be +used for characters specified numerically, for example [\e000-\e037]. In UTF-8 +mode, ranges can include characters whose values are greater than 255, for +example [\ex{100}-\ex{2ff}]. +.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 +[][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character +tables for a French locale are in use, [\exc8-\excb] 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. +.P +The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear +in a character class, and add the characters that they match to the class. For +example, [\edABCDEF] 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 [^\eW_] matches any letter or digit, but not underscore. +.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 - see the next section), and the terminating +closing square bracket. However, escaping other non-alphanumeric characters +does no harm. +. +. +.SH "POSIX CHARACTER CLASSES" +.rs +.sp +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, +.sp + [01[:alpha:]%] +.sp +matches "0", "1", any alphabetic character, or "%". The supported class names +are +.sp + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \ed) + 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 \es) + upper upper case letters + word "word" characters (same as \ew) + xdigit hexadecimal digits +.sp +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 \es, which does not include VT (for Perl +compatibility). +.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, +.sp + [12[:^digit:]] +.sp +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 +In UTF-8 mode, characters with values greater than 128 do not match any of +the POSIX character classes. +. +. +.SH "VERTICAL BAR" +.rs +.sp +Vertical bar characters are used to separate alternative patterns. For example, +the pattern +.sp + gilbert|sullivan +.sp +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 +.\" HTML +.\" +(defined below), +.\" +"succeeds" means matching the rest of the main pattern as well as the +alternative in the subpattern. +. +. +.SH "INTERNAL OPTION SETTING" +.rs +.sp +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 +.sp + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED +.sp +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 +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 +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 +\fBpcre_fullinfo()\fP function). +.P +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 +.sp + (a(?i)b)c +.sp +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, +.sp + (a(?i)b|c) +.sp +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 +\fBNote:\fP 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 +.\" HTML +.\" +"Newline sequences" +.\" +above. +. +. +.\" HTML +.SH SUBPATTERNS +.rs +.sp +Subpatterns are delimited by parentheses (round brackets), which can be nested. +Turning part of a pattern into a subpattern does two things: +.sp +1. It localizes a set of alternatives. For example, the pattern +.sp + cat(aract|erpillar|) +.sp +matches one of the words "cat", "cataract", or "caterpillar". Without the +parentheses, it would match "cataract", "erpillar" or an empty string. +.sp +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 \fIovector\fP argument of +\fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting +from 1) to obtain numbers for the capturing subpatterns. +.P +For example, if the string "the red king" is matched against the pattern +.sp + the ((red|white) (king|queen)) +.sp +the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3, respectively. +.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 +.sp + the ((?:red|white) (king|queen)) +.sp +the captured substrings are "white queen" and "queen", and are numbered 1 and +2. The maximum number of capturing subpatterns is 65535. +.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 +.sp + (?i:saturday|sunday) + (?:(?i)saturday|sunday) +.sp +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". +. +. +.SH "DUPLICATE SUBPATTERN NUMBERS" +.rs +.sp +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: +.sp + (?|(Sat)ur|(Sun))day +.sp +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. +.sp + # before ---------------branch-reset----------- after + / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x + # 1 2 2 3 2 3 4 +.sp +A backreference or a recursive call to a numbered subpattern always refers to +the first one in the pattern with the given number. +.P +An alternative approach to using this "branch reset" feature is to use +duplicate named subpatterns, as described in the next section. +. +. +.SH "NAMED SUBPATTERNS" +.rs +.sp +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. +.P +In PCRE, a subpattern can be named in one of three ways: (?...) or +(?'name'...) as in Perl, or (?P...) as in Python. References to capturing +parentheses from other parts of the pattern, such as +.\" HTML +.\" +backreferences, +.\" +.\" HTML +.\" +recursion, +.\" +and +.\" HTML +.\" +conditions, +.\" +can be made by name as well as by number. +.P +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. +.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. 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: +.sp + (?Mon|Fri|Sun)(?:day)?| + (?Tue)(?:sday)?| + (?Wed)(?:nesday)?| + (?Thu)(?:rsday)?| + (?Sat)(?:urday)? +.sp +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 +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 +.\" HREF +\fBpcreapi\fP +.\" +documentation. +. +. +.SH REPETITION +.rs +.sp +Repetition is specified by quantifiers, which can follow any of the following +items: +.sp + a literal data character + the dot metacharacter + the \eC escape sequence + the \eX escape sequence (in UTF-8 mode with Unicode properties) + the \eR escape sequence + an escape such as \ed that matches a single character + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion) +.sp +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: +.sp + z{2,4} +.sp +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 +.sp + [aeiou]{3,} +.sp +matches at least 3 successive vowels, but may match many more, while +.sp + \ed{8} +.sp +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 +In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual +bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of +which is represented by a two-byte sequence. Similarly, when Unicode property +support is available, \eX{3} matches three Unicode extended sequences, each of +which may be several bytes long (and they may be of different lengths). +.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 +.\" HTML +.\" +subroutines +.\" +from elsewhere in the pattern. Items other than subpatterns that have a {0} +quantifier are omitted from the compiled pattern. +.P +For convenience, the three most common quantifiers have single-character +abbreviations: +.sp + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} +.sp +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: +.sp + (a?)* +.sp +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 +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 +.sp + /\e*.*\e*/ +.sp +to the string +.sp + /* first comment */ not comment /* second comment */ +.sp +fails, because it matches the entire string owing to the greediness of the .* +item. +.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 +.sp + /\e*.*?\e*/ +.sp +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 +.sp + \ed??\ed +.sp +which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. +.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 +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 +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 \eA. +.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 +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: +.sp + (.*)abc\e1 +.sp +If the subject is "xyz123abc123" the match point is the fourth character. For +this reason, such a pattern is not implicitly anchored. +.P +When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after +.sp + (tweedle[dume]{3}\es*)+ +.sp +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 +.sp + /(a|(b))+/ +.sp +matches "aba" the value of the second captured substring is "b". +. +. +.\" HTML +.SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" +.rs +.sp +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 +Consider, for example, the pattern \ed+foo when applied to the subject line +.sp + 123456bar +.sp +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 \ed+ +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 +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: +.sp + (?>\ed+)foo +.sp +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 +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 +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 \ed+ and \ed+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\ed+) can only match an entire sequence of digits. +.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 +.sp + \ed++foo +.sp +Note that a possessive quantifier can be used with an entire group, for +example: +.sp + (abc|xyz){2,3}+ +.sp +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 +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 +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 +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 +.sp + (\eD+|<\ed+>)*[!?] +.sp +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 +.sp + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +.sp +it takes a long time before reporting failure. This is because the string can +be divided between the internal \eD+ 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: +.sp + ((?>\eD+)|<\ed+>)*[!?] +.sp +sequences of non-digits cannot be broken, and failure happens quickly. +. +. +.\" HTML +.SH "BACK REFERENCES" +.rs +.sp +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 +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 +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 \e50 is +interpreted as a character defined in octal. See the subsection entitled +"Non-printing characters" +.\" HTML +.\" +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). +.P +Another way of avoiding the ambiguity inherent in the use of digits following a +backslash is to use the \eg 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: +.sp + (ring), \e1 + (ring), \eg1 + (ring), \eg{1} +.sp +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: +.sp + (abc(def)ghi)\eg{-1} +.sp +The sequence \eg{-1} is a reference to the most recently started capturing +subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2} +would be equivalent to \e1. 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 +A back reference matches whatever actually matched the capturing subpattern in +the current subject string, rather than anything matching the subpattern +itself (see +.\" HTML +.\" +"Subpatterns as subroutines" +.\" +below for a way of doing that). So the pattern +.sp + (sens|respons)e and \e1ibility +.sp +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, +.sp + ((?i)rah)\es+\e1 +.sp +matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. +.P +There are several different ways of writing back references to named +subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek or +\ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified +back reference syntax, in which \eg can be used for both numeric and named +references, is also supported. We could rewrite the above example in any of +the following ways: +.sp + (?(?i)rah)\es+\ek + (?'p1'(?i)rah)\es+\ek{p1} + (?P(?i)rah)\es+(?P=p1) + (?(?i)rah)\es+\eg{p1} +.sp +A subpattern that is referenced by name may appear in the pattern before or +after the reference. +.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. For example, the pattern +.sp + (a|(bc))\e2 +.sp +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 +.\" HTML +.\" +"Comments" +.\" +below) can be used. +.P +A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\e1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern +.sp + (a|b\e1)+ +.sp +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. +. +. +.\" HTML +.SH ASSERTIONS +.rs +.sp +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 \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described +.\" HTML +.\" +above. +.\" +.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 +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. +. +. +.SS "Lookahead assertions" +.rs +.sp +Lookahead assertions start with (?= for positive assertions and (?! for +negative assertions. For example, +.sp + \ew+(?=;) +.sp +matches a word followed by a semicolon, but does not include the semicolon in +the match, and +.sp + foo(?!bar) +.sp +matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern +.sp + (?!foo)bar +.sp +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 +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. +. +. +.\" HTML +.SS "Lookbehind assertions" +.rs +.sp +Lookbehind assertions start with (?<= for positive assertions and (? +.\" +(see above) +.\" +can be used instead of a lookbehind assertion; this is not restricted to a +fixed-length. +.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 +PCRE does not allow the \eC 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 \eX and \eR escapes, which can match +different numbers of bytes, are also not permitted. +.P +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 +.sp + abcd$ +.sp +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 +.sp + ^.*abcd$ +.sp +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 +.sp + ^.*+(?<=abcd) +.sp +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. +. +. +.SS "Using multiple assertions" +.rs +.sp +Several assertions (of any sort) may occur in succession. For example, +.sp + (?<=\ed{3})(? +.SH "CONDITIONAL SUBPATTERNS" +.rs +.sp +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 +.sp + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) +.sp +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. +.P +There are four kinds of condition: references to subpatterns, references to +recursion, a pseudo-condition called DEFINE, and assertions. +. +.SS "Checking for a used subpattern by number" +.rs +.sp +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). +.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: +.sp + ( \e( )? [^()]+ (?(1) \e) ) +.sp +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. +.P +If you were embedding this pattern in a larger one, you could use a relative +reference: +.sp + ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ... +.sp +This makes the fragment independent of the parentheses in the larger pattern. +. +.SS "Checking for a used subpattern by name" +.rs +.sp +Perl uses the syntax (?()...) 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. +.P +Rewriting the above example to use a named subpattern gives this: +.sp + (? \e( )? [^()]+ (?() \e) ) +.sp +. +.SS "Checking for pattern recursion" +.rs +.sp +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: +.sp + (?(R3)...) or (?(R&name)...) +.sp +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. +.P +At "top level", all these recursion test conditions are false. Recursive +patterns are described below. +. +.SS "Defining subpatterns for use by reference only" +.rs +.sp +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): +.sp + (?(DEFINE) (? 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) + \eb (?&byte) (\e.(?&byte)){3} \eb +.sp +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. +.P +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. +. +.SS "Assertion conditions" +.rs +.sp +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: +.sp + (?(?=[^a-z]*[a-z]) + \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) +.sp +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. +. +. +.\" HTML +.SH COMMENTS +.rs +.sp +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. +.P +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. +. +. +.\" HTML +.SH "RECURSIVE PATTERNS" +.rs +.sp +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 +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: +.sp + $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; +.sp +The (?p{...}) item interpolates Perl code at run time, and in this case refers +recursively to the pattern in which it appears. +.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 introduced into Perl at release 5.10. +.P +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. +.P +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. +.P +This PCRE pattern solves the nested parentheses problem (assume the +PCRE_EXTENDED option is set so that white space is ignored): +.sp + \e( ( (?>[^()]+) | (?R) )* \e) +.sp +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. +.P +If this were part of a larger pattern, you would not want to recurse the entire +pattern, so instead you could use this: +.sp + ( \e( ( (?>[^()]+) | (?1) )* \e) ) +.sp +We have put the pattern into parentheses, and caused the recursion to refer to +them instead of the whole pattern. +.P +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. +.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 +"subroutine" calls, as described in the next section. +.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: +.sp + (? \e( ( (?>[^()]+) | (?&pn) )* \e) ) +.sp +If there is more than one subpattern with the same name, the earliest one is +used. +.P +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 +.sp + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() +.sp +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. +.P +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 +.\" HREF +\fBpcrecallout\fP +.\" +documentation). If the pattern above is matched against +.sp + (ab(cd)ef) +.sp +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 +.sp + \e( ( ( (?>[^()]+) | (?R) )* ) \e) + ^ ^ + ^ ^ +.sp +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 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no +memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. +.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. +.sp + < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > +.sp +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. +. +. +.\" HTML +.SH "SUBPATTERNS AS SUBROUTINES" +.rs +.sp +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: +.sp + (...(absolute)...)...(?2)... + (...(relative)...)...(?-1)... + (...(?+1)...(relative)... +.sp +An earlier example pointed out that the pattern +.sp + (sens|respons)e and \e1ibility +.sp +matches "sense and sensibility" and "response and responsibility", but not +"sense and responsibility". If instead the pattern +.sp + (sens|respons)e and (?1)ibility +.sp +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 +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. +.P +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: +.sp + (abc)(?i:(?-1)) +.sp +It matches "abcabc". It does not match "abcABC" because the change of +processing option does not affect the called subpattern. +. +. +.\" HTML +.SH "ONIGURUMA SUBROUTINE SYNTAX" +.rs +.sp +For compatibility with Oniguruma, the non-Perl syntax \eg 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: +.sp + (? \e( ( (?>[^()]+) | \eg )* \e) ) + (sens|respons)e and \eg'1'ibility +.sp +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: +.sp + (abc)(?i:\eg<-1>) +.sp +Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP +synonymous. The former is a back reference; the latter is a subroutine call. +. +. +.SH CALLOUTS +.rs +.sp +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 +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 \fIpcre_callout\fP. +By default, this variable contains NULL, which disables all calling out. +.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: +.sp + (?C1)abc(?C2)def +.sp +If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are +automatically installed before each item in the pattern. They are all numbered +255. +.P +During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP 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 \fBpcre_exec()\fP. 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 +.\" HREF +\fBpcrecallout\fP +.\" +documentation. +. +. +.SH "BACKTRACKING CONTROL" +.rs +.sp +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. +.P +Since these verbs are specifically related to backtracking, most of them can be +used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses +a backtracking algorithm. With the exception of (*FAIL), which behaves like a +failing negative assertion, they cause an error if encountered by +\fBpcre_dfa_exec()\fP. +.P +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: +. +.SS "Verbs that act immediately" +.rs +.sp +The following verbs act as soon as they are encountered: +.sp + (*ACCEPT) +.sp +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: +.sp + A(A|B(*ACCEPT)|C)D +.sp +This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is +captured. +.sp + (*FAIL) or (*F) +.sp +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: +.sp + a+(?C)(*FAIL) +.sp +A match with the string "aaaa" always fails, but the callout is taken before +each backtrack happens (in this example, 10 times). +. +.SS "Verbs that act after backtracking" +.rs +.sp +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. +.sp + (*COMMIT) +.sp +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, \fBpcre_exec()\fP is committed to finding a match at the current +starting point, or not at all. For example: +.sp + a+(*COMMIT)b +.sp +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." +.sp + (*PRUNE) +.sp +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. +.sp + (*SKIP) +.sp +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: +.sp + a+(*SKIP)b +.sp +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". +.sp + (*THEN) +.sp +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: +.sp + ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... +.sp +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). +. +. +.SH "SEE ALSO" +.rs +.sp +\fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3). +. +. +.SH AUTHOR +.rs +.sp +.nf +Philip Hazel +University Computing Service +Cambridge CB2 3QH, England. +.fi +. +. +.SH REVISION +.rs +.sp +.nf +Last updated: 19 April 2008 +Copyright (c) 1997-2008 University of Cambridge. +.fi