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diff --git a/lib/win32/pcre/doc/pcrepattern.3 b/lib/win32/pcre/doc/pcrepattern.3 deleted file mode 100644 index c4cb9d756b..0000000000 --- a/lib/win32/pcre/doc/pcrepattern.3 +++ /dev/null @@ -1,2420 +0,0 @@ -.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, -PCRE must be built to include UTF-8 support, and you must call -\fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There -is also a special sequence that can be given at the start of a pattern: -.sp - (*UTF8) -.sp -Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8 -option. This feature is not Perl-compatible. How setting UTF-8 mode affects -pattern matching is mentioned in several places below. There is also a summary -of UTF-8 features in the -.\" HTML <a href="pcre.html#utf8support"> -.\" </a> -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 <a href="pcreapi.html#newlines"> -.\" </a> -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 or -\fBpcre_compile2()\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 <a href="#newlineseq"> -.\" </a> -"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 <a name="digitsafterbackslash"></a> -.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 often 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 <a href="#backreferences"> -.\" </a> -later, -.\" -following the discussion of -.\" HTML <a href="#subpattern"> -.\" </a> -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 <a href="#uniextseq"> -.\" </a> -(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 <a href="#backreferences"> -.\" </a> -later, -.\" -following the discussion of -.\" HTML <a href="#subpattern"> -.\" </a> -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 <a href="#onigurumasubroutines"> -.\" </a> -later. -.\" -Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP -synonymous. The former is a back reference; the latter is a -.\" HTML <a href="#subpatternsassubroutines"> -.\" </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. Note that this also affects \eb, because it is defined in terms of \ew -and \eW. -.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 <a href="pcreapi.html#localesupport"> -.\" </a> -"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 <a name="newlineseq"></a> -.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 <a href="#atomicgroup"> -.\" </a> -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 or -\fBpcre_compile2()\fP, but they can be overridden by options given to -\fBpcre_exec()\fP or \fBpcre_dfa_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 <a name="uniextseq"></a> -.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). Perl does not support the Cs property. -.P -The long synonyms for property names 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 <a href="#atomicgroup"> -.\" </a> -(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 <a name="resetmatchstart"></a> -.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 <a href="#lookbehind"> -.\" </a> -(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 <a href="#subpattern"> -.\" </a> -captured substrings. -.\" -For example, when the pattern -.sp - (foo)\eKbar -.sp -matches "foobar", the first substring is still set to "foo". -. -. -.\" HTML <a name="smallassertions"></a> -.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 <a href="#bigassertions"> -.\" </a> -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. Neither PCRE nor Perl has a -separte "start of word" or "end of word" metasequence. However, whatever -follows \eb normally determines which it is. For example, the fragment -\eba matches "a" at the start of a word. -.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 <a href="#characterclass"> -.\" </a> -(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 <a href="#lookbehind"> -.\" </a> -(described below), -.\" -because in UTF-8 mode this would make it impossible to calculate the length of -the lookbehind. -. -. -.\" HTML <a name="characterclass"></a> -.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 by default. -However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square -bracket causes a compile-time error. If a closing square bracket is required as -a member of the class, it should be the first data character in the class -(after an initial circumflex, if present) or escaped with a backslash. -.P -A character class matches a single character in the subject. In UTF-8 mode, the -character may be more than one byte long. A matched character must be in the -set of characters defined by the class, unless the first character in the class -definition is a circumflex, in which case the subject character must not be in -the set defined by the class. If a circumflex is actually required as a member -of the class, ensure it is not the first character, or escape it with a -backslash. -.P -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 in UTF8-mode 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 <a href="#subpattern"> -.\" </a> -(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 one of these option changes occurs at top level (that is, not inside -subpattern parentheses), the change applies to the remainder of the pattern -that follows. If the change is placed right at the start of a pattern, PCRE -extracts it into the global options (and it will 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 such as (*CRLF) to override what -the application has set or what has been defaulted. Details are given in the -section entitled -.\" HTML <a href="#newlineseq"> -.\" </a> -"Newline sequences" -.\" -above. There is also the (*UTF8) leading sequence that can be used to set UTF-8 -mode; this is equivalent to setting the PCRE_UTF8 option. -. -. -.\" HTML <a name="subpattern"></a> -.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". -. -. -.\" HTML <a name="dupsubpatternnumber"></a> -.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 to a numbered subpattern uses the most recent value that is set -for that number by any subpattern. The following pattern matches "abcabc" or -"defdef": -.sp - /(?|(abc)|(def))\e1/ -.sp -In contrast, a recursive or "subroutine" call to a numbered subpattern always -refers to the first one in the pattern with the given number. The following -pattern matches "abcabc" or "defabc": -.sp - /(?|(abc)|(def))(?1)/ -.sp -If a -.\" HTML <a href="#conditions"> -.\" </a> -condition test -.\" -for a subpattern's having matched refers to a non-unique number, the test is -true if any of the subpatterns of that number have matched. -.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. Perl allows identically numbered subpatterns to -have different names, but PCRE does not. -.P -In PCRE, a subpattern can be named in one of three ways: (?<name>...) or -(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing -parentheses from other parts of the pattern, such as -.\" HTML <a href="#backreferences"> -.\" </a> -backreferences, -.\" -.\" HTML <a href="#recursion"> -.\" </a> -recursion, -.\" -and -.\" HTML <a href="#conditions"> -.\" </a> -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. (Duplicate -names are also always permitted for subpatterns with the same number, set up as -described in the previous section.) Duplicate names can be useful for patterns -where only one instance of the named parentheses can match. Suppose you want to -match the name of a weekday, either as a 3-letter abbreviation or as the full -name, and in both cases you want to extract the abbreviation. This pattern -(ignoring the line breaks) does the job: -.sp - (?<DN>Mon|Fri|Sun)(?:day)?| - (?<DN>Tue)(?:sday)?| - (?<DN>Wed)(?:nesday)?| - (?<DN>Thu)(?:rsday)?| - (?<DN>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. -.P -If you make a backreference to a non-unique named subpattern from elsewhere in -the pattern, the one that corresponds to the first occurrence of the name is -used. In the absence of duplicate numbers (see the previous section) this is -the one with the lowest number. If you use a named reference in a condition -test (see the -.\" -.\" HTML <a href="#conditions"> -.\" </a> -section about conditions -.\" -below), either to check whether a subpattern has matched, or to check for -recursion, all subpatterns with the same name are tested. If the condition is -true for any one of them, the overall condition is true. This is the same -behaviour as testing by number. For further details of the interfaces for -handling named subpatterns, see the -.\" HREF -\fBpcreapi\fP -.\" -documentation. -.P -\fBWarning:\fP You cannot use different names to distinguish between two -subpatterns with the same number because PCRE uses only the numbers when -matching. For this reason, an error is given at compile time if different names -are given to subpatterns with the same number. However, you can give the same -name to subpatterns with the same number, even when PCRE_DUPNAMES is not set. -. -. -.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) - a recursive or "subroutine" call to a subpattern -.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 <a href="#subpatternsassubroutines"> -.\" </a> -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 <a name="atomicgroup"></a> -.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 <a name="backreferences"></a> -.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 <a href="#digitsafterbackslash"> -.\" </a> -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 <a href="#subpatternsassubroutines"> -.\" </a> -"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<name> 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 - (?<p1>(?i)rah)\es+\ek<p1> - (?'p1'(?i)rah)\es+\ek{p1} - (?P<p1>(?i)rah)\es+(?P=p1) - (?<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 by default. For example, the pattern -.sp - (a|(bc))\e2 -.sp -always fails if it starts to match "a" rather than "bc". However, if the -PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an -unset value matches an empty string. -.P -Because there may be many capturing parentheses in a pattern, all digits -following a backslash are taken as part of a potential back reference number. -If the pattern continues with a digit character, some delimiter must be used to -terminate the back reference. If the PCRE_EXTENDED option is set, this can be -whitespace. Otherwise, the \eg{ syntax or an empty comment (see -.\" HTML <a href="#comments"> -.\" </a> -"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 <a name="bigassertions"></a> -.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 <a href="#smallassertions"> -.\" </a> -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. -The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a -synonym for (?!). -. -. -.\" HTML <a name="lookbehind"></a> -.SS "Lookbehind assertions" -.rs -.sp -Lookbehind assertions start with (?<= for positive assertions and (?<! for -negative assertions. For example, -.sp - (?<!foo)bar -.sp -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 -.sp - (?<=bullock|donkey) -.sp -is permitted, but -.sp - (?<!dogs?|cats?) -.sp -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 (5.8 and 5.10), which requires all branches to -match the same length of string. An assertion such as -.sp - (?<=ab(c|de)) -.sp -is not permitted, because its single top-level branch can match two different -lengths, but it is acceptable to PCRE if rewritten to use two top-level -branches: -.sp - (?<=abc|abde) -.sp -In some cases, the Perl 5.10 escape sequence \eK -.\" HTML <a href="#resetmatchstart"> -.\" </a> -(see above) -.\" -can be used instead of a lookbehind assertion to get round the fixed-length -restriction. -.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 -.\" HTML <a href="#subpatternsassubroutines"> -.\" </a> -"Subroutine" -.\" -calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long -as the subpattern matches a fixed-length string. -.\" HTML <a href="#recursion"> -.\" </a> -Recursion, -.\" -however, is not supported. -.P -Possessive quantifiers can be used in conjunction with lookbehind assertions to -specify efficient matching of fixed-length strings at the end of subject -strings. Consider a simple pattern such as -.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})(?<!999)foo -.sp -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 \fInot\fP 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 -.sp - (?<=\ed{3}...)(?<!999)foo -.sp -This time the first assertion looks at the preceding six characters, checking -that the first three are digits, and then the second assertion checks that the -preceding three characters are not "999". -.P -Assertions can be nested in any combination. For example, -.sp - (?<=(?<!foo)bar)baz -.sp -matches an occurrence of "baz" that is preceded by "bar" which in turn is not -preceded by "foo", while -.sp - (?<=\ed{3}(?!999)...)foo -.sp -is another pattern that matches "foo" preceded by three digits and any three -characters that are not "999". -. -. -.\" HTML <a name="conditions"></a> -.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 specific capturing subpattern has -already been matched. 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 a capturing subpattern of that number has previously -matched. If there is more than one capturing subpattern with the same number -(see the earlier -.\" -.\" HTML <a href="#recursion"> -.\" </a> -section about duplicate subpattern numbers), -.\" -the condition is true if any of them have been set. 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 (?(<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. -.P -Rewriting the above example to use a named subpattern gives this: -.sp - (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) ) -.sp -If the name used in a condition of this kind is a duplicate, the test is -applied to all subpatterns of the same name, and is true if any one of them has -matched. -. -.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 a subpattern whose -number or name is given. This condition does not check the entire recursion -stack. If the name used in a condition of this kind is a duplicate, the test is -applied to all subpatterns of the same name, and is true if any one of them is -the most recent recursion. -.P -At "top level", all these recursion test conditions are false. -.\" HTML <a href="#recursion"> -.\" </a> -The syntax for recursive patterns -.\" -is 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 -.\" HTML <a href="#subpatternsassubroutines"> -.\" </a> -"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) (?<byte> 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. 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 <a name="comments"></a> -.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 <a name="recursion"></a> -.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 subsequently 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 -.\" HTML <a href="#subpatternsassubroutines"> -.\" </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 -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. Note the use of a possessive quantifier -to avoid backtracking into sequences of non-parentheses. -.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 -.\" HTML <a href="#subpatternsassubroutines"> -.\" </a> -"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 - (?<pn> \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 a possessive quantifier for matching -strings of non-parentheses is important when applying the pattern to strings -that do not match. For example, when this pattern is applied to -.sp - (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() -.sp -it yields "no match" quickly. However, if a possessive quantifier is not used, -the match runs for a very long time indeed because there are so many different -ways the + and * repeats can carve up the subject, and all have to be tested -before failure can be reported. -.P -At the end of a match, the values of capturing parentheses are those from -the outermost level. If you want to obtain intermediate values, a callout -function can be used (see below and the -.\" HREF -\fBpcrecallout\fP -.\" -documentation). If the pattern above is matched against -.sp - (ab(cd)ef) -.sp -the value for the inner capturing parentheses (numbered 2) is "ef", which is -the last value taken on at the top level. If a capturing subpattern is not -matched at the top level, its final value is unset, even if it is (temporarily) -set at a deeper level. -.P -If there are more than 15 capturing parentheses in a pattern, PCRE has to -obtain extra memory to store data during a recursion, which it does by using -\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 <a name="recursiondifference"></a> -.SS "Recursion difference from Perl" -.rs -.sp -In PCRE (like Python, but unlike Perl), a recursive subpattern call is always -treated as an atomic group. That is, once it has matched some of the subject -string, it is never re-entered, even if it contains untried alternatives and -there is a subsequent matching failure. This can be illustrated by the -following pattern, which purports to match a palindromic string that contains -an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"): -.sp - ^(.|(.)(?1)\e2)$ -.sp -The idea is that it either matches a single character, or two identical -characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE -it does not if the pattern is longer than three characters. Consider the -subject string "abcba": -.P -At the top level, the first character is matched, but as it is not at the end -of the string, the first alternative fails; the second alternative is taken -and the recursion kicks in. The recursive call to subpattern 1 successfully -matches the next character ("b"). (Note that the beginning and end of line -tests are not part of the recursion). -.P -Back at the top level, the next character ("c") is compared with what -subpattern 2 matched, which was "a". This fails. Because the recursion is -treated as an atomic group, there are now no backtracking points, and so the -entire match fails. (Perl is able, at this point, to re-enter the recursion and -try the second alternative.) However, if the pattern is written with the -alternatives in the other order, things are different: -.sp - ^((.)(?1)\e2|.)$ -.sp -This time, the recursing alternative is tried first, and continues to recurse -until it runs out of characters, at which point the recursion fails. But this -time we do have another alternative to try at the higher level. That is the big -difference: in the previous case the remaining alternative is at a deeper -recursion level, which PCRE cannot use. -.P -To change the pattern so that matches all palindromic strings, not just those -with an odd number of characters, it is tempting to change the pattern to this: -.sp - ^((.)(?1)\e2|.?)$ -.sp -Again, this works in Perl, but not in PCRE, and for the same reason. When a -deeper recursion has matched a single character, it cannot be entered again in -order to match an empty string. The solution is to separate the two cases, and -write out the odd and even cases as alternatives at the higher level: -.sp - ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.)) -.sp -If you want to match typical palindromic phrases, the pattern has to ignore all -non-word characters, which can be done like this: -.sp - ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$ -.sp -If run with the PCRE_CASELESS option, this pattern matches phrases such as "A -man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note -the use of the possessive quantifier *+ to avoid backtracking into sequences of -non-word characters. Without this, PCRE takes a great deal longer (ten times or -more) to match typical phrases, and Perl takes so long that you think it has -gone into a loop. -.P -\fBWARNING\fP: The palindrome-matching patterns above work only if the subject -string does not start with a palindrome that is shorter than the entire string. -For example, although "abcba" is correctly matched, if the subject is "ababa", -PCRE finds the palindrome "aba" at the start, then fails at top level because -the end of the string does not follow. Once again, it cannot jump back into the -recursion to try other alternatives, so the entire match fails. -. -. -.\" HTML <a name="subpatternsassubroutines"></a> -.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. Any capturing parentheses that are set during the subroutine -call revert to their previous values afterwards. -.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 <a name="onigurumasubroutines"></a> -.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 - (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \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 -If any of these verbs are used in an assertion or subroutine subpattern -(including recursive subpatterns), their effect is confined to that subpattern; -it does not extend to the surrounding pattern. Note that such subpatterns are -processed as anchored at the point where they are tested. -.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. If (*ACCEPT) is inside capturing parentheses, the data so far is -captured. (This feature was added to PCRE at release 8.00.) For example: -.sp - A((?:A|B(*ACCEPT)|C)D) -.sp -This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by -the outer parentheses. -.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 starting 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 as this example; although it would suppress backtracking during the -first match attempt, the second attempt would start at the second character -instead of skipping on to "c". -.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), -\fBpcresyntax\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: 18 October 2009 -Copyright (c) 1997-2009 University of Cambridge. -.fi |