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revision 457 by ph10, Sat Oct 3 16:24:08 2009 UTC revision 555 by ph10, Tue Oct 26 08:26:20 2010 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 42  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 83  string with one of the following five se Line 93  string with one of the following five se
93    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
94    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
95  .sp  .sp
96  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
97  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
98  newline sequence, the pattern  newline sequence, the pattern
99  .sp  .sp
# Line 95  Perl-compatible, are recognized only at Line 105  Perl-compatible, are recognized only at
105  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
106  is used.  is used.
107  .P  .P
108  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
109  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
110  this can be changed; see the description of \eR in the section entitled  affect what the \eR escape sequence matches. By default, this is any Unicode
111    newline sequence, for Perl compatibility. However, this can be changed; see the
112    description of \eR in the section entitled
113  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
114  .\" </a>  .\" </a>
115  "Newline sequences"  "Newline sequences"
# Line 198  Perl, $ and @ cause variable interpolati Line 210  Perl, $ and @ cause variable interpolati
210    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
211  .sp  .sp
212  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
213    An isolated \eE that is not preceded by \eQ is ignored.
214  .  .
215  .  .
216  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 217  one of the following escape sequences th Line 230  one of the following escape sequences th
230    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
231    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
232    \et        tab (hex 09)    \et        tab (hex 09)
233    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
234    \exhh      character with hex code hh    \exhh      character with hex code hh
235    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
236  .sp  .sp
# Line 295  zero, because no more than three octal d Line 308  zero, because no more than three octal d
308  .P  .P
309  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
310  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
311  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
312  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
313  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
314  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
315  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
316  .  .
317  .  .
318  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 343  syntax for referencing a subpattern as a
343  later.  later.
344  .\"  .\"
345  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
346  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
347  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
348  .\" </a>  .\" </a>
349  subroutine  subroutine
# Line 341  subroutine Line 351  subroutine
351  call.  call.
352  .  .
353  .  .
354    .\" HTML <a name="genericchartypes"></a>
355  .SS "Generic character types"  .SS "Generic character types"
356  .rs  .rs
357  .sp  .sp
358  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
359  .sp  .sp
360    \ed     any decimal digit    \ed     any decimal digit
361    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 358  following are always recognized: Line 368  following are always recognized:
368    \ew     any "word" character    \ew     any "word" character
369    \eW     any "non-word" character    \eW     any "non-word" character
370  .sp  .sp
371  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
372  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
373    .\" HTML <a href="#fullstopdot">
374    .\" </a>
375    the "." metacharacter
376    .\"
377    when PCRE_DOTALL is not set.
378  .P  .P
379  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
380    of characters into two disjoint sets. Any given character matches one, and only
381    one, of each pair. The sequences can appear both inside and outside character
382  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
383  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
384  there is no character to match.  there is no character to match.
385  .P  .P
386  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 372  are HT (9), LF (10), FF (12), CR (13), a Line 389  are HT (9), LF (10), FF (12), CR (13), a
389  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
390  does.  does.
391  .P  .P
392  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
393  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
394  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
395  meanings from before UTF-8 support was available, mainly for efficiency  place (see
396  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
397  and \eW.  .\" </a>
398    "Locale support"
399    .\"
400    in the
401    .\" HREF
402    \fBpcreapi\fP
403    .\"
404    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
405    or "french" in Windows, some character codes greater than 128 are used for
406    accented letters, and these are then matched by \ew. The use of locales with
407    Unicode is discouraged.
408    .P
409    By default, in UTF-8 mode, characters with values greater than 128 never match
410    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
411    their original meanings from before UTF-8 support was available, mainly for
412    efficiency reasons. However, if PCRE is compiled with Unicode property support,
413    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
414    properties are used to determine character types, as follows:
415    .sp
416      \ed  any character that \ep{Nd} matches (decimal digit)
417      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
418      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
419    .sp
420    The upper case escapes match the inverse sets of characters. Note that \ed
421    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
422    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
423    \eB because they are defined in terms of \ew and \eW. Matching these sequences
424    is noticeably slower when PCRE_UCP is set.
425  .P  .P
426  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
427  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, which match only ASCII characters by default, these always
428  The horizontal space characters are:  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
429    set. The horizontal space characters are:
430  .sp  .sp
431    U+0009     Horizontal tab    U+0009     Horizontal tab
432    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 457  The vertical space characters are:
457    U+0085     Next line    U+0085     Next line
458    U+2028     Line separator    U+2028     Line separator
459    U+2029     Paragraph 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.  
460  .  .
461  .  .
462  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 468  one of the following sequences: Line 496  one of the following sequences:
496    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
497    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
498  .sp  .sp
499  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
500  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
501  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
502  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
503  pattern, and that they must be in upper case. If more than one of them is  pattern, and that they must be in upper case. If more than one of them is
504  present, the last one is used. They can be combined with a change of newline  present, the last one is used. They can be combined with a change of newline
505  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
506  .sp  .sp
507    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
508  .sp  .sp
509  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
510    a character class, \eR is treated as an unrecognized escape sequence, and so
511    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
512  .  .
513  .  .
514  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 526  The extra escape sequences are:
526    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
527  .sp  .sp
528  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
529  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
530  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
531  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
532  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
533    .\" </a>
534    next section).
535    .\"
536    Other Perl properties such as "InMusicalSymbols" are not currently supported by
537    PCRE. Note that \eP{Any} does not match any characters, so always causes a
538    match failure.
539  .P  .P
540  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
541  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 513  Those that are not part of an identified Line 549  Those that are not part of an identified
549  .P  .P
550  Arabic,  Arabic,
551  Armenian,  Armenian,
552    Avestan,
553  Balinese,  Balinese,
554    Bamum,
555  Bengali,  Bengali,
556  Bopomofo,  Bopomofo,
557  Braille,  Braille,
558  Buginese,  Buginese,
559  Buhid,  Buhid,
560  Canadian_Aboriginal,  Canadian_Aboriginal,
561    Carian,
562    Cham,
563  Cherokee,  Cherokee,
564  Common,  Common,
565  Coptic,  Coptic,
# Line 528  Cypriot, Line 568  Cypriot,
568  Cyrillic,  Cyrillic,
569  Deseret,  Deseret,
570  Devanagari,  Devanagari,
571    Egyptian_Hieroglyphs,
572  Ethiopic,  Ethiopic,
573  Georgian,  Georgian,
574  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 581  Hangul,
581  Hanunoo,  Hanunoo,
582  Hebrew,  Hebrew,
583  Hiragana,  Hiragana,
584    Imperial_Aramaic,
585  Inherited,  Inherited,
586    Inscriptional_Pahlavi,
587    Inscriptional_Parthian,
588    Javanese,
589    Kaithi,
590  Kannada,  Kannada,
591  Katakana,  Katakana,
592    Kayah_Li,
593  Kharoshthi,  Kharoshthi,
594  Khmer,  Khmer,
595  Lao,  Lao,
596  Latin,  Latin,
597    Lepcha,
598  Limbu,  Limbu,
599  Linear_B,  Linear_B,
600    Lisu,
601    Lycian,
602    Lydian,
603  Malayalam,  Malayalam,
604    Meetei_Mayek,
605  Mongolian,  Mongolian,
606  Myanmar,  Myanmar,
607  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 609  Nko,
609  Ogham,  Ogham,
610  Old_Italic,  Old_Italic,
611  Old_Persian,  Old_Persian,
612    Old_South_Arabian,
613    Old_Turkic,
614    Ol_Chiki,
615  Oriya,  Oriya,
616  Osmanya,  Osmanya,
617  Phags_Pa,  Phags_Pa,
618  Phoenician,  Phoenician,
619    Rejang,
620  Runic,  Runic,
621    Samaritan,
622    Saurashtra,
623  Shavian,  Shavian,
624  Sinhala,  Sinhala,
625    Sundanese,
626  Syloti_Nagri,  Syloti_Nagri,
627  Syriac,  Syriac,
628  Tagalog,  Tagalog,
629  Tagbanwa,  Tagbanwa,
630  Tai_Le,  Tai_Le,
631    Tai_Tham,
632    Tai_Viet,
633  Tamil,  Tamil,
634  Telugu,  Telugu,
635  Thaana,  Thaana,
# Line 576  Thai, Line 637  Thai,
637  Tibetan,  Tibetan,
638  Tifinagh,  Tifinagh,
639  Ugaritic,  Ugaritic,
640    Vai,
641  Yi.  Yi.
642  .P  .P
643  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
644  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
645  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
646  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
647  .P  .P
648  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
649  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 680  non-UTF-8 mode \eX matches any one chara Line 742  non-UTF-8 mode \eX matches any one chara
742  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
743  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
744  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
745  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
746    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
747    (*UCP).
748    .
749    .
750    .\" HTML <a name="extraprops"></a>
751    .SS PCRE's additional properties
752    .rs
753    .sp
754    As well as the standard Unicode properties described in the previous
755    section, PCRE supports four more that make it possible to convert traditional
756    escape sequences such as \ew and \es and POSIX character classes to use Unicode
757    properties. PCRE uses these non-standard, non-Perl properties internally when
758    PCRE_UCP is set. They are:
759    .sp
760      Xan   Any alphanumeric character
761      Xps   Any POSIX space character
762      Xsp   Any Perl space character
763      Xwd   Any Perl "word" character
764    .sp
765    Xan matches characters that have either the L (letter) or the N (number)
766    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
767    carriage return, and any other character that has the Z (separator) property.
768    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
769    same characters as Xan, plus underscore.
770  .  .
771  .  .
772  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 711  For example, when the pattern Line 797  For example, when the pattern
797    (foo)\eKbar    (foo)\eKbar
798  .sp  .sp
799  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
800    .P
801    Perl documents that the use of \eK within assertions is "not well defined". In
802    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
803    ignored in negative assertions.
804  .  .
805  .  .
806  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 825  The backslashed assertions are:
825    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
826    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
827  .sp  .sp
828  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
829  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
830    default it matches the corresponding literal character (for example, \eB
831    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
832    escape sequence" error is generated instead.
833  .P  .P
834  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
835  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
836  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
837  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
838  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
839  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
840  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
841    determines which it is. For example, the fragment \eba matches "a" at the start
842    of a word.
843  .P  .P
844  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
845  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 828  end of the subject in both modes, and if Line 923  end of the subject in both modes, and if
923  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
924  .  .
925  .  .
926  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
927    .SH "FULL STOP (PERIOD, DOT) AND \eN"
928  .rs  .rs
929  .sp  .sp
930  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 850  to match it. Line 946  to match it.
946  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
947  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
948  special meaning in a character class.  special meaning in a character class.
949    .P
950    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
951    set. In other words, it matches any one character except one that signifies the
952    end of a line.
953  .  .
954  .  .
955  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 876  the lookbehind. Line 976  the lookbehind.
976  .rs  .rs
977  .sp  .sp
978  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
979  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
980  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
981  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
982  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
983  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
# Line 944  characters in both cases. In UTF-8 mode, Line 1044  characters in both cases. In UTF-8 mode,
1044  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1045  property support.  property support.
1046  .P  .P
1047  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1048  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1049  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1050  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1051  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1052  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1053  .P  .P
1054  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1055  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 969  this notation. For example, Line 1069  this notation. For example,
1069    [01[:alpha:]%]    [01[:alpha:]%]
1070  .sp  .sp
1071  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1072  are  are:
1073  .sp  .sp
1074    alnum    letters and digits    alnum    letters and digits
1075    alpha    letters    alpha    letters
# Line 980  are Line 1080  are
1080    graph    printing characters, excluding space    graph    printing characters, excluding space
1081    lower    lower case letters    lower    lower case letters
1082    print    printing characters, including space    print    printing characters, including space
1083    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1084    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1085    upper    upper case letters    upper    upper case letters
1086    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1101  matches "1", "2", or any non-digit. PCRE
1101  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1102  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1103  .P  .P
1104  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1105  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1106    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1107    character properties are used. This is achieved by replacing the POSIX classes
1108    by other sequences, as follows:
1109    .sp
1110      [:alnum:]  becomes  \ep{Xan}
1111      [:alpha:]  becomes  \ep{L}
1112      [:blank:]  becomes  \eh
1113      [:digit:]  becomes  \ep{Nd}
1114      [:lower:]  becomes  \ep{Ll}
1115      [:space:]  becomes  \ep{Xps}
1116      [:upper:]  becomes  \ep{Lu}
1117      [:word:]   becomes  \ep{Xwd}
1118    .sp
1119    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1120    classes are unchanged, and match only characters with code points less than
1121    128.
1122  .  .
1123  .  .
1124  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1081  section entitled Line 1197  section entitled
1197  .\" </a>  .\" </a>
1198  "Newline sequences"  "Newline sequences"
1199  .\"  .\"
1200  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1201  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1202    PCRE_UTF8 and the PCRE_UCP options, respectively.
1203  .  .
1204  .  .
1205  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1163  stored. Line 1280  stored.
1280    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1281    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1282  .sp  .sp
1283  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1284  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1285  "defdef":  or "defdef":
1286  .sp  .sp
1287    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1288  .sp  .sp
1289  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1290  refers to the first one in the pattern with the given number. The following  refers to the first one in the pattern with the given number. The following
1291  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1292  .sp  .sp
1293    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1294  .sp  .sp
1295  An alternative approach to using the "branch reset" feature is to use  If a
1296    .\" HTML <a href="#conditions">
1297    .\" </a>
1298    condition test
1299    .\"
1300    for a subpattern's having matched refers to a non-unique number, the test is
1301    true if any of the subpatterns of that number have matched.
1302    .P
1303    An alternative approach to using this "branch reset" feature is to use
1304  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1305  .  .
1306  .  .
# Line 1188  if an expression is modified, the number Line 1313  if an expression is modified, the number
1313  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1314  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1315  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1316  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1317    have different names, but PCRE does not.
1318  .P  .P
1319  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1320  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1321  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1322  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1323  .\" </a>  .\" </a>
1324  backreferences,  back references,
1325  .\"  .\"
1326  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1327  .\" </a>  .\" </a>
# Line 1216  is also a convenience function for extra Line 1342  is also a convenience function for extra
1342  .P  .P
1343  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1344  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1345  names are also always permitted for subpatterns with the same number, set up as  names are also always permitted for subpatterns with the same number, set up as
1346  described in the previous section.) Duplicate names can be useful for patterns  described in the previous section.) Duplicate names can be useful for patterns
1347  where only one instance of the named parentheses can match. Suppose you want to  where only one instance of the named parentheses can match. Suppose you want to
1348  match the name of a weekday, either as a 3-letter abbreviation or as the full  match the name of a weekday, either as a 3-letter abbreviation or as the full
# Line 1235  subpattern, as described in the previous Line 1361  subpattern, as described in the previous
1361  .P  .P
1362  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1363  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1364  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1365  make a reference to a non-unique named subpattern from elsewhere in the  .P
1366  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1367  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1368    used. In the absence of duplicate numbers (see the previous section) this is
1369    the one with the lowest number. If you use a named reference in a condition
1370    test (see the
1371    .\"
1372    .\" HTML <a href="#conditions">
1373    .\" </a>
1374    section about conditions
1375    .\"
1376    below), either to check whether a subpattern has matched, or to check for
1377    recursion, all subpatterns with the same name are tested. If the condition is
1378    true for any one of them, the overall condition is true. This is the same
1379    behaviour as testing by number. For further details of the interfaces for
1380    handling named subpatterns, see the
1381  .\" HREF  .\" HREF
1382  \fBpcreapi\fP  \fBpcreapi\fP
1383  .\"  .\"
# Line 1266  items: Line 1405  items:
1405    a character class    a character class
1406    a back reference (see next section)    a back reference (see next section)
1407    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1408    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1409  .sp  .sp
1410  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1411  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1377  worth setting PCRE_DOTALL in order to ob Line 1516  worth setting PCRE_DOTALL in order to ob
1516  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1517  .P  .P
1518  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1519  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1520  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1521  succeeds. Consider, for example:  succeeds. Consider, for example:
1522  .sp  .sp
# Line 1592  references to it always fail by default. Line 1731  references to it always fail by default.
1731  .sp  .sp
1732    (a|(bc))\e2    (a|(bc))\e2
1733  .sp  .sp
1734  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1735  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1736  unset value matches an empty string.  unset value matches an empty string.
1737  .P  .P
1738  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
# Line 1606  whitespace. Otherwise, the \eg{ syntax o Line 1745  whitespace. Otherwise, the \eg{ syntax o
1745  "Comments"  "Comments"
1746  .\"  .\"
1747  below) can be used.  below) can be used.
1748  .P  .
1749    .SS "Recursive back references"
1750    .rs
1751    .sp
1752  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1753  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1754  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1620  to the previous iteration. In order for Line 1762  to the previous iteration. In order for
1762  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1763  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1764  minimum of zero.  minimum of zero.
1765    .P
1766    Back references of this type cause the group that they reference to be treated
1767    as an
1768    .\" HTML <a href="#atomicgroup">
1769    .\" </a>
1770    atomic group.
1771    .\"
1772    Once the whole group has been matched, a subsequent matching failure cannot
1773    cause backtracking into the middle of the group.
1774  .  .
1775  .  .
1776  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1715  In some cases, the Perl 5.10 escape sequ Line 1866  In some cases, the Perl 5.10 escape sequ
1866  .\" </a>  .\" </a>
1867  (see above)  (see above)
1868  .\"  .\"
1869  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
1870  restriction.  restriction.
1871  .P  .P
1872  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1733  different numbers of bytes, are also not Line 1884  different numbers of bytes, are also not
1884  "Subroutine"  "Subroutine"
1885  .\"  .\"
1886  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1887  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1888  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1889  .\" </a>  .\" </a>
1890  Recursion,  Recursion,
# Line 1806  characters that are not "999". Line 1957  characters that are not "999".
1957  .sp  .sp
1958  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1959  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1960  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1961  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1962  .sp  .sp
1963    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1824  recursion, a pseudo-condition called DEF Line 1975  recursion, a pseudo-condition called DEF
1975  .sp  .sp
1976  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1977  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1978  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
1979  (see the earlier  (see the earlier
1980  .\"  .\"
1981  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1982  .\" </a>  .\" </a>
# Line 1877  Rewriting the above example to use a nam Line 2028  Rewriting the above example to use a nam
2028  .sp  .sp
2029    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2030  .sp  .sp
2031    If the name used in a condition of this kind is a duplicate, the test is
2032    applied to all subpatterns of the same name, and is true if any one of them has
2033    matched.
2034  .  .
2035  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2036  .rs  .rs
# Line 1890  letter R, for example: Line 2044  letter R, for example:
2044  .sp  .sp
2045  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2046  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2047  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2048    applied to all subpatterns of the same name, and is true if any one of them is
2049    the most recent recursion.
2050  .P  .P
2051  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2052  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2053  .\" </a>  .\" </a>
2054  Recursive patterns  The syntax for recursive patterns
2055  .\"  .\"
2056  are described below.  is described below.
2057  .  .
2058  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2059  .rs  .rs
# Line 1906  If the condition is the string (DEFINE), Line 2062  If the condition is the string (DEFINE),
2062  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2063  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2064  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2065  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
2066  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2067  .\" </a>  .\" </a>
2068  "subroutines"  "subroutines"
# Line 1983  this kind of recursion was subsequently Line 2139  this kind of recursion was subsequently
2139  .P  .P
2140  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2141  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2142  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2143  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2144  .\" </a>  .\" </a>
2145  "subroutine"  "subroutine"
# Line 1999  PCRE_EXTENDED option is set so that whit Line 2155  PCRE_EXTENDED option is set so that whit
2155  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2156  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2157  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2158  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2159  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2160  .P  .P
2161  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 2047  the match runs for a very long time inde Line 2203  the match runs for a very long time inde
2203  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2204  before failure can be reported.  before failure can be reported.
2205  .P  .P
2206  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2207  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2208  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2209  .\" HREF  .\" HREF
2210  \fBpcrecallout\fP  \fBpcrecallout\fP
2211  .\"  .\"
# Line 2058  documentation). If the pattern above is Line 2213  documentation). If the pattern above is
2213  .sp  .sp
2214    (ab(cd)ef)    (ab(cd)ef)
2215  .sp  .sp
2216  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2217  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2218  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2219    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2220       ^                        ^  .P
2221       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2222  .sp  obtain extra memory to store data during a recursion, which it does by using
2223  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2224  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 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.  
2225  .P  .P
2226  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2227  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2090  is the actual recursive call. Line 2242  is the actual recursive call.
2242  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2243  treated as an atomic group. That is, once it has matched some of the subject  treated as an atomic group. That is, once it has matched some of the subject
2244  string, it is never re-entered, even if it contains untried alternatives and  string, it is never re-entered, even if it contains untried alternatives and
2245  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2246  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2247  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2248  .sp  .sp
2249    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2250  .sp  .sp
2251  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2252  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2253  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2254  subject string "abcba":  subject string "abcba":
2255  .P  .P
2256  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2257  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2258  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2259  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2260  tests are not part of the recursion).  tests are not part of the recursion).
2261  .P  .P
2262  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2263  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2264  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2265  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2266  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2116  alternatives in the other order, things Line 2268  alternatives in the other order, things
2268  .sp  .sp
2269    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2270  .sp  .sp
2271  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2272  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2273  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2274  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2275  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2276  .P  .P
2277  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that matches all palindromic strings, not just those
2278  with an odd number of characters, it is tempting to change the pattern to this:  with an odd number of characters, it is tempting to change the pattern to this:
2279  .sp  .sp
2280    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2281  .sp  .sp
2282  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2283  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2284  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2285  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2286  .sp  .sp
2287    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2288  .sp  .sp
2289  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2290  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2291  .sp  .sp
2292    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2293  .sp  .sp
2294  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2295  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2296  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2297  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2298  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2299  gone into a loop.  gone into a loop.
# Line 2180  matches "sense and sensibility" and "res Line 2332  matches "sense and sensibility" and "res
2332  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2333  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2334  .P  .P
2335  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2336  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2337  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2338  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2339    call revert to their previous values afterwards.
2340  .P  .P
2341  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2342  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 2252  description of the interface to the call Line 2405  description of the interface to the call
2405  documentation.  documentation.
2406  .  .
2407  .  .
2408    .\" HTML <a name="backtrackcontrol"></a>
2409  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2410  .rs  .rs
2411  .sp  .sp
# Line 2267  a backtracking algorithm. With the excep Line 2421  a backtracking algorithm. With the excep
2421  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2422  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2423  .P  .P
2424  If any of these verbs are used in an assertion subpattern, their effect is  If any of these verbs are used in an assertion or subroutine subpattern
2425  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2426  Note that assertion subpatterns are processed as anchored at the point where  it does not extend to the surrounding pattern. Note that such subpatterns are
2427  they are tested.  processed as anchored at the point where they are tested.
2428  .P  .P
2429  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2430  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2431  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2432  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. An name is a sequence of
2433  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2434    parenthesis immediately follows the colon, the effect is as if the colon were
2435    not there. Any number of these verbs may occur in a pattern.
2436    .P
2437    PCRE contains some optimizations that are used to speed up matching by running
2438    some checks at the start of each match attempt. For example, it may know the
2439    minimum length of matching subject, or that a particular character must be
2440    present. When one of these optimizations suppresses the running of a match, any
2441    included backtracking verbs will not, of course, be processed. You can suppress
2442    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2443    when calling \fBpcre_exec()\fP.
2444    .
2445  .  .
2446  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2447  .rs  .rs
2448  .sp  .sp
2449  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2450    followed by a name.
2451  .sp  .sp
2452     (*ACCEPT)     (*ACCEPT)
2453  .sp  .sp
# Line 2292  captured. (This feature was added to PCR Line 2458  captured. (This feature was added to PCR
2458  .sp  .sp
2459    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2460  .sp  .sp
2461  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2462  the outer parentheses.  the outer parentheses.
2463  .sp  .sp
2464    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2308  callout feature, as for example in this Line 2474  callout feature, as for example in this
2474  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2475  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2476  .  .
2477    .
2478    .SS "Recording which path was taken"
2479    .rs
2480    .sp
2481    There is one verb whose main purpose is to track how a match was arrived at,
2482    though it also has a secondary use in conjunction with advancing the match
2483    starting point (see (*SKIP) below).
2484    .sp
2485      (*MARK:NAME) or (*:NAME)
2486    .sp
2487    A name is always required with this verb. There may be as many instances of
2488    (*MARK) as you like in a pattern, and their names do not have to be unique.
2489    .P
2490    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2491    to the caller via the \fIpcre_extra\fP data structure, as described in the
2492    .\" HTML <a href="pcreapi.html#extradata">
2493    .\" </a>
2494    section on \fIpcre_extra\fP
2495    .\"
2496    in the
2497    .\" HREF
2498    \fBpcreapi\fP
2499    .\"
2500    documentation. No data is returned for a partial match. Here is an example of
2501    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2502    outputting of (*MARK) data:
2503    .sp
2504      /X(*MARK:A)Y|X(*MARK:B)Z/K
2505      XY
2506       0: XY
2507      MK: A
2508      XZ
2509       0: XZ
2510      MK: B
2511    .sp
2512    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2513    indicates which of the two alternatives matched. This is a more efficient way
2514    of obtaining this information than putting each alternative in its own
2515    capturing parentheses.
2516    .P
2517    A name may also be returned after a failed match if the final path through the
2518    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2519    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2520    starting point for matching is advanced, the final check is often with an empty
2521    string, causing a failure before (*MARK) is reached. For example:
2522    .sp
2523      /X(*MARK:A)Y|X(*MARK:B)Z/K
2524      XP
2525      No match
2526    .sp
2527    There are three potential starting points for this match (starting with X,
2528    starting with P, and with an empty string). If the pattern is anchored, the
2529    result is different:
2530    .sp
2531      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2532      XP
2533      No match, mark = B
2534    .sp
2535    PCRE's start-of-match optimizations can also interfere with this. For example,
2536    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2537    subject length for a match, a shorter subject will not be scanned at all.
2538    .P
2539    Note that similar anomalies (though different in detail) exist in Perl, no
2540    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2541    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2542    .
2543    .
2544  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2545  .rs  .rs
2546  .sp  .sp
2547  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2548  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2549  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2550    the verb. However, when one of these verbs appears inside an atomic group, its
2551    effect is confined to that group, because once the group has been matched,
2552    there is never any backtracking into it. In this situation, backtracking can
2553    "jump back" to the left of the entire atomic group. (Remember also, as stated
2554    above, that this localization also applies in subroutine calls and assertions.)
2555    .P
2556    These verbs differ in exactly what kind of failure occurs when backtracking
2557    reaches them.
2558  .sp  .sp
2559    (*COMMIT)    (*COMMIT)
2560  .sp  .sp
2561  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2562  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2563  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2564  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2565  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2566  .sp  .sp
2567    a+(*COMMIT)b    a+(*COMMIT)b
2568  .sp  .sp
2569  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2570  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2571  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2572    (*PRUNE)  match failure.
2573  .sp  .P
2574  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2575  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2576  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2577  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2578  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2579  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2580  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2581  be expressed in any other way.    xyzabc\eY
2582      No match
2583    .sp
2584    PCRE knows that any match must start with "a", so the optimization skips along
2585    the subject to "a" before running the first match attempt, which succeeds. When
2586    the optimization is disabled by the \eY escape in the second subject, the match
2587    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2588    starting points.
2589    .sp
2590      (*PRUNE) or (*PRUNE:NAME)
2591    .sp
2592    This verb causes the match to fail at the current starting position in the
2593    subject if the rest of the pattern does not match. If the pattern is
2594    unanchored, the normal "bumpalong" advance to the next starting character then
2595    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2596    reached, or when matching to the right of (*PRUNE), but if there is no match to
2597    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2598    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2599    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2600    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2601    match fails completely; the name is passed back if this is the final attempt.
2602    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2603    pattern (*PRUNE) has the same effect as (*COMMIT).
2604  .sp  .sp
2605    (*SKIP)    (*SKIP)
2606  .sp  .sp
2607  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2608  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2609  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2610  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2611    successful match. Consider:
2612  .sp  .sp
2613    a+(*SKIP)b    a+(*SKIP)b
2614  .sp  .sp
# Line 2355  effect as this example; although it woul Line 2619  effect as this example; although it woul
2619  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2620  instead of skipping on to "c".  instead of skipping on to "c".
2621  .sp  .sp
2622    (*THEN)    (*SKIP:NAME)
2623  .sp  .sp
2624  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2625  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2626  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2627  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2628    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2629    matching name is found, normal "bumpalong" of one character happens (the
2630    (*SKIP) is ignored).
2631    .sp
2632      (*THEN) or (*THEN:NAME)
2633    .sp
2634    This verb causes a skip to the next alternation in the innermost enclosing
2635    group if the rest of the pattern does not match. That is, it cancels pending
2636    backtracking, but only within the current alternation. Its name comes from the
2637    observation that it can be used for a pattern-based if-then-else block:
2638  .sp  .sp
2639    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2640  .sp  .sp
2641  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2642  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure the matcher skips to the
2643  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2644  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2645    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2646    like (*PRUNE).
2647    .
2648    .P
2649    The above verbs provide four different "strengths" of control when subsequent
2650    matching fails. (*THEN) is the weakest, carrying on the match at the next
2651    alternation. (*PRUNE) comes next, failing the match at the current starting
2652    position, but allowing an advance to the next character (for an unanchored
2653    pattern). (*SKIP) is similar, except that the advance may be more than one
2654    character. (*COMMIT) is the strongest, causing the entire match to fail.
2655    .P
2656    If more than one is present in a pattern, the "stongest" one wins. For example,
2657    consider this pattern, where A, B, etc. are complex pattern fragments:
2658    .sp
2659      (A(*COMMIT)B(*THEN)C|D)
2660    .sp
2661    Once A has matched, PCRE is committed to this match, at the current starting
2662    position. If subsequently B matches, but C does not, the normal (*THEN) action
2663    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2664    overrides.
2665  .  .
2666  .  .
2667  .SH "SEE ALSO"  .SH "SEE ALSO"
2668  .rs  .rs
2669  .sp  .sp
2670  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2671  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2672  .  .
2673  .  .
# Line 2391  Cambridge CB2 3QH, England. Line 2685  Cambridge CB2 3QH, England.
2685  .rs  .rs
2686  .sp  .sp
2687  .nf  .nf
2688  Last updated: 03 October 2009  Last updated: 26 October 2010
2689  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2690  .fi  .fi

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