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revision 451 by ph10, Wed Sep 16 11:05:51 2009 UTC revision 628 by ph10, Wed Jul 20 18:03:20 2011 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, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. There is also a special sequence that can be given at the  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  start of a pattern:  is also a special sequence that can be given at the start of a pattern:
28  .sp  .sp
29    (*UTF8)    (*UTF8)
30  .sp  .sp
# 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    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
61  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 56  discussed in the Line 71  discussed in the
71  page.  page.
72  .  .
73  .  .
74    .\" HTML <a name="newlines"></a>
75  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
76  .rs  .rs
77  .sp  .sp
# Line 83  string with one of the following five se Line 99  string with one of the following five se
99    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
100    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
101  .sp  .sp
102  These override the default and the options given to \fBpcre_compile()\fP. For  These override the default and the options given to \fBpcre_compile()\fP or
103  example, on a Unix system where LF is the default newline sequence, the pattern  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
104    newline sequence, the pattern
105  .sp  .sp
106    (*CR)a.b    (*CR)a.b
107  .sp  .sp
# Line 94  Perl-compatible, are recognized only at Line 111  Perl-compatible, are recognized only at
111  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
112  is used.  is used.
113  .P  .P
114  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
115  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
116  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
117    newline sequence, for Perl compatibility. However, this can be changed; see the
118    description of \eR in the section entitled
119  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
120  .\" </a>  .\" </a>
121  "Newline sequences"  "Newline sequences"
# Line 168  The following sections describe the use Line 187  The following sections describe the use
187  .rs  .rs
188  .sp  .sp
189  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
190  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
191  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
192  outside character classes.  both inside and outside character classes.
193  .P  .P
194  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
195  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 178  otherwise be interpreted as a metacharac Line 197  otherwise be interpreted as a metacharac
197  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
198  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
199  .P  .P
200    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201    backslash. All other characters (in particular, those whose codepoints are
202    greater than 127) are treated as literals.
203    .P
204  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
205  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
206  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 197  Perl, $ and @ cause variable interpolati Line 220  Perl, $ and @ cause variable interpolati
220    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
221  .sp  .sp
222  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
223    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224    by \eE later in the pattern, the literal interpretation continues to the end of
225    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226    a character class, this causes an error, because the character class is not
227    terminated.
228  .  .
229  .  .
230  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 206  The \eQ...\eE sequence is recognized bot Line 234  The \eQ...\eE sequence is recognized bot
234  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
235  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
236  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
237  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
238  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
239  .sp  .sp
240    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
241    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
242    \ee        escape (hex 1B)    \ee        escape (hex 1B)
243    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
244    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
245    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
246    \et        tab (hex 09)    \et        tab (hex 09)
247    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
248    \exhh      character with hex code hh    \exhh      character with hex code hh
249    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
250  .sp  .sp
251  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
252  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
253  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
254  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255    than 127, a compile-time error occurs. This locks out non-ASCII characters in
256    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257    values are valid. A lower case letter is converted to upper case, and then the
258    0xc0 bits are flipped.)
259  .P  .P
260  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
261  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
# Line 295  zero, because no more than three octal d Line 326  zero, because no more than three octal d
326  .P  .P
327  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
328  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
329  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
330  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
331  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
332  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
333  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
334  .  .
335  .  .
336  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 361  syntax for referencing a subpattern as a
361  later.  later.
362  .\"  .\"
363  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
364  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
365    .\" HTML <a href="#subpatternsassubroutines">
366    .\" </a>
367    subroutine
368    .\"
369    call.
370  .  .
371  .  .
372    .\" HTML <a name="genericchartypes"></a>
373  .SS "Generic character types"  .SS "Generic character types"
374  .rs  .rs
375  .sp  .sp
376  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:  
377  .sp  .sp
378    \ed     any decimal digit    \ed     any decimal digit
379    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 353  following are always recognized: Line 386  following are always recognized:
386    \ew     any "word" character    \ew     any "word" character
387    \eW     any "non-word" character    \eW     any "non-word" character
388  .sp  .sp
389  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.
390  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
391    .\" HTML <a href="#fullstopdot">
392    .\" </a>
393    the "." metacharacter
394    .\"
395    when PCRE_DOTALL is not set.
396  .P  .P
397  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
398    of characters into two disjoint sets. Any given character matches one, and only
399    one, of each pair. The sequences can appear both inside and outside character
400  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
401  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
402  there is no character to match.  there is no character to match.
403  .P  .P
404  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 367  are HT (9), LF (10), FF (12), CR (13), a Line 407  are HT (9), LF (10), FF (12), CR (13), a
407  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
408  does.  does.
409  .P  .P
410  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.
411  \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
412  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
413  meanings from before UTF-8 support was available, mainly for efficiency  place (see
414  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
415  and \eW.  .\" </a>
416  .P  "Locale support"
417  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
418  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
419  The horizontal space characters are:  .\" HREF
420    \fBpcreapi\fP
421    .\"
422    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
423    or "french" in Windows, some character codes greater than 128 are used for
424    accented letters, and these are then matched by \ew. The use of locales with
425    Unicode is discouraged.
426    .P
427    By default, in UTF-8 mode, characters with values greater than 128 never match
428    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
429    their original meanings from before UTF-8 support was available, mainly for
430    efficiency reasons. However, if PCRE is compiled with Unicode property support,
431    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
432    properties are used to determine character types, as follows:
433    .sp
434      \ed  any character that \ep{Nd} matches (decimal digit)
435      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
436      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
437    .sp
438    The upper case escapes match the inverse sets of characters. Note that \ed
439    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
440    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
441    \eB because they are defined in terms of \ew and \eW. Matching these sequences
442    is noticeably slower when PCRE_UCP is set.
443    .P
444    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
445    release 5.10. In contrast to the other sequences, which match only ASCII
446    characters by default, these always match certain high-valued codepoints in
447    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448    are:
449  .sp  .sp
450    U+0009     Horizontal tab    U+0009     Horizontal tab
451    U+0020     Space    U+0020     Space
# Line 407  The vertical space characters are: Line 476  The vertical space characters are:
476    U+0085     Next line    U+0085     Next line
477    U+2028     Line separator    U+2028     Line separator
478    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.  
479  .  .
480  .  .
481  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 431  is discouraged. Line 483  is discouraged.
483  .rs  .rs
484  .sp  .sp
485  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
486  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
487  .sp  .sp
488    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489  .sp  .sp
# Line 463  one of the following sequences: Line 514  one of the following sequences:
514    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
515    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
516  .sp  .sp
517  These override the default and the options given to \fBpcre_compile()\fP, but  These override the default and the options given to \fBpcre_compile()\fP or
518  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  \fBpcre_compile2()\fP, but they can be overridden by options given to
519  special settings, which are not Perl-compatible, are recognized only at the  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
520  very start of a pattern, and that they must be in upper case. If more than one  which are not Perl-compatible, are recognized only at the very start of a
521  of them is present, the last one is used. They can be combined with a change of  pattern, and that they must be in upper case. If more than one of them is
522  newline convention, for example, a pattern can start with:  present, the last one is used. They can be combined with a change of newline
523    convention; for example, a pattern can start with:
524  .sp  .sp
525    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
526  .sp  .sp
527  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
528    a character class, \eR is treated as an unrecognized escape sequence, and so
529    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
530  .  .
531  .  .
532  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 490  The extra escape sequences are: Line 544  The extra escape sequences are:
544    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
545  .sp  .sp
546  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
547  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
548  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
549  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
550  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
551    .\" </a>
552    next section).
553    .\"
554    Other Perl properties such as "InMusicalSymbols" are not currently supported by
555    PCRE. Note that \eP{Any} does not match any characters, so always causes a
556    match failure.
557  .P  .P
558  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
559  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 507  Those that are not part of an identified Line 567  Those that are not part of an identified
567  .P  .P
568  Arabic,  Arabic,
569  Armenian,  Armenian,
570    Avestan,
571  Balinese,  Balinese,
572    Bamum,
573  Bengali,  Bengali,
574  Bopomofo,  Bopomofo,
575  Braille,  Braille,
576  Buginese,  Buginese,
577  Buhid,  Buhid,
578  Canadian_Aboriginal,  Canadian_Aboriginal,
579    Carian,
580    Cham,
581  Cherokee,  Cherokee,
582  Common,  Common,
583  Coptic,  Coptic,
# Line 522  Cypriot, Line 586  Cypriot,
586  Cyrillic,  Cyrillic,
587  Deseret,  Deseret,
588  Devanagari,  Devanagari,
589    Egyptian_Hieroglyphs,
590  Ethiopic,  Ethiopic,
591  Georgian,  Georgian,
592  Glagolitic,  Glagolitic,
# Line 534  Hangul, Line 599  Hangul,
599  Hanunoo,  Hanunoo,
600  Hebrew,  Hebrew,
601  Hiragana,  Hiragana,
602    Imperial_Aramaic,
603  Inherited,  Inherited,
604    Inscriptional_Pahlavi,
605    Inscriptional_Parthian,
606    Javanese,
607    Kaithi,
608  Kannada,  Kannada,
609  Katakana,  Katakana,
610    Kayah_Li,
611  Kharoshthi,  Kharoshthi,
612  Khmer,  Khmer,
613  Lao,  Lao,
614  Latin,  Latin,
615    Lepcha,
616  Limbu,  Limbu,
617  Linear_B,  Linear_B,
618    Lisu,
619    Lycian,
620    Lydian,
621  Malayalam,  Malayalam,
622    Meetei_Mayek,
623  Mongolian,  Mongolian,
624  Myanmar,  Myanmar,
625  New_Tai_Lue,  New_Tai_Lue,
# Line 551  Nko, Line 627  Nko,
627  Ogham,  Ogham,
628  Old_Italic,  Old_Italic,
629  Old_Persian,  Old_Persian,
630    Old_South_Arabian,
631    Old_Turkic,
632    Ol_Chiki,
633  Oriya,  Oriya,
634  Osmanya,  Osmanya,
635  Phags_Pa,  Phags_Pa,
636  Phoenician,  Phoenician,
637    Rejang,
638  Runic,  Runic,
639    Samaritan,
640    Saurashtra,
641  Shavian,  Shavian,
642  Sinhala,  Sinhala,
643    Sundanese,
644  Syloti_Nagri,  Syloti_Nagri,
645  Syriac,  Syriac,
646  Tagalog,  Tagalog,
647  Tagbanwa,  Tagbanwa,
648  Tai_Le,  Tai_Le,
649    Tai_Tham,
650    Tai_Viet,
651  Tamil,  Tamil,
652  Telugu,  Telugu,
653  Thaana,  Thaana,
# Line 570  Thai, Line 655  Thai,
655  Tibetan,  Tibetan,
656  Tifinagh,  Tifinagh,
657  Ugaritic,  Ugaritic,
658    Vai,
659  Yi.  Yi.
660  .P  .P
661  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
662  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
663  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
664  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
665  .P  .P
666  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
667  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 671  Characters with the "mark" property are Line 757  Characters with the "mark" property are
757  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
758  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
759  .P  .P
760    Note that recent versions of Perl have changed \eX to match what Unicode calls
761    an "extended grapheme cluster", which has a more complicated definition.
762    .P
763  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
764  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
765  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
766  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
767    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
768    (*UCP).
769    .
770    .
771    .\" HTML <a name="extraprops"></a>
772    .SS PCRE's additional properties
773    .rs
774    .sp
775    As well as the standard Unicode properties described in the previous
776    section, PCRE supports four more that make it possible to convert traditional
777    escape sequences such as \ew and \es and POSIX character classes to use Unicode
778    properties. PCRE uses these non-standard, non-Perl properties internally when
779    PCRE_UCP is set. They are:
780    .sp
781      Xan   Any alphanumeric character
782      Xps   Any POSIX space character
783      Xsp   Any Perl space character
784      Xwd   Any Perl "word" character
785    .sp
786    Xan matches characters that have either the L (letter) or the N (number)
787    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
788    carriage return, and any other character that has the Z (separator) property.
789    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
790    same characters as Xan, plus underscore.
791  .  .
792  .  .
793  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
794  .SS "Resetting the match start"  .SS "Resetting the match start"
795  .rs  .rs
796  .sp  .sp
797  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
798  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
799  .sp  .sp
800    foo\eKbar    foo\eKbar
801  .sp  .sp
# Line 705  For example, when the pattern Line 817  For example, when the pattern
817    (foo)\eKbar    (foo)\eKbar
818  .sp  .sp
819  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
820    .P
821    Perl documents that the use of \eK within assertions is "not well defined". In
822    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
823    ignored in negative assertions.
824  .  .
825  .  .
826  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 729  The backslashed assertions are: Line 845  The backslashed assertions are:
845    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
846    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
847  .sp  .sp
848  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
849  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
850    default it matches the corresponding literal character (for example, \eB
851    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
852    escape sequence" error is generated instead.
853  .P  .P
854  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
855  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
856  \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
857  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
858    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
859    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
860    of word" or "end of word" metasequence. However, whatever follows \eb normally
861    determines which it is. For example, the fragment \eba matches "a" at the start
862    of a word.
863  .P  .P
864  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
865  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 819  end of the subject in both modes, and if Line 943  end of the subject in both modes, and if
943  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
944  .  .
945  .  .
946  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
947    .SH "FULL STOP (PERIOD, DOT) AND \eN"
948  .rs  .rs
949  .sp  .sp
950  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 841  to match it. Line 966  to match it.
966  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
967  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
968  special meaning in a character class.  special meaning in a character class.
969    .P
970    The escape sequence \eN behaves like a dot, except that it is not affected by
971    the PCRE_DOTALL option. In other words, it matches any character except one
972    that signifies the end of a line.
973  .  .
974  .  .
975  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 849  special meaning in a character class. Line 978  special meaning in a character class.
978  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
979  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
980  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
981  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
982  what remains in the string may be a malformed UTF-8 string. For this reason,  rest of the string may start with a malformed UTF-8 character. For this reason,
983  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
984  .P  .P
985  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 867  the lookbehind. Line 996  the lookbehind.
996  .rs  .rs
997  .sp  .sp
998  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
999  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
1000  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1001  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
1002  escaped with a backslash.  a member of the class, it should be the first data character in the class
1003    (after an initial circumflex, if present) or escaped with a backslash.
1004  .P  .P
1005  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
1006  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
1007  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
1008  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
1009  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
1010  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 884  For example, the character class [aeiou] Line 1014  For example, the character class [aeiou]
1014  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1015  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1016  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1017  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1018  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1019  string.  string.
1020  .P  .P
# Line 898  caseful version would. In UTF-8 mode, PC Line 1028  caseful version would. In UTF-8 mode, PC
1028  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1029  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1030  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1031  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1032  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
1033  UTF-8 support.  with UTF-8 support.
1034  .P  .P
1035  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1036  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 934  characters in both cases. In UTF-8 mode, Line 1064  characters in both cases. In UTF-8 mode,
1064  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
1065  property support.  property support.
1066  .P  .P
1067  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1068  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1069  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1070  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1071  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1072  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1073    .\" HTML <a href="#genericchartypes">
1074    .\" </a>
1075    "Generic character types"
1076    .\"
1077    above. The escape sequence \eb has a different meaning inside a character
1078    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1079    are not special inside a character class. Like any other unrecognized escape
1080    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1081    default, but cause an error if the PCRE_EXTRA option is set.
1082    .P
1083    A circumflex can conveniently be used with the upper case character types to
1084    specify a more restricted set of characters than the matching lower case type.
1085    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1086    whereas [\ew] includes underscore. A positive character class should be read as
1087    "something OR something OR ..." and a negative class as "NOT something AND NOT
1088    something AND NOT ...".
1089  .P  .P
1090  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1091  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 959  this notation. For example, Line 1105  this notation. For example,
1105    [01[:alpha:]%]    [01[:alpha:]%]
1106  .sp  .sp
1107  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1108  are  are:
1109  .sp  .sp
1110    alnum    letters and digits    alnum    letters and digits
1111    alpha    letters    alpha    letters
# Line 970  are Line 1116  are
1116    graph    printing characters, excluding space    graph    printing characters, excluding space
1117    lower    lower case letters    lower    lower case letters
1118    print    printing characters, including space    print    printing characters, including space
1119    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1120    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1121    upper    upper case letters    upper    upper case letters
1122    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 991  matches "1", "2", or any non-digit. PCRE Line 1137  matches "1", "2", or any non-digit. PCRE
1137  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
1138  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1139  .P  .P
1140  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
1141  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1142    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1143    character properties are used. This is achieved by replacing the POSIX classes
1144    by other sequences, as follows:
1145    .sp
1146      [:alnum:]  becomes  \ep{Xan}
1147      [:alpha:]  becomes  \ep{L}
1148      [:blank:]  becomes  \eh
1149      [:digit:]  becomes  \ep{Nd}
1150      [:lower:]  becomes  \ep{Ll}
1151      [:space:]  becomes  \ep{Xps}
1152      [:upper:]  becomes  \ep{Lu}
1153      [:word:]   becomes  \ep{Xwd}
1154    .sp
1155    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1156    classes are unchanged, and match only characters with code points less than
1157    128.
1158  .  .
1159  .  .
1160  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1046  extracts it into the global options (and Line 1208  extracts it into the global options (and
1208  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1209  .P  .P
1210  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1211  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1212  .sp  .sp
1213    (a(?i)b)c    (a(?i)b)c
1214  .sp  .sp
# Line 1071  section entitled Line 1233  section entitled
1233  .\" </a>  .\" </a>
1234  "Newline sequences"  "Newline sequences"
1235  .\"  .\"
1236  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
1237  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1238    PCRE_UTF8 and the PCRE_UCP options, respectively.
1239  .  .
1240  .  .
1241  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1086  Turning part of a pattern into a subpatt Line 1249  Turning part of a pattern into a subpatt
1249  .sp  .sp
1250    cat(aract|erpillar|)    cat(aract|erpillar|)
1251  .sp  .sp
1252  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1253  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1254  .sp  .sp
1255  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1256  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1257  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1258  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1259  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1260  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1261  .sp  .sp
1262    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1263  .sp  .sp
# Line 1127  is reached, an option setting in one bra Line 1289  is reached, an option setting in one bra
1289  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1290  .  .
1291  .  .
1292    .\" HTML <a name="dupsubpatternnumber"></a>
1293  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1294  .rs  .rs
1295  .sp  .sp
# Line 1143  at captured substring number one, whiche Line 1306  at captured substring number one, whiche
1306  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1307  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1308  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1309  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1310  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1311  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1312  .sp  .sp
1313    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1314    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1315    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1316  .sp  .sp
1317  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1318  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1319    or "defdef":
1320    .sp
1321      /(?|(abc)|(def))\e1/
1322    .sp
1323    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1324    refers to the first one in the pattern with the given number. The following
1325    pattern matches "abcabc" or "defabc":
1326    .sp
1327      /(?|(abc)|(def))(?1)/
1328    .sp
1329    If a
1330    .\" HTML <a href="#conditions">
1331    .\" </a>
1332    condition test
1333    .\"
1334    for a subpattern's having matched refers to a non-unique number, the test is
1335    true if any of the subpatterns of that number have matched.
1336  .P  .P
1337  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1338  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1168  if an expression is modified, the number Line 1347  if an expression is modified, the number
1347  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1348  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
1349  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
1350  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1351    have different names, but PCRE does not.
1352  .P  .P
1353  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
1354  (?'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
1355  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1356  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1357  .\" </a>  .\" </a>
1358  backreferences,  back references,
1359  .\"  .\"
1360  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1361  .\" </a>  .\" </a>
# Line 1195  extracting the name-to-number translatio Line 1375  extracting the name-to-number translatio
1375  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1376  .P  .P
1377  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
1378  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1379  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1380  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1381  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1382  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1383    name, and in both cases you want to extract the abbreviation. This pattern
1384    (ignoring the line breaks) does the job:
1385  .sp  .sp
1386    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1387    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1213  subpattern, as described in the previous Line 1395  subpattern, as described in the previous
1395  .P  .P
1396  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1397  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
1398  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1399  make a reference to a non-unique named subpattern from elsewhere in the  .P
1400  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
1401  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1402    used. In the absence of duplicate numbers (see the previous section) this is
1403    the one with the lowest number. If you use a named reference in a condition
1404    test (see the
1405    .\"
1406    .\" HTML <a href="#conditions">
1407    .\" </a>
1408    section about conditions
1409    .\"
1410    below), either to check whether a subpattern has matched, or to check for
1411    recursion, all subpatterns with the same name are tested. If the condition is
1412    true for any one of them, the overall condition is true. This is the same
1413    behaviour as testing by number. For further details of the interfaces for
1414    handling named subpatterns, see the
1415  .\" HREF  .\" HREF
1416  \fBpcreapi\fP  \fBpcreapi\fP
1417  .\"  .\"
1418  documentation.  documentation.
1419  .P  .P
1420  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1421  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1422  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1423    are given to subpatterns with the same number. However, you can give the same
1424    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1425  .  .
1426  .  .
1427  .SH REPETITION  .SH REPETITION
# Line 1238  items: Line 1435  items:
1435    the \eC escape sequence    the \eC escape sequence
1436    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1437    the \eR escape sequence    the \eR escape sequence
1438    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1439    a character class    a character class
1440    a back reference (see next section)    a back reference (see next section)
1441    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1442      a recursive or "subroutine" call to a subpattern
1443  .sp  .sp
1444  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1445  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1279  subpatterns that are referenced as Line 1477  subpatterns that are referenced as
1477  .\" </a>  .\" </a>
1478  subroutines  subroutines
1479  .\"  .\"
1480  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1481  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1482    .\" </a>
1483    "Defining subpatterns for use by reference only"
1484    .\"
1485    below). Items other than subpatterns that have a {0} quantifier are omitted
1486    from the compiled pattern.
1487  .P  .P
1488  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1489  abbreviations:  abbreviations:
# Line 1352  worth setting PCRE_DOTALL in order to ob Line 1555  worth setting PCRE_DOTALL in order to ob
1555  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1556  .P  .P
1557  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1558  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1559  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
1560  succeeds. Consider, for example:  succeeds. Consider, for example:
1561  .sp  .sp
# Line 1505  no such problem when named parentheses a Line 1708  no such problem when named parentheses a
1708  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1709  .P  .P
1710  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1711  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1712  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1713  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1714  .sp  .sp
1715    (ring), \e1    (ring), \e1
1716    (ring), \eg1    (ring), \eg1
# Line 1521  example: Line 1724  example:
1724    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1725  .sp  .sp
1726  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1727  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1728  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1729  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1730  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1731  .P  .P
1732  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1733  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1563  after the reference. Line 1766  after the reference.
1766  .P  .P
1767  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1768  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1769  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1770  .sp  .sp
1771    (a|(bc))\e2    (a|(bc))\e2
1772  .sp  .sp
1773  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1774  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1775  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1776  with a digit character, some delimiter must be used to terminate the back  .P
1777  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1778  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1779    If the pattern continues with a digit character, some delimiter must be used to
1780    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1781    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1782  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1783  .\" </a>  .\" </a>
1784  "Comments"  "Comments"
1785  .\"  .\"
1786  below) can be used.  below) can be used.
1787  .P  .
1788    .SS "Recursive back references"
1789    .rs
1790    .sp
1791  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
1792  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.
1793  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1592  to the previous iteration. In order for Line 1801  to the previous iteration. In order for
1801  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
1802  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
1803  minimum of zero.  minimum of zero.
1804    .P
1805    Back references of this type cause the group that they reference to be treated
1806    as an
1807    .\" HTML <a href="#atomicgroup">
1808    .\" </a>
1809    atomic group.
1810    .\"
1811    Once the whole group has been matched, a subsequent matching failure cannot
1812    cause backtracking into the middle of the group.
1813  .  .
1814  .  .
1815  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1645  lookbehind assertion is needed to achiev Line 1863  lookbehind assertion is needed to achiev
1863  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1864  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1865  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1866    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1867  .  .
1868  .  .
1869  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1669  is permitted, but Line 1888  is permitted, but
1888  .sp  .sp
1889  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1890  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1891  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1892  match the same length of string. An assertion such as  length of string. An assertion such as
1893  .sp  .sp
1894    (?<=ab(c|de))    (?<=ab(c|de))
1895  .sp  .sp
1896  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1897  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1898    branches:
1899  .sp  .sp
1900    (?<=abc|abde)    (?<=abc|abde)
1901  .sp  .sp
1902  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1903  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1904  .\" </a>  .\" </a>
1905  (see above)  (see above)
1906  .\"  .\"
1907  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1908  fixed-length.  restriction.
1909  .P  .P
1910  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1911  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1697  to appear in lookbehind assertions, beca Line 1917  to appear in lookbehind assertions, beca
1917  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1918  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1919  .P  .P
1920    .\" HTML <a href="#subpatternsassubroutines">
1921    .\" </a>
1922    "Subroutine"
1923    .\"
1924    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1925    as the subpattern matches a fixed-length string.
1926    .\" HTML <a href="#recursion">
1927    .\" </a>
1928    Recursion,
1929    .\"
1930    however, is not supported.
1931    .P
1932  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1933  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1934  pattern such as  strings. Consider a simple pattern such as
1935  .sp  .sp
1936    abcd$    abcd$
1937  .sp  .sp
# Line 1763  characters that are not "999". Line 1995  characters that are not "999".
1995  .sp  .sp
1996  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1997  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1998  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1999  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2000  .sp  .sp
2001    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2002    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2003  .sp  .sp
2004  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2005  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2006  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2007    itself contain nested subpatterns of any form, including conditional
2008    subpatterns; the restriction to two alternatives applies only at the level of
2009    the condition. This pattern fragment is an example where the alternatives are
2010    complex:
2011    .sp
2012      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2013    .sp
2014  .P  .P
2015  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2016  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1780  recursion, a pseudo-condition called DEF Line 2019  recursion, a pseudo-condition called DEF
2019  .rs  .rs
2020  .sp  .sp
2021  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
2022  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2023  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2024  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2025  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2026  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2027  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2028    section about duplicate subpattern numbers),
2029    .\"
2030    the condition is true if any of them have matched. An alternative notation is
2031    to precede the digits with a plus or minus sign. In this case, the subpattern
2032    number is relative rather than absolute. The most recently opened parentheses
2033    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2034    loops it can also make sense to refer to subsequent groups. The next
2035    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2036    zero in any of these forms is not used; it provokes a compile-time error.)
2037  .P  .P
2038  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2039  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1796  three parts for ease of discussion: Line 2044  three parts for ease of discussion:
2044  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2045  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2046  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2047  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2048  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2049  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2050  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2051  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1826  Rewriting the above example to use a nam Line 2074  Rewriting the above example to use a nam
2074  .sp  .sp
2075    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2076  .sp  .sp
2077    If the name used in a condition of this kind is a duplicate, the test is
2078    applied to all subpatterns of the same name, and is true if any one of them has
2079    matched.
2080  .  .
2081  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2082  .rs  .rs
# Line 1837  letter R, for example: Line 2088  letter R, for example:
2088  .sp  .sp
2089    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2090  .sp  .sp
2091  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2092  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
2093  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2094    applied to all subpatterns of the same name, and is true if any one of them is
2095    the most recent recursion.
2096  .P  .P
2097  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2098  patterns are described below.  .\" HTML <a href="#recursion">
2099    .\" </a>
2100    The syntax for recursive patterns
2101    .\"
2102    is described below.
2103  .  .
2104    .\" HTML <a name="subdefine"></a>
2105  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2106  .rs  .rs
2107  .sp  .sp
# Line 1851  If the condition is the string (DEFINE), Line 2109  If the condition is the string (DEFINE),
2109  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
2110  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2111  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
2112  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
2113  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2114  written like this (ignore whitespace and line breaks):  .\" </a>
2115    "subroutines"
2116    .\"
2117    is described below.) For example, a pattern to match an IPv4 address such as
2118    "192.168.23.245" could be written like this (ignore whitespace and line
2119    breaks):
2120  .sp  .sp
2121    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2122    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1861  written like this (ignore whitespace and Line 2124  written like this (ignore whitespace and
2124  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2125  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2126  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2127  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2128  .P  pattern uses references to the named group to match the four dot-separated
2129  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2130  .  .
2131  .SS "Assertion conditions"  .SS "Assertion conditions"
2132  .rs  .rs
# Line 1890  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2151  dd-aaa-dd or dd-dd-dd, where aaa are let
2151  .SH COMMENTS  .SH COMMENTS
2152  .rs  .rs
2153  .sp  .sp
2154  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2155  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2156  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2157    subpattern name or number. The characters that make up a comment play no part
2158    in the pattern matching.
2159  .P  .P
2160  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2161  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2162  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2163    this case continues to immediately after the next newline character or
2164    character sequence in the pattern. Which characters are interpreted as newlines
2165    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2166    sequence at the start of the pattern, as described in the section entitled
2167    .\" HTML <a href="#newlines">
2168    .\" </a>
2169    "Newline conventions"
2170    .\"
2171    above. Note that the end of this type of comment is a literal newline sequence
2172    in the pattern; escape sequences that happen to represent a newline do not
2173    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2174    default newline convention is in force:
2175    .sp
2176      abc #comment \en still comment
2177    .sp
2178    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2179    a newline in the pattern. The sequence \en is still literal at this stage, so
2180    it does not terminate the comment. Only an actual character with the code value
2181    0x0a (the default newline) does so.
2182  .  .
2183  .  .
2184  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1922  recursively to the pattern in which it a Line 2204  recursively to the pattern in which it a
2204  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2205  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2206  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2207  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2208  .P  .P
2209  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
2210  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,
2211  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
2212    .\" HTML <a href="#subpatternsassubroutines">
2213    .\" </a>
2214    "subroutine"
2215    .\"
2216  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2217  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2218  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2219  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2220  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2221  .sp  .sp
2222    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2223  .sp  .sp
2224  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2225  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
2226  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2227  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2228    to avoid backtracking into sequences of non-parentheses.
2229  .P  .P
2230  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
2231  pattern, so instead you could use this:  pattern, so instead you could use this:
2232  .sp  .sp
2233    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2234  .sp  .sp
2235  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2236  them instead of the whole pattern.  them instead of the whole pattern.
2237  .P  .P
2238  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2239  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. Instead of (?1) in the
2240  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2241  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2242  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2243  .P  .P
2244  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2245  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2246  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2247  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2248    .\" </a>
2249    "subroutine"
2250    .\"
2251    calls, as described in the next section.
2252  .P  .P
2253  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2254  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2255  could rewrite the above example as follows:  could rewrite the above example as follows:
2256  .sp  .sp
2257    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2258  .sp  .sp
2259  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2260  used.  used.
2261  .P  .P
2262  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2263  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2264  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2265  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2266  .sp  .sp
2267    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2268  .sp  .sp
2269  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2270  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2271  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
2272  before failure can be reported.  before failure can be reported.
2273  .P  .P
2274  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
2275  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
2276  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  
2277  .\" HREF  .\" HREF
2278  \fBpcrecallout\fP  \fBpcrecallout\fP
2279  .\"  .\"
# Line 1997  documentation). If the pattern above is Line 2281  documentation). If the pattern above is
2281  .sp  .sp
2282    (ab(cd)ef)    (ab(cd)ef)
2283  .sp  .sp
2284  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
2285  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
2286  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2287    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2288       ^                        ^  .P
2289       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2290  .sp  obtain extra memory to store data during a recursion, which it does by using
2291  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
2292  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.  
2293  .P  .P
2294  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.
2295  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2022  different alternatives for the recursive Line 2303  different alternatives for the recursive
2303  is the actual recursive call.  is the actual recursive call.
2304  .  .
2305  .  .
2306    .\" HTML <a name="recursiondifference"></a>
2307    .SS "Recursion difference from Perl"
2308    .rs
2309    .sp
2310    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2311    treated as an atomic group. That is, once it has matched some of the subject
2312    string, it is never re-entered, even if it contains untried alternatives and
2313    there is a subsequent matching failure. This can be illustrated by the
2314    following pattern, which purports to match a palindromic string that contains
2315    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2316    .sp
2317      ^(.|(.)(?1)\e2)$
2318    .sp
2319    The idea is that it either matches a single character, or two identical
2320    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2321    it does not if the pattern is longer than three characters. Consider the
2322    subject string "abcba":
2323    .P
2324    At the top level, the first character is matched, but as it is not at the end
2325    of the string, the first alternative fails; the second alternative is taken
2326    and the recursion kicks in. The recursive call to subpattern 1 successfully
2327    matches the next character ("b"). (Note that the beginning and end of line
2328    tests are not part of the recursion).
2329    .P
2330    Back at the top level, the next character ("c") is compared with what
2331    subpattern 2 matched, which was "a". This fails. Because the recursion is
2332    treated as an atomic group, there are now no backtracking points, and so the
2333    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2334    try the second alternative.) However, if the pattern is written with the
2335    alternatives in the other order, things are different:
2336    .sp
2337      ^((.)(?1)\e2|.)$
2338    .sp
2339    This time, the recursing alternative is tried first, and continues to recurse
2340    until it runs out of characters, at which point the recursion fails. But this
2341    time we do have another alternative to try at the higher level. That is the big
2342    difference: in the previous case the remaining alternative is at a deeper
2343    recursion level, which PCRE cannot use.
2344    .P
2345    To change the pattern so that it matches all palindromic strings, not just
2346    those with an odd number of characters, it is tempting to change the pattern to
2347    this:
2348    .sp
2349      ^((.)(?1)\e2|.?)$
2350    .sp
2351    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2352    deeper recursion has matched a single character, it cannot be entered again in
2353    order to match an empty string. The solution is to separate the two cases, and
2354    write out the odd and even cases as alternatives at the higher level:
2355    .sp
2356      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2357    .sp
2358    If you want to match typical palindromic phrases, the pattern has to ignore all
2359    non-word characters, which can be done like this:
2360    .sp
2361      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2362    .sp
2363    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2364    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2365    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2366    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2367    more) to match typical phrases, and Perl takes so long that you think it has
2368    gone into a loop.
2369    .P
2370    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2371    string does not start with a palindrome that is shorter than the entire string.
2372    For example, although "abcba" is correctly matched, if the subject is "ababa",
2373    PCRE finds the palindrome "aba" at the start, then fails at top level because
2374    the end of the string does not follow. Once again, it cannot jump back into the
2375    recursion to try other alternatives, so the entire match fails.
2376    .
2377    .
2378  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2379  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2380  .rs  .rs
# Line 2048  matches "sense and sensibility" and "res Line 2401  matches "sense and sensibility" and "res
2401  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
2402  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2403  .P  .P
2404  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2405  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
2406  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
2407  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2408    call revert to their previous values afterwards.
2409  .P  .P
2410  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2411  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 2120  description of the interface to the call Line 2474  description of the interface to the call
2474  documentation.  documentation.
2475  .  .
2476  .  .
2477    .\" HTML <a name="backtrackcontrol"></a>
2478  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2479  .rs  .rs
2480  .sp  .sp
# Line 2135  a backtracking algorithm. With the excep Line 2490  a backtracking algorithm. With the excep
2490  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2491  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2492  .P  .P
2493  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
2494  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2495  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
2496  they are tested.  processed as anchored at the point where they are tested.
2497  .P  .P
2498  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
2499  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2500  (*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,
2501  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
2502  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2503    parenthesis immediately follows the colon, the effect is as if the colon were
2504    not there. Any number of these verbs may occur in a pattern.
2505    .P
2506    PCRE contains some optimizations that are used to speed up matching by running
2507    some checks at the start of each match attempt. For example, it may know the
2508    minimum length of matching subject, or that a particular character must be
2509    present. When one of these optimizations suppresses the running of a match, any
2510    included backtracking verbs will not, of course, be processed. You can suppress
2511    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2512    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2513    pattern with (*NO_START_OPT).
2514    .
2515  .  .
2516  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2517  .rs  .rs
2518  .sp  .sp
2519  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2520    followed by a name.
2521  .sp  .sp
2522     (*ACCEPT)     (*ACCEPT)
2523  .sp  .sp
2524  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2525  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2526  immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2527  is captured. (This feature was added to PCRE at release 8.00.) For example:  captured. (This feature was added to PCRE at release 8.00.) For example:
2528  .sp  .sp
2529    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2530  .sp  .sp
2531  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
2532  the outer parentheses.  the outer parentheses.
2533  .sp  .sp
2534    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2176  callout feature, as for example in this Line 2544  callout feature, as for example in this
2544  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
2545  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2546  .  .
2547    .
2548    .SS "Recording which path was taken"
2549    .rs
2550    .sp
2551    There is one verb whose main purpose is to track how a match was arrived at,
2552    though it also has a secondary use in conjunction with advancing the match
2553    starting point (see (*SKIP) below).
2554    .sp
2555      (*MARK:NAME) or (*:NAME)
2556    .sp
2557    A name is always required with this verb. There may be as many instances of
2558    (*MARK) as you like in a pattern, and their names do not have to be unique.
2559    .P
2560    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2561    to the caller via the \fIpcre_extra\fP data structure, as described in the
2562    .\" HTML <a href="pcreapi.html#extradata">
2563    .\" </a>
2564    section on \fIpcre_extra\fP
2565    .\"
2566    in the
2567    .\" HREF
2568    \fBpcreapi\fP
2569    .\"
2570    documentation. No data is returned for a partial match. Here is an example of
2571    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2572    outputting of (*MARK) data:
2573    .sp
2574      /X(*MARK:A)Y|X(*MARK:B)Z/K
2575      XY
2576       0: XY
2577      MK: A
2578      XZ
2579       0: XZ
2580      MK: B
2581    .sp
2582    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2583    indicates which of the two alternatives matched. This is a more efficient way
2584    of obtaining this information than putting each alternative in its own
2585    capturing parentheses.
2586    .P
2587    A name may also be returned after a failed match if the final path through the
2588    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2589    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2590    starting point for matching is advanced, the final check is often with an empty
2591    string, causing a failure before (*MARK) is reached. For example:
2592    .sp
2593      /X(*MARK:A)Y|X(*MARK:B)Z/K
2594      XP
2595      No match
2596    .sp
2597    There are three potential starting points for this match (starting with X,
2598    starting with P, and with an empty string). If the pattern is anchored, the
2599    result is different:
2600    .sp
2601      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2602      XP
2603      No match, mark = B
2604    .sp
2605    PCRE's start-of-match optimizations can also interfere with this. For example,
2606    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2607    subject length for a match, a shorter subject will not be scanned at all.
2608    .P
2609    Note that similar anomalies (though different in detail) exist in Perl, no
2610    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2611    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2612    .
2613    .
2614  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2615  .rs  .rs
2616  .sp  .sp
2617  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2618  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
2619  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
2620    the verb. However, when one of these verbs appears inside an atomic group, its
2621    effect is confined to that group, because once the group has been matched,
2622    there is never any backtracking into it. In this situation, backtracking can
2623    "jump back" to the left of the entire atomic group. (Remember also, as stated
2624    above, that this localization also applies in subroutine calls and assertions.)
2625    .P
2626    These verbs differ in exactly what kind of failure occurs when backtracking
2627    reaches them.
2628  .sp  .sp
2629    (*COMMIT)    (*COMMIT)
2630  .sp  .sp
2631  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
2632  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
2633  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2634  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
2635  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2636  .sp  .sp
2637    a+(*COMMIT)b    a+(*COMMIT)b
2638  .sp  .sp
2639  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
2640  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
2641  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2642    (*PRUNE)  match failure.
2643  .sp  .P
2644  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,
2645  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
2646  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2647  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2648  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2649  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2650  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2651  be expressed in any other way.    xyzabc\eY
2652      No match
2653    .sp
2654    PCRE knows that any match must start with "a", so the optimization skips along
2655    the subject to "a" before running the first match attempt, which succeeds. When
2656    the optimization is disabled by the \eY escape in the second subject, the match
2657    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2658    starting points.
2659    .sp
2660      (*PRUNE) or (*PRUNE:NAME)
2661    .sp
2662    This verb causes the match to fail at the current starting position in the
2663    subject if the rest of the pattern does not match. If the pattern is
2664    unanchored, the normal "bumpalong" advance to the next starting character then
2665    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2666    reached, or when matching to the right of (*PRUNE), but if there is no match to
2667    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2668    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2669    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2670    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2671    match fails completely; the name is passed back if this is the final attempt.
2672    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2673    pattern (*PRUNE) has the same effect as (*COMMIT).
2674  .sp  .sp
2675    (*SKIP)    (*SKIP)
2676  .sp  .sp
2677  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
2678  "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,
2679  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2680  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
2681    successful match. Consider:
2682  .sp  .sp
2683    a+(*SKIP)b    a+(*SKIP)b
2684  .sp  .sp
2685  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2686  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2687  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2688  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2689  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2690  instead of skipping on to "c".  instead of skipping on to "c".
2691  .sp  .sp
2692    (*THEN)    (*SKIP:NAME)
2693  .sp  .sp
2694  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
2695  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2696  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,
2697  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2698    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2699    matching name is found, normal "bumpalong" of one character happens (the
2700    (*SKIP) is ignored).
2701    .sp
2702      (*THEN) or (*THEN:NAME)
2703    .sp
2704    This verb causes a skip to the next alternation in the innermost enclosing
2705    group if the rest of the pattern does not match. That is, it cancels pending
2706    backtracking, but only within the current alternation. Its name comes from the
2707    observation that it can be used for a pattern-based if-then-else block:
2708  .sp  .sp
2709    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2710  .sp  .sp
2711  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
2712  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
2713  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2714  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
2715    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2716    like (*PRUNE).
2717    .
2718    .P
2719    The above verbs provide four different "strengths" of control when subsequent
2720    matching fails. (*THEN) is the weakest, carrying on the match at the next
2721    alternation. (*PRUNE) comes next, failing the match at the current starting
2722    position, but allowing an advance to the next character (for an unanchored
2723    pattern). (*SKIP) is similar, except that the advance may be more than one
2724    character. (*COMMIT) is the strongest, causing the entire match to fail.
2725    .P
2726    If more than one is present in a pattern, the "stongest" one wins. For example,
2727    consider this pattern, where A, B, etc. are complex pattern fragments:
2728    .sp
2729      (A(*COMMIT)B(*THEN)C|D)
2730    .sp
2731    Once A has matched, PCRE is committed to this match, at the current starting
2732    position. If subsequently B matches, but C does not, the normal (*THEN) action
2733    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2734    overrides.
2735  .  .
2736  .  .
2737  .SH "SEE ALSO"  .SH "SEE ALSO"
2738  .rs  .rs
2739  .sp  .sp
2740  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2741    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2742  .  .
2743  .  .
2744  .SH AUTHOR  .SH AUTHOR
# Line 2258  Cambridge CB2 3QH, England. Line 2755  Cambridge CB2 3QH, England.
2755  .rs  .rs
2756  .sp  .sp
2757  .nf  .nf
2758  Last updated: 16 September 2009  Last updated: 20 July 2011
2759  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2760  .fi  .fi

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