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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 607 by ph10, Sun Jun 12 15:09:49 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,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 42  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55    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 or  These override the default and the options given to \fBpcre_compile()\fP or
103  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
104  newline sequence, the pattern  newline sequence, the pattern
105  .sp  .sp
# Line 95  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 169  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 179  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 198  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 211  but when a pattern is being prepared by Line 238  but when a pattern is being prepared by
238  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it 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  synonymous. The former is a back reference; the latter is a
365  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
366  .\" </a>  .\" </a>
367  subroutine  subroutine
# Line 341  subroutine Line 369  subroutine
369  call.  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 358  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 372  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 412  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 436  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 468  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 or  These override the default and the options given to \fBpcre_compile()\fP or
518  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
519  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
520  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
521  pattern, and that they must be in upper case. If more than one of them is  pattern, and that they must be in upper case. If more than one of them is
522  present, the last one is used. They can be combined with a change of newline  present, the last one is used. They can be combined with a change of newline
523  convention, for example, a pattern can start with:  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 496  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 513  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 528  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 540  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 557  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 576  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 680  non-UTF-8 mode \eX matches any one chara Line 760  non-UTF-8 mode \eX matches any one chara
760  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
761  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
762  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
763  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
764    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
765    (*UCP).
766    .
767    .
768    .\" HTML <a name="extraprops"></a>
769    .SS PCRE's additional properties
770    .rs
771    .sp
772    As well as the standard Unicode properties described in the previous
773    section, PCRE supports four more that make it possible to convert traditional
774    escape sequences such as \ew and \es and POSIX character classes to use Unicode
775    properties. PCRE uses these non-standard, non-Perl properties internally when
776    PCRE_UCP is set. They are:
777    .sp
778      Xan   Any alphanumeric character
779      Xps   Any POSIX space character
780      Xsp   Any Perl space character
781      Xwd   Any Perl "word" character
782    .sp
783    Xan matches characters that have either the L (letter) or the N (number)
784    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
785    carriage return, and any other character that has the Z (separator) property.
786    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
787    same characters as Xan, plus underscore.
788  .  .
789  .  .
790  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
791  .SS "Resetting the match start"  .SS "Resetting the match start"
792  .rs  .rs
793  .sp  .sp
794  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
795  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:  
796  .sp  .sp
797    foo\eKbar    foo\eKbar
798  .sp  .sp
# Line 711  For example, when the pattern Line 814  For example, when the pattern
814    (foo)\eKbar    (foo)\eKbar
815  .sp  .sp
816  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
817    .P
818    Perl documents that the use of \eK within assertions is "not well defined". In
819    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
820    ignored in negative assertions.
821  .  .
822  .  .
823  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 842  The backslashed assertions are:
842    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
843    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
844  .sp  .sp
845  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
846  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
847    default it matches the corresponding literal character (for example, \eB
848    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
849    escape sequence" error is generated instead.
850  .P  .P
851  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
852  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
853  \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
854  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
855  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
856  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
857  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
858    determines which it is. For example, the fragment \eba matches "a" at the start
859    of a word.
860  .P  .P
861  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
862  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 828  end of the subject in both modes, and if Line 940  end of the subject in both modes, and if
940  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
941  .  .
942  .  .
943  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
944    .SH "FULL STOP (PERIOD, DOT) AND \eN"
945  .rs  .rs
946  .sp  .sp
947  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 850  to match it. Line 963  to match it.
963  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
964  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
965  special meaning in a character class.  special meaning in a character class.
966    .P
967    The escape sequence \eN behaves like a dot, except that it is not affected by
968    the PCRE_DOTALL option. In other words, it matches any character except one
969    that signifies the end of a line.
970  .  .
971  .  .
972  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 858  special meaning in a character class. Line 975  special meaning in a character class.
975  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
976  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
977  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
978  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
979  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,
980  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
981  .P  .P
982  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 876  the lookbehind. Line 993  the lookbehind.
993  .rs  .rs
994  .sp  .sp
995  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
996  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
997  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
998  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
999  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1000  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
# Line 944  characters in both cases. In UTF-8 mode, Line 1061  characters in both cases. In UTF-8 mode,
1061  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
1062  property support.  property support.
1063  .P  .P
1064  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,
1065  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
1066  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1067  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
1068  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
1069  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1070    .\" HTML <a href="#genericchartypes">
1071    .\" </a>
1072    "Generic character types"
1073    .\"
1074    above. The escape sequence \eb has a different meaning inside a character
1075    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1076    are not special inside a character class. Like any other unrecognized escape
1077    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1078    default, but cause an error if the PCRE_EXTRA option is set.
1079    .P
1080    A circumflex can conveniently be used with the upper case character types to
1081    specify a more restricted set of characters than the matching lower case type.
1082    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1083    whereas [\ew] includes underscore. A positive character class should be read as
1084    "something OR something OR ..." and a negative class as "NOT something AND NOT
1085    something AND NOT ...".
1086  .P  .P
1087  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1088  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 969  this notation. For example, Line 1102  this notation. For example,
1102    [01[:alpha:]%]    [01[:alpha:]%]
1103  .sp  .sp
1104  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1105  are  are:
1106  .sp  .sp
1107    alnum    letters and digits    alnum    letters and digits
1108    alpha    letters    alpha    letters
# Line 980  are Line 1113  are
1113    graph    printing characters, excluding space    graph    printing characters, excluding space
1114    lower    lower case letters    lower    lower case letters
1115    print    printing characters, including space    print    printing characters, including space
1116    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1117    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1118    upper    upper case letters    upper    upper case letters
1119    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1134  matches "1", "2", or any non-digit. PCRE
1134  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
1135  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1136  .P  .P
1137  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
1138  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1139    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1140    character properties are used. This is achieved by replacing the POSIX classes
1141    by other sequences, as follows:
1142    .sp
1143      [:alnum:]  becomes  \ep{Xan}
1144      [:alpha:]  becomes  \ep{L}
1145      [:blank:]  becomes  \eh
1146      [:digit:]  becomes  \ep{Nd}
1147      [:lower:]  becomes  \ep{Ll}
1148      [:space:]  becomes  \ep{Xps}
1149      [:upper:]  becomes  \ep{Lu}
1150      [:word:]   becomes  \ep{Xwd}
1151    .sp
1152    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1153    classes are unchanged, and match only characters with code points less than
1154    128.
1155  .  .
1156  .  .
1157  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1056  extracts it into the global options (and Line 1205  extracts it into the global options (and
1205  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1206  .P  .P
1207  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1208  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
1209  .sp  .sp
1210    (a(?i)b)c    (a(?i)b)c
1211  .sp  .sp
# Line 1081  section entitled Line 1230  section entitled
1230  .\" </a>  .\" </a>
1231  "Newline sequences"  "Newline sequences"
1232  .\"  .\"
1233  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
1234  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1235    PCRE_UTF8 and the PCRE_UCP options, respectively.
1236  .  .
1237  .  .
1238  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1096  Turning part of a pattern into a subpatt Line 1246  Turning part of a pattern into a subpatt
1246  .sp  .sp
1247    cat(aract|erpillar|)    cat(aract|erpillar|)
1248  .sp  .sp
1249  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1250  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1251  .sp  .sp
1252  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
1253  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
1254  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
1255  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1256  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1257  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1258  .sp  .sp
1259    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1260  .sp  .sp
# Line 1154  at captured substring number one, whiche Line 1303  at captured substring number one, whiche
1303  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
1304  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1305  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
1306  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
1307  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1308  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.  
1309  .sp  .sp
1310    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1311    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1312    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1313  .sp  .sp
1314  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1315  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1316  "defdef":  or "defdef":
1317  .sp  .sp
1318    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1319  .sp  .sp
1320  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1321  refers to the first one in the pattern with the given number. The following  refers to the first one in the pattern with the given number. The following
1322  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1323  .sp  .sp
1324    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1325  .sp  .sp
1326    If a
1327    .\" HTML <a href="#conditions">
1328    .\" </a>
1329    condition test
1330    .\"
1331    for a subpattern's having matched refers to a non-unique number, the test is
1332    true if any of the subpatterns of that number have matched.
1333  .P  .P
1334  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1335  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1336  .  .
1337  .  .
# Line 1189  if an expression is modified, the number Line 1344  if an expression is modified, the number
1344  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1345  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
1346  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
1347  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1348    have different names, but PCRE does not.
1349  .P  .P
1350  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
1351  (?'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
1352  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1353  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1354  .\" </a>  .\" </a>
1355  backreferences,  back references,
1356  .\"  .\"
1357  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1358  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1372  extracting the name-to-number translatio
1372  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1373  .P  .P
1374  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
1375  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
1376  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
1377  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
1378  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
1379  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
1380    name, and in both cases you want to extract the abbreviation. This pattern
1381    (ignoring the line breaks) does the job:
1382  .sp  .sp
1383    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1384    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1392  subpattern, as described in the previous
1392  .P  .P
1393  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1394  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
1395  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1396  make a reference to a non-unique named subpattern from elsewhere in the  .P
1397  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
1398  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1399    used. In the absence of duplicate numbers (see the previous section) this is
1400    the one with the lowest number. If you use a named reference in a condition
1401    test (see the
1402    .\"
1403    .\" HTML <a href="#conditions">
1404    .\" </a>
1405    section about conditions
1406    .\"
1407    below), either to check whether a subpattern has matched, or to check for
1408    recursion, all subpatterns with the same name are tested. If the condition is
1409    true for any one of them, the overall condition is true. This is the same
1410    behaviour as testing by number. For further details of the interfaces for
1411    handling named subpatterns, see the
1412  .\" HREF  .\" HREF
1413  \fBpcreapi\fP  \fBpcreapi\fP
1414  .\"  .\"
1415  documentation.  documentation.
1416  .P  .P
1417  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1418  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1419  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1420    are given to subpatterns with the same number. However, you can give the same
1421    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1422  .  .
1423  .  .
1424  .SH REPETITION  .SH REPETITION
# Line 1259  items: Line 1432  items:
1432    the \eC escape sequence    the \eC escape sequence
1433    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1434    the \eR escape sequence    the \eR escape sequence
1435    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1436    a character class    a character class
1437    a back reference (see next section)    a back reference (see next section)
1438    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1439    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1440  .sp  .sp
1441  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1442  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1301  subpatterns that are referenced as Line 1474  subpatterns that are referenced as
1474  .\" </a>  .\" </a>
1475  subroutines  subroutines
1476  .\"  .\"
1477  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1478  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1479    .\" </a>
1480    "Defining subpatterns for use by reference only"
1481    .\"
1482    below). Items other than subpatterns that have a {0} quantifier are omitted
1483    from the compiled pattern.
1484  .P  .P
1485  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1486  abbreviations:  abbreviations:
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1552  worth setting PCRE_DOTALL in order to ob
1552  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1553  .P  .P
1554  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1555  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1556  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
1557  succeeds. Consider, for example:  succeeds. Consider, for example:
1558  .sp  .sp
# Line 1527  no such problem when named parentheses a Line 1705  no such problem when named parentheses a
1705  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1706  .P  .P
1707  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
1708  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
1709  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
1710  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1711  .sp  .sp
1712    (ring), \e1    (ring), \e1
1713    (ring), \eg1    (ring), \eg1
# Line 1543  example: Line 1721  example:
1721    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1722  .sp  .sp
1723  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
1724  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.
1725  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
1726  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
1727  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1728  .P  .P
1729  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1730  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1589  references to it always fail by default. Line 1767  references to it always fail by default.
1767  .sp  .sp
1768    (a|(bc))\e2    (a|(bc))\e2
1769  .sp  .sp
1770  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1771  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1772  unset value matches an empty string.  unset value matches an empty string.
1773  .P  .P
1774  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
# Line 1603  whitespace. Otherwise, the \eg{ syntax o Line 1781  whitespace. Otherwise, the \eg{ syntax o
1781  "Comments"  "Comments"
1782  .\"  .\"
1783  below) can be used.  below) can be used.
1784  .P  .
1785    .SS "Recursive back references"
1786    .rs
1787    .sp
1788  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
1789  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.
1790  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1617  to the previous iteration. In order for Line 1798  to the previous iteration. In order for
1798  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
1799  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
1800  minimum of zero.  minimum of zero.
1801    .P
1802    Back references of this type cause the group that they reference to be treated
1803    as an
1804    .\" HTML <a href="#atomicgroup">
1805    .\" </a>
1806    atomic group.
1807    .\"
1808    Once the whole group has been matched, a subsequent matching failure cannot
1809    cause backtracking into the middle of the group.
1810  .  .
1811  .  .
1812  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1670  lookbehind assertion is needed to achiev Line 1860  lookbehind assertion is needed to achiev
1860  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
1861  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
1862  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.
1863  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1864  .  .
1865  .  .
1866  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1696  is permitted, but Line 1885  is permitted, but
1885  .sp  .sp
1886  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1887  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
1888  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1889  match the same length of string. An assertion such as  length of string. An assertion such as
1890  .sp  .sp
1891    (?<=ab(c|de))    (?<=ab(c|de))
1892  .sp  .sp
# Line 1707  branches: Line 1896  branches:
1896  .sp  .sp
1897    (?<=abc|abde)    (?<=abc|abde)
1898  .sp  .sp
1899  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1900  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1901  .\" </a>  .\" </a>
1902  (see above)  (see above)
1903  .\"  .\"
1904  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
1905  restriction.  restriction.
1906  .P  .P
1907  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1730  different numbers of bytes, are also not Line 1919  different numbers of bytes, are also not
1919  "Subroutine"  "Subroutine"
1920  .\"  .\"
1921  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1922  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1923  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1924  .\" </a>  .\" </a>
1925  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 1992  characters that are not "999".
1992  .sp  .sp
1993  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1994  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1995  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1996  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1997  .sp  .sp
1998    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1811  already been matched. The two possible f Line 2000  already been matched. The two possible f
2000  .sp  .sp
2001  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2002  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
2003  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2004    itself contain nested subpatterns of any form, including conditional
2005    subpatterns; the restriction to two alternatives applies only at the level of
2006    the condition. This pattern fragment is an example where the alternatives are
2007    complex:
2008    .sp
2009      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2010    .sp
2011  .P  .P
2012  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2013  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1821  recursion, a pseudo-condition called DEF Line 2017  recursion, a pseudo-condition called DEF
2017  .sp  .sp
2018  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
2019  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2020  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
2021  (see the earlier  (see the earlier
2022  .\"  .\"
2023  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2024  .\" </a>  .\" </a>
2025  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2026  .\"  .\"
2027  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2028  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2029  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2030  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2031  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2032  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2033    zero in any of these forms is not used; it provokes a compile-time error.)
2034  .P  .P
2035  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2036  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 1844  three parts for ease of discussion: Line 2041  three parts for ease of discussion:
2041  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2042  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
2043  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
2044  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2045  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,
2046  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
2047  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2048  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 1874  Rewriting the above example to use a nam Line 2071  Rewriting the above example to use a nam
2071  .sp  .sp
2072    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2073  .sp  .sp
2074    If the name used in a condition of this kind is a duplicate, the test is
2075    applied to all subpatterns of the same name, and is true if any one of them has
2076    matched.
2077  .  .
2078  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2079  .rs  .rs
# Line 1887  letter R, for example: Line 2087  letter R, for example:
2087  .sp  .sp
2088  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2089  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
2090  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2091    applied to all subpatterns of the same name, and is true if any one of them is
2092    the most recent recursion.
2093  .P  .P
2094  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2095  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2096  .\" </a>  .\" </a>
2097  Recursive patterns  The syntax for recursive patterns
2098  .\"  .\"
2099  are described below.  is described below.
2100  .  .
2101    .\" HTML <a name="subdefine"></a>
2102  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2103  .rs  .rs
2104  .sp  .sp
# Line 1903  If the condition is the string (DEFINE), Line 2106  If the condition is the string (DEFINE),
2106  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
2107  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2108  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
2109  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
2110  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2111  .\" </a>  .\" </a>
2112  "subroutines"  "subroutines"
2113  .\"  .\"
2114  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2115  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2116    breaks):
2117  .sp  .sp
2118    (?(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) )
2119    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1944  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2148  dd-aaa-dd or dd-dd-dd, where aaa are let
2148  .SH COMMENTS  .SH COMMENTS
2149  .rs  .rs
2150  .sp  .sp
2151  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
2152  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,
2153  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
2154    subpattern name or number. The characters that make up a comment play no part
2155    in the pattern matching.
2156  .P  .P
2157  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
2158  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2159  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2160    this case continues to immediately after the next newline character or
2161    character sequence in the pattern. Which characters are interpreted as newlines
2162    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2163    sequence at the start of the pattern, as described in the section entitled
2164    .\" HTML <a href="#newlines">
2165    .\" </a>
2166    "Newline conventions"
2167    .\"
2168    above. Note that the end of this type of comment is a literal newline sequence
2169    in the pattern; escape sequences that happen to represent a newline do not
2170    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2171    default newline convention is in force:
2172    .sp
2173      abc #comment \en still comment
2174    .sp
2175    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2176    a newline in the pattern. The sequence \en is still literal at this stage, so
2177    it does not terminate the comment. Only an actual character with the code value
2178    0x0a (the default newline) does so.
2179  .  .
2180  .  .
2181  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1980  this kind of recursion was subsequently Line 2205  this kind of recursion was subsequently
2205  .P  .P
2206  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
2207  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,
2208  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2209  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2210  .\" </a>  .\" </a>
2211  "subroutine"  "subroutine"
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2221  PCRE_EXTENDED option is set so that whit
2221  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2222  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
2223  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2224  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2225  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2226  .P  .P
2227  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 2008  We have put the pattern into parentheses Line 2233  We have put the pattern into parentheses
2233  them instead of the whole pattern.  them instead of the whole pattern.
2234  .P  .P
2235  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
2236  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
2237  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
2238  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2239  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.  
2240  .P  .P
2241  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2242  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
# Line 2044  the match runs for a very long time inde Line 2268  the match runs for a very long time inde
2268  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
2269  before failure can be reported.  before failure can be reported.
2270  .P  .P
2271  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
2272  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
2273  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  
2274  .\" HREF  .\" HREF
2275  \fBpcrecallout\fP  \fBpcrecallout\fP
2276  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2278  documentation). If the pattern above is
2278  .sp  .sp
2279    (ab(cd)ef)    (ab(cd)ef)
2280  .sp  .sp
2281  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
2282  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
2283  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2284    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2285       ^                        ^  .P
2286       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2287  .sp  obtain extra memory to store data during a recursion, which it does by using
2288  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
2289  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.  
2290  .P  .P
2291  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.
2292  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2087  is the actual recursive call. Line 2307  is the actual recursive call.
2307  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2308  treated as an atomic group. That is, once it has matched some of the subject  treated as an atomic group. That is, once it has matched some of the subject
2309  string, it is never re-entered, even if it contains untried alternatives and  string, it is never re-entered, even if it contains untried alternatives and
2310  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2311  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2312  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2313  .sp  .sp
2314    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2315  .sp  .sp
2316  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2317  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2318  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2319  subject string "abcba":  subject string "abcba":
2320  .P  .P
2321  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2322  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2323  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2324  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2325  tests are not part of the recursion).  tests are not part of the recursion).
2326  .P  .P
2327  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2328  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2329  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2330  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2331  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2113  alternatives in the other order, things Line 2333  alternatives in the other order, things
2333  .sp  .sp
2334    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2335  .sp  .sp
2336  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2337  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2338  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2339  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2340  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2341  .P  .P
2342  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2343  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2344    this:
2345  .sp  .sp
2346    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2347  .sp  .sp
2348  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2349  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2350  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2351  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2352  .sp  .sp
2353    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2354  .sp  .sp
2355  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2356  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2357  .sp  .sp
2358    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2359  .sp  .sp
2360  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2361  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2362  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2363  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2364  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2365  gone into a loop.  gone into a loop.
# Line 2177  matches "sense and sensibility" and "res Line 2398  matches "sense and sensibility" and "res
2398  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
2399  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2400  .P  .P
2401  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2402  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
2403  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
2404  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2405    call revert to their previous values afterwards.
2406  .P  .P
2407  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2408  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 2249  description of the interface to the call Line 2471  description of the interface to the call
2471  documentation.  documentation.
2472  .  .
2473  .  .
2474    .\" HTML <a name="backtrackcontrol"></a>
2475  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2476  .rs  .rs
2477  .sp  .sp
# Line 2264  a backtracking algorithm. With the excep Line 2487  a backtracking algorithm. With the excep
2487  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2488  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2489  .P  .P
2490  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
2491  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2492  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
2493  they are tested.  processed as anchored at the point where they are tested.
2494  .P  .P
2495  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
2496  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2497  (*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,
2498  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
2499  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2500    parenthesis immediately follows the colon, the effect is as if the colon were
2501    not there. Any number of these verbs may occur in a pattern.
2502    .P
2503    PCRE contains some optimizations that are used to speed up matching by running
2504    some checks at the start of each match attempt. For example, it may know the
2505    minimum length of matching subject, or that a particular character must be
2506    present. When one of these optimizations suppresses the running of a match, any
2507    included backtracking verbs will not, of course, be processed. You can suppress
2508    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2509    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2510    pattern with (*NO_START_OPT).
2511    .
2512  .  .
2513  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2514  .rs  .rs
2515  .sp  .sp
2516  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2517    followed by a name.
2518  .sp  .sp
2519     (*ACCEPT)     (*ACCEPT)
2520  .sp  .sp
# Line 2289  captured. (This feature was added to PCR Line 2525  captured. (This feature was added to PCR
2525  .sp  .sp
2526    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2527  .sp  .sp
2528  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
2529  the outer parentheses.  the outer parentheses.
2530  .sp  .sp
2531    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2305  callout feature, as for example in this Line 2541  callout feature, as for example in this
2541  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
2542  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2543  .  .
2544    .
2545    .SS "Recording which path was taken"
2546    .rs
2547    .sp
2548    There is one verb whose main purpose is to track how a match was arrived at,
2549    though it also has a secondary use in conjunction with advancing the match
2550    starting point (see (*SKIP) below).
2551    .sp
2552      (*MARK:NAME) or (*:NAME)
2553    .sp
2554    A name is always required with this verb. There may be as many instances of
2555    (*MARK) as you like in a pattern, and their names do not have to be unique.
2556    .P
2557    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2558    to the caller via the \fIpcre_extra\fP data structure, as described in the
2559    .\" HTML <a href="pcreapi.html#extradata">
2560    .\" </a>
2561    section on \fIpcre_extra\fP
2562    .\"
2563    in the
2564    .\" HREF
2565    \fBpcreapi\fP
2566    .\"
2567    documentation. No data is returned for a partial match. Here is an example of
2568    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2569    outputting of (*MARK) data:
2570    .sp
2571      /X(*MARK:A)Y|X(*MARK:B)Z/K
2572      XY
2573       0: XY
2574      MK: A
2575      XZ
2576       0: XZ
2577      MK: B
2578    .sp
2579    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2580    indicates which of the two alternatives matched. This is a more efficient way
2581    of obtaining this information than putting each alternative in its own
2582    capturing parentheses.
2583    .P
2584    A name may also be returned after a failed match if the final path through the
2585    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2586    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2587    starting point for matching is advanced, the final check is often with an empty
2588    string, causing a failure before (*MARK) is reached. For example:
2589    .sp
2590      /X(*MARK:A)Y|X(*MARK:B)Z/K
2591      XP
2592      No match
2593    .sp
2594    There are three potential starting points for this match (starting with X,
2595    starting with P, and with an empty string). If the pattern is anchored, the
2596    result is different:
2597    .sp
2598      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2599      XP
2600      No match, mark = B
2601    .sp
2602    PCRE's start-of-match optimizations can also interfere with this. For example,
2603    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2604    subject length for a match, a shorter subject will not be scanned at all.
2605    .P
2606    Note that similar anomalies (though different in detail) exist in Perl, no
2607    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2608    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2609    .
2610    .
2611  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2612  .rs  .rs
2613  .sp  .sp
2614  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2615  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
2616  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
2617    the verb. However, when one of these verbs appears inside an atomic group, its
2618    effect is confined to that group, because once the group has been matched,
2619    there is never any backtracking into it. In this situation, backtracking can
2620    "jump back" to the left of the entire atomic group. (Remember also, as stated
2621    above, that this localization also applies in subroutine calls and assertions.)
2622    .P
2623    These verbs differ in exactly what kind of failure occurs when backtracking
2624    reaches them.
2625  .sp  .sp
2626    (*COMMIT)    (*COMMIT)
2627  .sp  .sp
2628  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
2629  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
2630  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2631  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
2632  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2633  .sp  .sp
2634    a+(*COMMIT)b    a+(*COMMIT)b
2635  .sp  .sp
2636  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
2637  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
2638  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2639    (*PRUNE)  match failure.
2640  .sp  .P
2641  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,
2642  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
2643  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2644  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2645  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2646  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2647  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2648  be expressed in any other way.    xyzabc\eY
2649      No match
2650    .sp
2651    PCRE knows that any match must start with "a", so the optimization skips along
2652    the subject to "a" before running the first match attempt, which succeeds. When
2653    the optimization is disabled by the \eY escape in the second subject, the match
2654    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2655    starting points.
2656    .sp
2657      (*PRUNE) or (*PRUNE:NAME)
2658    .sp
2659    This verb causes the match to fail at the current starting position in the
2660    subject if the rest of the pattern does not match. If the pattern is
2661    unanchored, the normal "bumpalong" advance to the next starting character then
2662    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2663    reached, or when matching to the right of (*PRUNE), but if there is no match to
2664    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2665    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2666    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2667    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2668    match fails completely; the name is passed back if this is the final attempt.
2669    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2670    pattern (*PRUNE) has the same effect as (*COMMIT).
2671  .sp  .sp
2672    (*SKIP)    (*SKIP)
2673  .sp  .sp
2674  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
2675  "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,
2676  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2677  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
2678    successful match. Consider:
2679  .sp  .sp
2680    a+(*SKIP)b    a+(*SKIP)b
2681  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2686  effect as this example; although it woul
2686  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2687  instead of skipping on to "c".  instead of skipping on to "c".
2688  .sp  .sp
2689    (*THEN)    (*SKIP:NAME)
2690  .sp  .sp
2691  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
2692  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2693  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,
2694  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2695    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2696    matching name is found, normal "bumpalong" of one character happens (the
2697    (*SKIP) is ignored).
2698    .sp
2699      (*THEN) or (*THEN:NAME)
2700    .sp
2701    This verb causes a skip to the next alternation in the innermost enclosing
2702    group if the rest of the pattern does not match. That is, it cancels pending
2703    backtracking, but only within the current alternation. Its name comes from the
2704    observation that it can be used for a pattern-based if-then-else block:
2705  .sp  .sp
2706    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2707  .sp  .sp
2708  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
2709  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
2710  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2711  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
2712    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2713    like (*PRUNE).
2714    .
2715    .P
2716    The above verbs provide four different "strengths" of control when subsequent
2717    matching fails. (*THEN) is the weakest, carrying on the match at the next
2718    alternation. (*PRUNE) comes next, failing the match at the current starting
2719    position, but allowing an advance to the next character (for an unanchored
2720    pattern). (*SKIP) is similar, except that the advance may be more than one
2721    character. (*COMMIT) is the strongest, causing the entire match to fail.
2722    .P
2723    If more than one is present in a pattern, the "stongest" one wins. For example,
2724    consider this pattern, where A, B, etc. are complex pattern fragments:
2725    .sp
2726      (A(*COMMIT)B(*THEN)C|D)
2727    .sp
2728    Once A has matched, PCRE is committed to this match, at the current starting
2729    position. If subsequently B matches, but C does not, the normal (*THEN) action
2730    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2731    overrides.
2732  .  .
2733  .  .
2734  .SH "SEE ALSO"  .SH "SEE ALSO"
2735  .rs  .rs
2736  .sp  .sp
2737  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2738  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2739  .  .
2740  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2752  Cambridge CB2 3QH, England.
2752  .rs  .rs
2753  .sp  .sp
2754  .nf  .nf
2755  Last updated: 30 September 2009  Last updated: 12 June 2011
2756  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2757  .fi  .fi

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