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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 579 by ph10, Wed Nov 24 17:39:25 2010 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 42  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55    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.
224  .  .
225  .  .
226  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 211  but when a pattern is being prepared by Line 234  but when a pattern is being prepared by
234  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
235  .sp  .sp
236    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
237    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
238    \ee        escape (hex 1B)    \ee        escape (hex 1B)
239    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
240    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
241    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
242    \et        tab (hex 09)    \et        tab (hex 09)
243    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
244    \exhh      character with hex code hh    \exhh      character with hex code hh
245    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
246  .sp  .sp
247  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
248  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.
249  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
250  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251    than 127, a compile-time error occurs. This locks out non-ASCII characters in
252    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253    values are valid. A lower case letter is converted to upper case, and then the
254    0xc0 bits are flipped.)
255  .P  .P
256  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
257  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 322  zero, because no more than three octal d
322  .P  .P
323  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
324  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
325  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
326  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
327  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
328  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
329  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
330  .  .
331  .  .
332  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 357  syntax for referencing a subpattern as a
357  later.  later.
358  .\"  .\"
359  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
360  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
361  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
362  .\" </a>  .\" </a>
363  subroutine  subroutine
# Line 341  subroutine Line 365  subroutine
365  call.  call.
366  .  .
367  .  .
368    .\" HTML <a name="genericchartypes"></a>
369  .SS "Generic character types"  .SS "Generic character types"
370  .rs  .rs
371  .sp  .sp
372  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:  
373  .sp  .sp
374    \ed     any decimal digit    \ed     any decimal digit
375    \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 382  following are always recognized:
382    \ew     any "word" character    \ew     any "word" character
383    \eW     any "non-word" character    \eW     any "non-word" character
384  .sp  .sp
385  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.
386  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
387    .\" HTML <a href="#fullstopdot">
388    .\" </a>
389    the "." metacharacter
390    .\"
391    when PCRE_DOTALL is not set.
392  .P  .P
393  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
394    of characters into two disjoint sets. Any given character matches one, and only
395    one, of each pair. The sequences can appear both inside and outside character
396  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
397  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
398  there is no character to match.  there is no character to match.
399  .P  .P
400  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 403  are HT (9), LF (10), FF (12), CR (13), a
403  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
404  does.  does.
405  .P  .P
406  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.
407  \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
408  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
409  meanings from before UTF-8 support was available, mainly for efficiency  place (see
410  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
411  and \eW.  .\" </a>
412  .P  "Locale support"
413  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
414  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
415  The horizontal space characters are:  .\" HREF
416    \fBpcreapi\fP
417    .\"
418    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
419    or "french" in Windows, some character codes greater than 128 are used for
420    accented letters, and these are then matched by \ew. The use of locales with
421    Unicode is discouraged.
422    .P
423    By default, in UTF-8 mode, characters with values greater than 128 never match
424    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
425    their original meanings from before UTF-8 support was available, mainly for
426    efficiency reasons. However, if PCRE is compiled with Unicode property support,
427    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
428    properties are used to determine character types, as follows:
429    .sp
430      \ed  any character that \ep{Nd} matches (decimal digit)
431      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
432      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
433    .sp
434    The upper case escapes match the inverse sets of characters. Note that \ed
435    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
436    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
437    \eB because they are defined in terms of \ew and \eW. Matching these sequences
438    is noticeably slower when PCRE_UCP is set.
439    .P
440    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
441    release 5.10. In contrast to the other sequences, which match only ASCII
442    characters by default, these always match certain high-valued codepoints in
443    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
444    are:
445  .sp  .sp
446    U+0009     Horizontal tab    U+0009     Horizontal tab
447    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 472  The vertical space characters are:
472    U+0085     Next line    U+0085     Next line
473    U+2028     Line separator    U+2028     Line separator
474    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.  
475  .  .
476  .  .
477  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 436  is discouraged. Line 479  is discouraged.
479  .rs  .rs
480  .sp  .sp
481  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
482  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:  
483  .sp  .sp
484    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
485  .sp  .sp
# Line 468  one of the following sequences: Line 510  one of the following sequences:
510    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
511    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
512  .sp  .sp
513  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
514  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
515  \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,
516  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
517  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
518  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
519  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
520  .sp  .sp
521    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
522  .sp  .sp
523  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
524    a character class, \eR is treated as an unrecognized escape sequence, and so
525    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
526  .  .
527  .  .
528  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 540  The extra escape sequences are:
540    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
541  .sp  .sp
542  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
543  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
544  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
545  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
546  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
547    .\" </a>
548    next section).
549    .\"
550    Other Perl properties such as "InMusicalSymbols" are not currently supported by
551    PCRE. Note that \eP{Any} does not match any characters, so always causes a
552    match failure.
553  .P  .P
554  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
555  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 563  Those that are not part of an identified
563  .P  .P
564  Arabic,  Arabic,
565  Armenian,  Armenian,
566    Avestan,
567  Balinese,  Balinese,
568    Bamum,
569  Bengali,  Bengali,
570  Bopomofo,  Bopomofo,
571  Braille,  Braille,
572  Buginese,  Buginese,
573  Buhid,  Buhid,
574  Canadian_Aboriginal,  Canadian_Aboriginal,
575    Carian,
576    Cham,
577  Cherokee,  Cherokee,
578  Common,  Common,
579  Coptic,  Coptic,
# Line 528  Cypriot, Line 582  Cypriot,
582  Cyrillic,  Cyrillic,
583  Deseret,  Deseret,
584  Devanagari,  Devanagari,
585    Egyptian_Hieroglyphs,
586  Ethiopic,  Ethiopic,
587  Georgian,  Georgian,
588  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 595  Hangul,
595  Hanunoo,  Hanunoo,
596  Hebrew,  Hebrew,
597  Hiragana,  Hiragana,
598    Imperial_Aramaic,
599  Inherited,  Inherited,
600    Inscriptional_Pahlavi,
601    Inscriptional_Parthian,
602    Javanese,
603    Kaithi,
604  Kannada,  Kannada,
605  Katakana,  Katakana,
606    Kayah_Li,
607  Kharoshthi,  Kharoshthi,
608  Khmer,  Khmer,
609  Lao,  Lao,
610  Latin,  Latin,
611    Lepcha,
612  Limbu,  Limbu,
613  Linear_B,  Linear_B,
614    Lisu,
615    Lycian,
616    Lydian,
617  Malayalam,  Malayalam,
618    Meetei_Mayek,
619  Mongolian,  Mongolian,
620  Myanmar,  Myanmar,
621  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 623  Nko,
623  Ogham,  Ogham,
624  Old_Italic,  Old_Italic,
625  Old_Persian,  Old_Persian,
626    Old_South_Arabian,
627    Old_Turkic,
628    Ol_Chiki,
629  Oriya,  Oriya,
630  Osmanya,  Osmanya,
631  Phags_Pa,  Phags_Pa,
632  Phoenician,  Phoenician,
633    Rejang,
634  Runic,  Runic,
635    Samaritan,
636    Saurashtra,
637  Shavian,  Shavian,
638  Sinhala,  Sinhala,
639    Sundanese,
640  Syloti_Nagri,  Syloti_Nagri,
641  Syriac,  Syriac,
642  Tagalog,  Tagalog,
643  Tagbanwa,  Tagbanwa,
644  Tai_Le,  Tai_Le,
645    Tai_Tham,
646    Tai_Viet,
647  Tamil,  Tamil,
648  Telugu,  Telugu,
649  Thaana,  Thaana,
# Line 576  Thai, Line 651  Thai,
651  Tibetan,  Tibetan,
652  Tifinagh,  Tifinagh,
653  Ugaritic,  Ugaritic,
654    Vai,
655  Yi.  Yi.
656  .P  .P
657  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
658  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
659  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
660  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
661  .P  .P
662  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
663  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 756  non-UTF-8 mode \eX matches any one chara
756  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
757  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
758  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
759  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
760    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
761    (*UCP).
762    .
763    .
764    .\" HTML <a name="extraprops"></a>
765    .SS PCRE's additional properties
766    .rs
767    .sp
768    As well as the standard Unicode properties described in the previous
769    section, PCRE supports four more that make it possible to convert traditional
770    escape sequences such as \ew and \es and POSIX character classes to use Unicode
771    properties. PCRE uses these non-standard, non-Perl properties internally when
772    PCRE_UCP is set. They are:
773    .sp
774      Xan   Any alphanumeric character
775      Xps   Any POSIX space character
776      Xsp   Any Perl space character
777      Xwd   Any Perl "word" character
778    .sp
779    Xan matches characters that have either the L (letter) or the N (number)
780    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
781    carriage return, and any other character that has the Z (separator) property.
782    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
783    same characters as Xan, plus underscore.
784  .  .
785  .  .
786  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
787  .SS "Resetting the match start"  .SS "Resetting the match start"
788  .rs  .rs
789  .sp  .sp
790  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
791  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:  
792  .sp  .sp
793    foo\eKbar    foo\eKbar
794  .sp  .sp
# Line 711  For example, when the pattern Line 810  For example, when the pattern
810    (foo)\eKbar    (foo)\eKbar
811  .sp  .sp
812  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
813    .P
814    Perl documents that the use of \eK within assertions is "not well defined". In
815    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
816    ignored in negative assertions.
817  .  .
818  .  .
819  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 838  The backslashed assertions are:
838    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
839    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
840  .sp  .sp
841  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
842  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
843    default it matches the corresponding literal character (for example, \eB
844    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
845    escape sequence" error is generated instead.
846  .P  .P
847  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
848  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
849  \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
850  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
851  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
852  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
853  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
854    determines which it is. For example, the fragment \eba matches "a" at the start
855    of a word.
856  .P  .P
857  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
858  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 936  end of the subject in both modes, and if
936  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
937  .  .
938  .  .
939  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
940    .SH "FULL STOP (PERIOD, DOT) AND \eN"
941  .rs  .rs
942  .sp  .sp
943  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 959  to match it.
959  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
960  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
961  special meaning in a character class.  special meaning in a character class.
962    .P
963    The escape sequence \eN behaves like a dot, except that it is not affected by
964    the PCRE_DOTALL option. In other words, it matches any character except one
965    that signifies the end of a line.
966  .  .
967  .  .
968  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 858  special meaning in a character class. Line 971  special meaning in a character class.
971  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
972  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
973  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
974  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
975  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,
976  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
977  .P  .P
978  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 989  the lookbehind.
989  .rs  .rs
990  .sp  .sp
991  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
992  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.
993  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
994  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
995  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
996  (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 1057  characters in both cases. In UTF-8 mode,
1057  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
1058  property support.  property support.
1059  .P  .P
1060  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,
1061  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
1062  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1063  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
1064  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
1065  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1066    .\" HTML <a href="#genericchartypes">
1067    .\" </a>
1068    "Generic character types"
1069    .\"
1070    above. The escape sequence \eb has a different meaning inside a character
1071    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1072    are not special inside a character class. Like any other unrecognized escape
1073    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1074    default, but cause an error if the PCRE_EXTRA option is set.
1075    .P
1076    A circumflex can conveniently be used with the upper case character types to
1077    specify a more restricted set of characters than the matching lower case type.
1078    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1079    whereas [\ew] includes underscore. A positive character class should be read as
1080    "something OR something OR ..." and a negative class as "NOT something AND NOT
1081    something AND NOT ...".
1082  .P  .P
1083  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1084  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 1098  this notation. For example,
1098    [01[:alpha:]%]    [01[:alpha:]%]
1099  .sp  .sp
1100  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1101  are  are:
1102  .sp  .sp
1103    alnum    letters and digits    alnum    letters and digits
1104    alpha    letters    alpha    letters
# Line 980  are Line 1109  are
1109    graph    printing characters, excluding space    graph    printing characters, excluding space
1110    lower    lower case letters    lower    lower case letters
1111    print    printing characters, including space    print    printing characters, including space
1112    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1113    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1114    upper    upper case letters    upper    upper case letters
1115    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1130  matches "1", "2", or any non-digit. PCRE
1130  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
1131  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1132  .P  .P
1133  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
1134  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1135    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1136    character properties are used. This is achieved by replacing the POSIX classes
1137    by other sequences, as follows:
1138    .sp
1139      [:alnum:]  becomes  \ep{Xan}
1140      [:alpha:]  becomes  \ep{L}
1141      [:blank:]  becomes  \eh
1142      [:digit:]  becomes  \ep{Nd}
1143      [:lower:]  becomes  \ep{Ll}
1144      [:space:]  becomes  \ep{Xps}
1145      [:upper:]  becomes  \ep{Lu}
1146      [:word:]   becomes  \ep{Xwd}
1147    .sp
1148    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1149    classes are unchanged, and match only characters with code points less than
1150    128.
1151  .  .
1152  .  .
1153  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1056  extracts it into the global options (and Line 1201  extracts it into the global options (and
1201  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1202  .P  .P
1203  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1204  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
1205  .sp  .sp
1206    (a(?i)b)c    (a(?i)b)c
1207  .sp  .sp
# Line 1081  section entitled Line 1226  section entitled
1226  .\" </a>  .\" </a>
1227  "Newline sequences"  "Newline sequences"
1228  .\"  .\"
1229  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
1230  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1231    PCRE_UTF8 and the PCRE_UCP options, respectively.
1232  .  .
1233  .  .
1234  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1096  Turning part of a pattern into a subpatt Line 1242  Turning part of a pattern into a subpatt
1242  .sp  .sp
1243    cat(aract|erpillar|)    cat(aract|erpillar|)
1244  .sp  .sp
1245  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1246  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1247  .sp  .sp
1248  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
1249  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
1250  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
1251  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1252  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1253  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1254  .sp  .sp
1255    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1256  .sp  .sp
# Line 1154  at captured substring number one, whiche Line 1299  at captured substring number one, whiche
1299  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
1300  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1301  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
1302  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
1303  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1304  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.  
1305  .sp  .sp
1306    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1307    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1308    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1309  .sp  .sp
1310  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
1311  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1312  "defdef":  or "defdef":
1313  .sp  .sp
1314    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1315  .sp  .sp
1316  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1317  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
1318  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1319  .sp  .sp
1320    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1321  .sp  .sp
1322    If a
1323    .\" HTML <a href="#conditions">
1324    .\" </a>
1325    condition test
1326    .\"
1327    for a subpattern's having matched refers to a non-unique number, the test is
1328    true if any of the subpatterns of that number have matched.
1329  .P  .P
1330  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1331  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1332  .  .
1333  .  .
# Line 1189  if an expression is modified, the number Line 1340  if an expression is modified, the number
1340  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1341  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
1342  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
1343  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1344    have different names, but PCRE does not.
1345  .P  .P
1346  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
1347  (?'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
1348  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1349  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1350  .\" </a>  .\" </a>
1351  backreferences,  back references,
1352  .\"  .\"
1353  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1354  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1368  extracting the name-to-number translatio
1368  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1369  .P  .P
1370  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
1371  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
1372  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
1373  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
1374  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
1375  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
1376    name, and in both cases you want to extract the abbreviation. This pattern
1377    (ignoring the line breaks) does the job:
1378  .sp  .sp
1379    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1380    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1388  subpattern, as described in the previous
1388  .P  .P
1389  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1390  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
1391  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1392  make a reference to a non-unique named subpattern from elsewhere in the  .P
1393  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
1394  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1395    used. In the absence of duplicate numbers (see the previous section) this is
1396    the one with the lowest number. If you use a named reference in a condition
1397    test (see the
1398    .\"
1399    .\" HTML <a href="#conditions">
1400    .\" </a>
1401    section about conditions
1402    .\"
1403    below), either to check whether a subpattern has matched, or to check for
1404    recursion, all subpatterns with the same name are tested. If the condition is
1405    true for any one of them, the overall condition is true. This is the same
1406    behaviour as testing by number. For further details of the interfaces for
1407    handling named subpatterns, see the
1408  .\" HREF  .\" HREF
1409  \fBpcreapi\fP  \fBpcreapi\fP
1410  .\"  .\"
1411  documentation.  documentation.
1412  .P  .P
1413  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1414  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1415  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1416    are given to subpatterns with the same number. However, you can give the same
1417    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1418  .  .
1419  .  .
1420  .SH REPETITION  .SH REPETITION
# Line 1259  items: Line 1428  items:
1428    the \eC escape sequence    the \eC escape sequence
1429    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1430    the \eR escape sequence    the \eR escape sequence
1431    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1432    a character class    a character class
1433    a back reference (see next section)    a back reference (see next section)
1434    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1435    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1436  .sp  .sp
1437  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1438  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 1470  subpatterns that are referenced as
1470  .\" </a>  .\" </a>
1471  subroutines  subroutines
1472  .\"  .\"
1473  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1474  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1475    .\" </a>
1476    "Defining subpatterns for use by reference only"
1477    .\"
1478    below). Items other than subpatterns that have a {0} quantifier are omitted
1479    from the compiled pattern.
1480  .P  .P
1481  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1482  abbreviations:  abbreviations:
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1548  worth setting PCRE_DOTALL in order to ob
1548  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1549  .P  .P
1550  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1551  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1552  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
1553  succeeds. Consider, for example:  succeeds. Consider, for example:
1554  .sp  .sp
# Line 1527  no such problem when named parentheses a Line 1701  no such problem when named parentheses a
1701  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1702  .P  .P
1703  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
1704  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
1705  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
1706  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1707  .sp  .sp
1708    (ring), \e1    (ring), \e1
1709    (ring), \eg1    (ring), \eg1
# Line 1543  example: Line 1717  example:
1717    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1718  .sp  .sp
1719  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
1720  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.
1721  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
1722  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
1723  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1724  .P  .P
1725  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1726  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 1763  references to it always fail by default.
1763  .sp  .sp
1764    (a|(bc))\e2    (a|(bc))\e2
1765  .sp  .sp
1766  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
1767  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
1768  unset value matches an empty string.  unset value matches an empty string.
1769  .P  .P
1770  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 1777  whitespace. Otherwise, the \eg{ syntax o
1777  "Comments"  "Comments"
1778  .\"  .\"
1779  below) can be used.  below) can be used.
1780  .P  .
1781    .SS "Recursive back references"
1782    .rs
1783    .sp
1784  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
1785  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.
1786  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 1794  to the previous iteration. In order for
1794  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
1795  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
1796  minimum of zero.  minimum of zero.
1797    .P
1798    Back references of this type cause the group that they reference to be treated
1799    as an
1800    .\" HTML <a href="#atomicgroup">
1801    .\" </a>
1802    atomic group.
1803    .\"
1804    Once the whole group has been matched, a subsequent matching failure cannot
1805    cause backtracking into the middle of the group.
1806  .  .
1807  .  .
1808  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1670  lookbehind assertion is needed to achiev Line 1856  lookbehind assertion is needed to achiev
1856  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
1857  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
1858  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.
1859  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 (?!).  
1860  .  .
1861  .  .
1862  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1696  is permitted, but Line 1881  is permitted, but
1881  .sp  .sp
1882  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1883  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
1884  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
1885  match the same length of string. An assertion such as  length of string. An assertion such as
1886  .sp  .sp
1887    (?<=ab(c|de))    (?<=ab(c|de))
1888  .sp  .sp
# Line 1707  branches: Line 1892  branches:
1892  .sp  .sp
1893    (?<=abc|abde)    (?<=abc|abde)
1894  .sp  .sp
1895  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1896  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1897  .\" </a>  .\" </a>
1898  (see above)  (see above)
1899  .\"  .\"
1900  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
1901  restriction.  restriction.
1902  .P  .P
1903  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 1915  different numbers of bytes, are also not
1915  "Subroutine"  "Subroutine"
1916  .\"  .\"
1917  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
1918  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1919  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1920  .\" </a>  .\" </a>
1921  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 1988  characters that are not "999".
1988  .sp  .sp
1989  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1990  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1991  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1992  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1993  .sp  .sp
1994    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1811  already been matched. The two possible f Line 1996  already been matched. The two possible f
1996  .sp  .sp
1997  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1998  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
1999  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2000    itself contain nested subpatterns of any form, including conditional
2001    subpatterns; the restriction to two alternatives applies only at the level of
2002    the condition. This pattern fragment is an example where the alternatives are
2003    complex:
2004    .sp
2005      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2006    .sp
2007  .P  .P
2008  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2009  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 2013  recursion, a pseudo-condition called DEF
2013  .sp  .sp
2014  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
2015  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2016  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
2017  (see the earlier  (see the earlier
2018  .\"  .\"
2019  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2020  .\" </a>  .\" </a>
2021  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2022  .\"  .\"
2023  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
2024  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
2025  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2026  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
2027  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
2028  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2029    zero in any of these forms is not used; it provokes a compile-time error.)
2030  .P  .P
2031  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2032  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 2037  three parts for ease of discussion:
2037  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2038  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
2039  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
2040  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2041  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,
2042  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
2043  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2044  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 2067  Rewriting the above example to use a nam
2067  .sp  .sp
2068    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2069  .sp  .sp
2070    If the name used in a condition of this kind is a duplicate, the test is
2071    applied to all subpatterns of the same name, and is true if any one of them has
2072    matched.
2073  .  .
2074  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2075  .rs  .rs
# Line 1887  letter R, for example: Line 2083  letter R, for example:
2083  .sp  .sp
2084  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
2085  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
2086  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2087    applied to all subpatterns of the same name, and is true if any one of them is
2088    the most recent recursion.
2089  .P  .P
2090  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2091  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2092  .\" </a>  .\" </a>
2093  Recursive patterns  The syntax for recursive patterns
2094  .\"  .\"
2095  are described below.  is described below.
2096  .  .
2097    .\" HTML <a name="subdefine"></a>
2098  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2099  .rs  .rs
2100  .sp  .sp
# Line 1903  If the condition is the string (DEFINE), Line 2102  If the condition is the string (DEFINE),
2102  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
2103  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2104  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
2105  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
2106  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2107  .\" </a>  .\" </a>
2108  "subroutines"  "subroutines"
2109  .\"  .\"
2110  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
2111  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2112    breaks):
2113  .sp  .sp
2114    (?(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) )
2115    \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 2144  dd-aaa-dd or dd-dd-dd, where aaa are let
2144  .SH COMMENTS  .SH COMMENTS
2145  .rs  .rs
2146  .sp  .sp
2147  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
2148  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,
2149  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
2150    subpattern name or number. The characters that make up a comment play no part
2151    in the pattern matching.
2152  .P  .P
2153  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
2154  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2155  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2156    this case continues to immediately after the next newline character or
2157    character sequence in the pattern. Which characters are interpreted as newlines
2158    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2159    sequence at the start of the pattern, as described in the section entitled
2160    .\" HTML <a href="#newlines">
2161    .\" </a>
2162    "Newline conventions"
2163    .\"
2164    above. Note that the end of this type of comment is a literal newline sequence
2165    in the pattern; escape sequences that happen to represent a newline do not
2166    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2167    default newline convention is in force:
2168    .sp
2169      abc #comment \en still comment
2170    .sp
2171    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2172    a newline in the pattern. The sequence \en is still literal at this stage, so
2173    it does not terminate the comment. Only an actual character with the code value
2174    0x0a (the default newline) does so.
2175  .  .
2176  .  .
2177  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1980  this kind of recursion was subsequently Line 2201  this kind of recursion was subsequently
2201  .P  .P
2202  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
2203  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,
2204  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2205  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2206  .\" </a>  .\" </a>
2207  "subroutine"  "subroutine"
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2217  PCRE_EXTENDED option is set so that whit
2217  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2218  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
2219  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2220  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
2221  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2222  .P  .P
2223  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 2229  We have put the pattern into parentheses
2229  them instead of the whole pattern.  them instead of the whole pattern.
2230  .P  .P
2231  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
2232  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
2233  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
2234  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2235  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.  
2236  .P  .P
2237  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2238  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 2264  the match runs for a very long time inde
2264  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
2265  before failure can be reported.  before failure can be reported.
2266  .P  .P
2267  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
2268  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
2269  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  
2270  .\" HREF  .\" HREF
2271  \fBpcrecallout\fP  \fBpcrecallout\fP
2272  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2274  documentation). If the pattern above is
2274  .sp  .sp
2275    (ab(cd)ef)    (ab(cd)ef)
2276  .sp  .sp
2277  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
2278  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
2279  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2280    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2281       ^                        ^  .P
2282       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2283  .sp  obtain extra memory to store data during a recursion, which it does by using
2284  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
2285  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.  
2286  .P  .P
2287  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.
2288  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 2303  is the actual recursive call.
2303  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
2304  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
2305  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
2306  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2307  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2308  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2309  .sp  .sp
2310    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2311  .sp  .sp
2312  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
2313  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2314  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
2315  subject string "abcba":  subject string "abcba":
2316  .P  .P
2317  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
2318  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2319  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2320  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
2321  tests are not part of the recursion).  tests are not part of the recursion).
2322  .P  .P
2323  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
2324  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2325  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
2326  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
2327  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 2329  alternatives in the other order, things
2329  .sp  .sp
2330    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2331  .sp  .sp
2332  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2333  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
2334  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
2335  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2336  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2337  .P  .P
2338  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
2339  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
2340    this:
2341  .sp  .sp
2342    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2343  .sp  .sp
2344  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
2345  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
2346  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
2347  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:
2348  .sp  .sp
2349    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2350  .sp  .sp
2351  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
2352  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2353  .sp  .sp
2354    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2355  .sp  .sp
2356  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
2357  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
2358  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2359  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
2360  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
2361  gone into a loop.  gone into a loop.
# Line 2177  matches "sense and sensibility" and "res Line 2394  matches "sense and sensibility" and "res
2394  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
2395  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2396  .P  .P
2397  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2398  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
2399  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
2400  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2401    call revert to their previous values afterwards.
2402  .P  .P
2403  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2404  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 2467  description of the interface to the call
2467  documentation.  documentation.
2468  .  .
2469  .  .
2470    .\" HTML <a name="backtrackcontrol"></a>
2471  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2472  .rs  .rs
2473  .sp  .sp
# Line 2264  a backtracking algorithm. With the excep Line 2483  a backtracking algorithm. With the excep
2483  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2484  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2485  .P  .P
2486  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
2487  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2488  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
2489  they are tested.  processed as anchored at the point where they are tested.
2490  .P  .P
2491  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
2492  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2493  (*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,
2494  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
2495  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2496    parenthesis immediately follows the colon, the effect is as if the colon were
2497    not there. Any number of these verbs may occur in a pattern.
2498    .P
2499    PCRE contains some optimizations that are used to speed up matching by running
2500    some checks at the start of each match attempt. For example, it may know the
2501    minimum length of matching subject, or that a particular character must be
2502    present. When one of these optimizations suppresses the running of a match, any
2503    included backtracking verbs will not, of course, be processed. You can suppress
2504    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2505    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2506    pattern with (*NO_START_OPT).
2507    .
2508  .  .
2509  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2510  .rs  .rs
2511  .sp  .sp
2512  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2513    followed by a name.
2514  .sp  .sp
2515     (*ACCEPT)     (*ACCEPT)
2516  .sp  .sp
# Line 2289  captured. (This feature was added to PCR Line 2521  captured. (This feature was added to PCR
2521  .sp  .sp
2522    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2523  .sp  .sp
2524  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
2525  the outer parentheses.  the outer parentheses.
2526  .sp  .sp
2527    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2305  callout feature, as for example in this Line 2537  callout feature, as for example in this
2537  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
2538  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2539  .  .
2540    .
2541    .SS "Recording which path was taken"
2542    .rs
2543    .sp
2544    There is one verb whose main purpose is to track how a match was arrived at,
2545    though it also has a secondary use in conjunction with advancing the match
2546    starting point (see (*SKIP) below).
2547    .sp
2548      (*MARK:NAME) or (*:NAME)
2549    .sp
2550    A name is always required with this verb. There may be as many instances of
2551    (*MARK) as you like in a pattern, and their names do not have to be unique.
2552    .P
2553    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2554    to the caller via the \fIpcre_extra\fP data structure, as described in the
2555    .\" HTML <a href="pcreapi.html#extradata">
2556    .\" </a>
2557    section on \fIpcre_extra\fP
2558    .\"
2559    in the
2560    .\" HREF
2561    \fBpcreapi\fP
2562    .\"
2563    documentation. No data is returned for a partial match. Here is an example of
2564    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2565    outputting of (*MARK) data:
2566    .sp
2567      /X(*MARK:A)Y|X(*MARK:B)Z/K
2568      XY
2569       0: XY
2570      MK: A
2571      XZ
2572       0: XZ
2573      MK: B
2574    .sp
2575    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2576    indicates which of the two alternatives matched. This is a more efficient way
2577    of obtaining this information than putting each alternative in its own
2578    capturing parentheses.
2579    .P
2580    A name may also be returned after a failed match if the final path through the
2581    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2582    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2583    starting point for matching is advanced, the final check is often with an empty
2584    string, causing a failure before (*MARK) is reached. For example:
2585    .sp
2586      /X(*MARK:A)Y|X(*MARK:B)Z/K
2587      XP
2588      No match
2589    .sp
2590    There are three potential starting points for this match (starting with X,
2591    starting with P, and with an empty string). If the pattern is anchored, the
2592    result is different:
2593    .sp
2594      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2595      XP
2596      No match, mark = B
2597    .sp
2598    PCRE's start-of-match optimizations can also interfere with this. For example,
2599    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2600    subject length for a match, a shorter subject will not be scanned at all.
2601    .P
2602    Note that similar anomalies (though different in detail) exist in Perl, no
2603    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2604    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2605    .
2606    .
2607  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2608  .rs  .rs
2609  .sp  .sp
2610  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2611  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
2612  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
2613    the verb. However, when one of these verbs appears inside an atomic group, its
2614    effect is confined to that group, because once the group has been matched,
2615    there is never any backtracking into it. In this situation, backtracking can
2616    "jump back" to the left of the entire atomic group. (Remember also, as stated
2617    above, that this localization also applies in subroutine calls and assertions.)
2618    .P
2619    These verbs differ in exactly what kind of failure occurs when backtracking
2620    reaches them.
2621  .sp  .sp
2622    (*COMMIT)    (*COMMIT)
2623  .sp  .sp
2624  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
2625  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
2626  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
2627  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
2628  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2629  .sp  .sp
2630    a+(*COMMIT)b    a+(*COMMIT)b
2631  .sp  .sp
2632  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
2633  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
2634  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2635    (*PRUNE)  match failure.
2636  .sp  .P
2637  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,
2638  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
2639  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2640  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2641  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2642  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2643  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2644  be expressed in any other way.    xyzabc\eY
2645      No match
2646    .sp
2647    PCRE knows that any match must start with "a", so the optimization skips along
2648    the subject to "a" before running the first match attempt, which succeeds. When
2649    the optimization is disabled by the \eY escape in the second subject, the match
2650    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2651    starting points.
2652    .sp
2653      (*PRUNE) or (*PRUNE:NAME)
2654    .sp
2655    This verb causes the match to fail at the current starting position in the
2656    subject if the rest of the pattern does not match. If the pattern is
2657    unanchored, the normal "bumpalong" advance to the next starting character then
2658    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2659    reached, or when matching to the right of (*PRUNE), but if there is no match to
2660    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2661    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2662    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2663    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2664    match fails completely; the name is passed back if this is the final attempt.
2665    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2666    pattern (*PRUNE) has the same effect as (*COMMIT).
2667  .sp  .sp
2668    (*SKIP)    (*SKIP)
2669  .sp  .sp
2670  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
2671  "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,
2672  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2673  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
2674    successful match. Consider:
2675  .sp  .sp
2676    a+(*SKIP)b    a+(*SKIP)b
2677  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2682  effect as this example; although it woul
2682  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2683  instead of skipping on to "c".  instead of skipping on to "c".
2684  .sp  .sp
2685    (*THEN)    (*SKIP:NAME)
2686  .sp  .sp
2687  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
2688  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2689  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,
2690  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2691    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2692    matching name is found, normal "bumpalong" of one character happens (the
2693    (*SKIP) is ignored).
2694    .sp
2695      (*THEN) or (*THEN:NAME)
2696    .sp
2697    This verb causes a skip to the next alternation in the innermost enclosing
2698    group if the rest of the pattern does not match. That is, it cancels pending
2699    backtracking, but only within the current alternation. Its name comes from the
2700    observation that it can be used for a pattern-based if-then-else block:
2701  .sp  .sp
2702    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2703  .sp  .sp
2704  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
2705  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
2706  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2707  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
2708    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2709    like (*PRUNE).
2710    .
2711    .P
2712    The above verbs provide four different "strengths" of control when subsequent
2713    matching fails. (*THEN) is the weakest, carrying on the match at the next
2714    alternation. (*PRUNE) comes next, failing the match at the current starting
2715    position, but allowing an advance to the next character (for an unanchored
2716    pattern). (*SKIP) is similar, except that the advance may be more than one
2717    character. (*COMMIT) is the strongest, causing the entire match to fail.
2718    .P
2719    If more than one is present in a pattern, the "stongest" one wins. For example,
2720    consider this pattern, where A, B, etc. are complex pattern fragments:
2721    .sp
2722      (A(*COMMIT)B(*THEN)C|D)
2723    .sp
2724    Once A has matched, PCRE is committed to this match, at the current starting
2725    position. If subsequently B matches, but C does not, the normal (*THEN) action
2726    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2727    overrides.
2728  .  .
2729  .  .
2730  .SH "SEE ALSO"  .SH "SEE ALSO"
2731  .rs  .rs
2732  .sp  .sp
2733  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2734  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2735  .  .
2736  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2748  Cambridge CB2 3QH, England.
2748  .rs  .rs
2749  .sp  .sp
2750  .nf  .nf
2751  Last updated: 30 September 2009  Last updated: 21 November 2010
2752  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2753  .fi  .fi

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