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revision 211 by ph10, Thu Aug 9 09:52:43 2007 UTC revision 574 by ph10, Sat Nov 20 17:47:27 2010 UTC
# Line 9  are described in detail below. There is Line 9  are described in detail below. There is
9  .\" HREF  .\" HREF
10  \fBpcresyntax\fP  \fBpcresyntax\fP
11  .\"  .\"
12  page. Perl's regular expressions are described in its own documentation, and  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18  regular expressions in general are covered in a number of books, some of which  regular expressions in general are covered in a number of books, some of which
19  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 30  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 44  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70    .SH "NEWLINE CONVENTIONS"
71    .rs
72    .sp
73    PCRE supports five different conventions for indicating line breaks in
74    strings: a single CR (carriage return) character, a single LF (linefeed)
75    character, the two-character sequence CRLF, any of the three preceding, or any
76    Unicode newline sequence. The
77    .\" HREF
78    \fBpcreapi\fP
79    .\"
80    page has
81    .\" HTML <a href="pcreapi.html#newlines">
82    .\" </a>
83    further discussion
84    .\"
85    about newlines, and shows how to set the newline convention in the
86    \fIoptions\fP arguments for the compiling and matching functions.
87    .P
88    It is also possible to specify a newline convention by starting a pattern
89    string with one of the following five sequences:
90    .sp
91      (*CR)        carriage return
92      (*LF)        linefeed
93      (*CRLF)      carriage return, followed by linefeed
94      (*ANYCRLF)   any of the three above
95      (*ANY)       all Unicode newline sequences
96    .sp
97    These override the default and the options given to \fBpcre_compile()\fP or
98    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99    newline sequence, the pattern
100    .sp
101      (*CR)a.b
102    .sp
103    changes the convention to CR. That pattern matches "a\enb" because LF is no
104    longer a newline. Note that these special settings, which are not
105    Perl-compatible, are recognized only at the very start of a pattern, and that
106    they must be in upper case. If more than one of them is present, the last one
107    is used.
108    .P
109    The newline convention affects the interpretation of the dot metacharacter when
110    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111    affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114    .\" HTML <a href="#newlineseq">
115    .\" </a>
116    "Newline sequences"
117    .\"
118    below. A change of \eR setting can be combined with a change of newline
119    convention.
120    .
121    .
122  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
123  .rs  .rs
124  .sp  .sp
# Line 107  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  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
185  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
186  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
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  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.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 117  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  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
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  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 136  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored.
219  .  .
220  .  .
221  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 145  The \eQ...\eE sequence is recognized bot Line 225  The \eQ...\eE sequence is recognized bot
225  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
226  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
227  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
228  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
229  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
230  .sp  .sp
231    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
232    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
233    \ee        escape (hex 1B)    \ee        escape (hex 1B)
234    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
235    \en        newline (hex 0A)    \en        linefeed (hex 0A)
236    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
237    \et        tab (hex 09)    \et        tab (hex 09)
238    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
239    \exhh      character with hex code hh    \exhh      character with hex code hh
240    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
241  .sp  .sp
242  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
243  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.
244  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
245  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
246    than 127, a compile-time error occurs. This locks out non-ASCII characters in
247    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
248    values are valid. A lower case letter is converted to upper case, and then the
249    0xc0 bits are flipped.)
250  .P  .P
251  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
252  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 234  zero, because no more than three octal d Line 317  zero, because no more than three octal d
317  .P  .P
318  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
319  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
320  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
321  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
322  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
323  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
324  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
325  .  .
326  .  .
327  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 261  parenthesized subpatterns. Line 341  parenthesized subpatterns.
341  .\"  .\"
342  .  .
343  .  .
344    .SS "Absolute and relative subroutine calls"
345    .rs
346    .sp
347    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
348    a number enclosed either in angle brackets or single quotes, is an alternative
349    syntax for referencing a subpattern as a "subroutine". Details are discussed
350    .\" HTML <a href="#onigurumasubroutines">
351    .\" </a>
352    later.
353    .\"
354    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
355    synonymous. The former is a back reference; the latter is a
356    .\" HTML <a href="#subpatternsassubroutines">
357    .\" </a>
358    subroutine
359    .\"
360    call.
361    .
362    .
363    .\" HTML <a name="genericchartypes"></a>
364  .SS "Generic character types"  .SS "Generic character types"
365  .rs  .rs
366  .sp  .sp
367  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:  
368  .sp  .sp
369    \ed     any decimal digit    \ed     any decimal digit
370    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 278  following are always recognized: Line 377  following are always recognized:
377    \ew     any "word" character    \ew     any "word" character
378    \eW     any "non-word" character    \eW     any "non-word" character
379  .sp  .sp
380  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.
381  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
382    .\" HTML <a href="#fullstopdot">
383    .\" </a>
384    the "." metacharacter
385    .\"
386    when PCRE_DOTALL is not set.
387  .P  .P
388  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
389    of characters into two disjoint sets. Any given character matches one, and only
390    one, of each pair. The sequences can appear both inside and outside character
391  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
392  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
393  there is no character to match.  there is no character to match.
394  .P  .P
395  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 292  are HT (9), LF (10), FF (12), CR (13), a Line 398  are HT (9), LF (10), FF (12), CR (13), a
398  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
399  does.  does.
400  .P  .P
401  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.
402  \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
403  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
404  meanings from before UTF-8 support was available, mainly for efficiency  place (see
405  reasons.  .\" HTML <a href="pcreapi.html#localesupport">
406  .P  .\" </a>
407  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  "Locale support"
408  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\"
409  The horizontal space characters are:  in the
410    .\" HREF
411    \fBpcreapi\fP
412    .\"
413    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
414    or "french" in Windows, some character codes greater than 128 are used for
415    accented letters, and these are then matched by \ew. The use of locales with
416    Unicode is discouraged.
417    .P
418    By default, in UTF-8 mode, characters with values greater than 128 never match
419    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
420    their original meanings from before UTF-8 support was available, mainly for
421    efficiency reasons. However, if PCRE is compiled with Unicode property support,
422    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
423    properties are used to determine character types, as follows:
424    .sp
425      \ed  any character that \ep{Nd} matches (decimal digit)
426      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
427      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
428    .sp
429    The upper case escapes match the inverse sets of characters. Note that \ed
430    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
431    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
432    \eB because they are defined in terms of \ew and \eW. Matching these sequences
433    is noticeably slower when PCRE_UCP is set.
434    .P
435    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
436    release 5.10. In contrast to the other sequences, which match only ASCII
437    characters by default, these always match certain high-valued codepoints in
438    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
439    are:
440  .sp  .sp
441    U+0009     Horizontal tab    U+0009     Horizontal tab
442    U+0020     Space    U+0020     Space
# Line 331  The vertical space characters are: Line 467  The vertical space characters are:
467    U+0085     Next line    U+0085     Next line
468    U+2028     Line separator    U+2028     Line separator
469    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.  
470  .  .
471  .  .
472    .\" HTML <a name="newlineseq"></a>
473  .SS "Newline sequences"  .SS "Newline sequences"
474  .rs  .rs
475  .sp  .sp
476  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
477  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 the following:  
478  .sp  .sp
479    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
480  .sp  .sp
# Line 375  are added: LS (line separator, U+2028) a Line 494  are added: LS (line separator, U+2028) a
494  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
495  recognized.  recognized.
496  .P  .P
497  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
498    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
499    either at compile time or when the pattern is matched. (BSR is an abbrevation
500    for "backslash R".) This can be made the default when PCRE is built; if this is
501    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
502    It is also possible to specify these settings by starting a pattern string with
503    one of the following sequences:
504    .sp
505      (*BSR_ANYCRLF)   CR, LF, or CRLF only
506      (*BSR_UNICODE)   any Unicode newline sequence
507    .sp
508    These override the default and the options given to \fBpcre_compile()\fP or
509    \fBpcre_compile2()\fP, but they can be overridden by options given to
510    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
511    which are not Perl-compatible, are recognized only at the very start of a
512    pattern, and that they must be in upper case. If more than one of them is
513    present, the last one is used. They can be combined with a change of newline
514    convention; for example, a pattern can start with:
515    .sp
516      (*ANY)(*BSR_ANYCRLF)
517    .sp
518    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
519    a character class, \eR is treated as an unrecognized escape sequence, and so
520    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
521  .  .
522  .  .
523  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 393  The extra escape sequences are: Line 535  The extra escape sequences are:
535    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
536  .sp  .sp
537  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
538  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
539  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
540  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
541  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
542    .\" </a>
543    next section).
544    .\"
545    Other Perl properties such as "InMusicalSymbols" are not currently supported by
546    PCRE. Note that \eP{Any} does not match any characters, so always causes a
547    match failure.
548  .P  .P
549  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
550  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 410  Those that are not part of an identified Line 558  Those that are not part of an identified
558  .P  .P
559  Arabic,  Arabic,
560  Armenian,  Armenian,
561    Avestan,
562  Balinese,  Balinese,
563    Bamum,
564  Bengali,  Bengali,
565  Bopomofo,  Bopomofo,
566  Braille,  Braille,
567  Buginese,  Buginese,
568  Buhid,  Buhid,
569  Canadian_Aboriginal,  Canadian_Aboriginal,
570    Carian,
571    Cham,
572  Cherokee,  Cherokee,
573  Common,  Common,
574  Coptic,  Coptic,
# Line 425  Cypriot, Line 577  Cypriot,
577  Cyrillic,  Cyrillic,
578  Deseret,  Deseret,
579  Devanagari,  Devanagari,
580    Egyptian_Hieroglyphs,
581  Ethiopic,  Ethiopic,
582  Georgian,  Georgian,
583  Glagolitic,  Glagolitic,
# Line 437  Hangul, Line 590  Hangul,
590  Hanunoo,  Hanunoo,
591  Hebrew,  Hebrew,
592  Hiragana,  Hiragana,
593    Imperial_Aramaic,
594  Inherited,  Inherited,
595    Inscriptional_Pahlavi,
596    Inscriptional_Parthian,
597    Javanese,
598    Kaithi,
599  Kannada,  Kannada,
600  Katakana,  Katakana,
601    Kayah_Li,
602  Kharoshthi,  Kharoshthi,
603  Khmer,  Khmer,
604  Lao,  Lao,
605  Latin,  Latin,
606    Lepcha,
607  Limbu,  Limbu,
608  Linear_B,  Linear_B,
609    Lisu,
610    Lycian,
611    Lydian,
612  Malayalam,  Malayalam,
613    Meetei_Mayek,
614  Mongolian,  Mongolian,
615  Myanmar,  Myanmar,
616  New_Tai_Lue,  New_Tai_Lue,
# Line 454  Nko, Line 618  Nko,
618  Ogham,  Ogham,
619  Old_Italic,  Old_Italic,
620  Old_Persian,  Old_Persian,
621    Old_South_Arabian,
622    Old_Turkic,
623    Ol_Chiki,
624  Oriya,  Oriya,
625  Osmanya,  Osmanya,
626  Phags_Pa,  Phags_Pa,
627  Phoenician,  Phoenician,
628    Rejang,
629  Runic,  Runic,
630    Samaritan,
631    Saurashtra,
632  Shavian,  Shavian,
633  Sinhala,  Sinhala,
634    Sundanese,
635  Syloti_Nagri,  Syloti_Nagri,
636  Syriac,  Syriac,
637  Tagalog,  Tagalog,
638  Tagbanwa,  Tagbanwa,
639  Tai_Le,  Tai_Le,
640    Tai_Tham,
641    Tai_Viet,
642  Tamil,  Tamil,
643  Telugu,  Telugu,
644  Thaana,  Thaana,
# Line 473  Thai, Line 646  Thai,
646  Tibetan,  Tibetan,
647  Tifinagh,  Tifinagh,
648  Ugaritic,  Ugaritic,
649    Vai,
650  Yi.  Yi.
651  .P  .P
652  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
653  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
654  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
655  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
656  .P  .P
657  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
658  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 545  cannot be tested by PCRE, unless UTF-8 v Line 719  cannot be tested by PCRE, unless UTF-8 v
719  .\" HREF  .\" HREF
720  \fBpcreapi\fP  \fBpcreapi\fP
721  .\"  .\"
722  page).  page). Perl does not support the Cs property.
723  .P  .P
724  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
725  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
726  properties with "Is".  properties with "Is".
727  .P  .P
# Line 577  non-UTF-8 mode \eX matches any one chara Line 751  non-UTF-8 mode \eX matches any one chara
751  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
752  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
753  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
754  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
755    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
756    (*UCP).
757    .
758    .
759    .\" HTML <a name="extraprops"></a>
760    .SS PCRE's additional properties
761    .rs
762    .sp
763    As well as the standard Unicode properties described in the previous
764    section, PCRE supports four more that make it possible to convert traditional
765    escape sequences such as \ew and \es and POSIX character classes to use Unicode
766    properties. PCRE uses these non-standard, non-Perl properties internally when
767    PCRE_UCP is set. They are:
768    .sp
769      Xan   Any alphanumeric character
770      Xps   Any POSIX space character
771      Xsp   Any Perl space character
772      Xwd   Any Perl "word" character
773    .sp
774    Xan matches characters that have either the L (letter) or the N (number)
775    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
776    carriage return, and any other character that has the Z (separator) property.
777    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
778    same characters as Xan, plus underscore.
779  .  .
780  .  .
781  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
782  .SS "Resetting the match start"  .SS "Resetting the match start"
783  .rs  .rs
784  .sp  .sp
785  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
786  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:  
787  .sp  .sp
788    foo\eKbar    foo\eKbar
789  .sp  .sp
# Line 608  For example, when the pattern Line 805  For example, when the pattern
805    (foo)\eKbar    (foo)\eKbar
806  .sp  .sp
807  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
808    .P
809    Perl documents that the use of \eK within assertions is "not well defined". In
810    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
811    ignored in negative assertions.
812  .  .
813  .  .
814  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 632  The backslashed assertions are: Line 833  The backslashed assertions are:
833    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
834    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
835  .sp  .sp
836  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
837  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
838    default it matches the corresponding literal character (for example, \eB
839    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
840    escape sequence" error is generated instead.
841  .P  .P
842  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
843  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
844  \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
845  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
846    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
847    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
848    of word" or "end of word" metasequence. However, whatever follows \eb normally
849    determines which it is. For example, the fragment \eba matches "a" at the start
850    of a word.
851  .P  .P
852  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
853  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 722  end of the subject in both modes, and if Line 931  end of the subject in both modes, and if
931  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
932  .  .
933  .  .
934  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
935    .SH "FULL STOP (PERIOD, DOT) AND \eN"
936  .rs  .rs
937  .sp  .sp
938  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 744  to match it. Line 954  to match it.
954  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
955  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
956  special meaning in a character class.  special meaning in a character class.
957    .P
958    The escape sequence \eN behaves like a dot, except that it is not affected by
959    the PCRE_DOTALL option. In other words, it matches any character except one
960    that signifies the end of a line.
961  .  .
962  .  .
963  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 752  special meaning in a character class. Line 966  special meaning in a character class.
966  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
967  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
968  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
969  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
970  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,
971  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
972  .P  .P
973  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 770  the lookbehind. Line 984  the lookbehind.
984  .rs  .rs
985  .sp  .sp
986  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
987  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
988  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
989  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
990  escaped with a backslash.  a member of the class, it should be the first data character in the class
991    (after an initial circumflex, if present) or escaped with a backslash.
992  .P  .P
993  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
994  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
995  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
996  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
997  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
998  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 787  For example, the character class [aeiou] Line 1002  For example, the character class [aeiou]
1002  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1003  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1004  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1005  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1006  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1007  string.  string.
1008  .P  .P
# Line 801  caseful version would. In UTF-8 mode, PC Line 1016  caseful version would. In UTF-8 mode, PC
1016  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1017  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1018  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1019  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1020  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
1021  UTF-8 support.  with UTF-8 support.
1022  .P  .P
1023  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1024  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 837  characters in both cases. In UTF-8 mode, Line 1052  characters in both cases. In UTF-8 mode,
1052  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
1053  property support.  property support.
1054  .P  .P
1055  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1056  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1057  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1058  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1059  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1060  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1061  .P  .P
1062  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1063  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 862  this notation. For example, Line 1077  this notation. For example,
1077    [01[:alpha:]%]    [01[:alpha:]%]
1078  .sp  .sp
1079  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1080  are  are:
1081  .sp  .sp
1082    alnum    letters and digits    alnum    letters and digits
1083    alpha    letters    alpha    letters
# Line 873  are Line 1088  are
1088    graph    printing characters, excluding space    graph    printing characters, excluding space
1089    lower    lower case letters    lower    lower case letters
1090    print    printing characters, including space    print    printing characters, including space
1091    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1092    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1093    upper    upper case letters    upper    upper case letters
1094    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 894  matches "1", "2", or any non-digit. PCRE Line 1109  matches "1", "2", or any non-digit. PCRE
1109  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
1110  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1111  .P  .P
1112  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
1113  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1114    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1115    character properties are used. This is achieved by replacing the POSIX classes
1116    by other sequences, as follows:
1117    .sp
1118      [:alnum:]  becomes  \ep{Xan}
1119      [:alpha:]  becomes  \ep{L}
1120      [:blank:]  becomes  \eh
1121      [:digit:]  becomes  \ep{Nd}
1122      [:lower:]  becomes  \ep{Ll}
1123      [:space:]  becomes  \ep{Xps}
1124      [:upper:]  becomes  \ep{Lu}
1125      [:word:]   becomes  \ep{Xwd}
1126    .sp
1127    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1128    classes are unchanged, and match only characters with code points less than
1129    128.
1130  .  .
1131  .  .
1132  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 922  alternative in the subpattern. Line 1153  alternative in the subpattern.
1153  .rs  .rs
1154  .sp  .sp
1155  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1156  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1157  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1158    The option letters are
1159  .sp  .sp
1160    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1161    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 937  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1169  PCRE_MULTILINE while unsetting PCRE_DOTA
1169  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1170  unset.  unset.
1171  .P  .P
1172  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1173  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1174  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1175  the global options (and it will therefore show up in data extracted by the  .P
1176  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1177    subpattern parentheses), the change applies to the remainder of the pattern
1178    that follows. If the change is placed right at the start of a pattern, PCRE
1179    extracts it into the global options (and it will therefore show up in data
1180    extracted by the \fBpcre_fullinfo()\fP function).
1181  .P  .P
1182  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1183  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
1184  .sp  .sp
1185    (a(?i)b)c    (a(?i)b)c
1186  .sp  .sp
# Line 960  branch is abandoned before the option se Line 1196  branch is abandoned before the option se
1196  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1197  behaviour otherwise.  behaviour otherwise.
1198  .P  .P
1199  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1200  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1201  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1202    the application has set or what has been defaulted. Details are given in the
1203    section entitled
1204    .\" HTML <a href="#newlineseq">
1205    .\" </a>
1206    "Newline sequences"
1207    .\"
1208    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1209    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1210    PCRE_UTF8 and the PCRE_UCP options, respectively.
1211  .  .
1212  .  .
1213  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 976  Turning part of a pattern into a subpatt Line 1221  Turning part of a pattern into a subpatt
1221  .sp  .sp
1222    cat(aract|erpillar|)    cat(aract|erpillar|)
1223  .sp  .sp
1224  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1225  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1226  .sp  .sp
1227  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
1228  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
1229  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
1230  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1231  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1232  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1233  .sp  .sp
1234    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1235  .sp  .sp
# Line 1017  is reached, an option setting in one bra Line 1261  is reached, an option setting in one bra
1261  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1262  .  .
1263  .  .
1264    .\" HTML <a name="dupsubpatternnumber"></a>
1265  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1266  .rs  .rs
1267  .sp  .sp
# Line 1033  at captured substring number one, whiche Line 1278  at captured substring number one, whiche
1278  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
1279  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1280  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
1281  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
1282  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1283  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.  
1284  .sp  .sp
1285    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1286    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1287    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1288  .sp  .sp
1289  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1290  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1291    or "defdef":
1292    .sp
1293      /(?|(abc)|(def))\e1/
1294    .sp
1295    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1296    refers to the first one in the pattern with the given number. The following
1297    pattern matches "abcabc" or "defabc":
1298    .sp
1299      /(?|(abc)|(def))(?1)/
1300    .sp
1301    If a
1302    .\" HTML <a href="#conditions">
1303    .\" </a>
1304    condition test
1305    .\"
1306    for a subpattern's having matched refers to a non-unique number, the test is
1307    true if any of the subpatterns of that number have matched.
1308  .P  .P
1309  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1310  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1058  if an expression is modified, the number Line 1319  if an expression is modified, the number
1319  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1320  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
1321  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
1322  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1323    have different names, but PCRE does not.
1324  .P  .P
1325  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
1326  (?'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
1327  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1328  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1329  .\" </a>  .\" </a>
1330  backreferences,  back references,
1331  .\"  .\"
1332  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1333  .\" </a>  .\" </a>
# Line 1085  extracting the name-to-number translatio Line 1347  extracting the name-to-number translatio
1347  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1348  .P  .P
1349  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
1350  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
1351  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
1352  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
1353  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
1354  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
1355    name, and in both cases you want to extract the abbreviation. This pattern
1356    (ignoring the line breaks) does the job:
1357  .sp  .sp
1358    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1359    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1103  subpattern, as described in the previous Line 1367  subpattern, as described in the previous
1367  .P  .P
1368  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1369  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
1370  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1371  make a reference to a non-unique named subpattern from elsewhere in the  .P
1372  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
1373  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1374    used. In the absence of duplicate numbers (see the previous section) this is
1375    the one with the lowest number. If you use a named reference in a condition
1376    test (see the
1377    .\"
1378    .\" HTML <a href="#conditions">
1379    .\" </a>
1380    section about conditions
1381    .\"
1382    below), either to check whether a subpattern has matched, or to check for
1383    recursion, all subpatterns with the same name are tested. If the condition is
1384    true for any one of them, the overall condition is true. This is the same
1385    behaviour as testing by number. For further details of the interfaces for
1386    handling named subpatterns, see the
1387  .\" HREF  .\" HREF
1388  \fBpcreapi\fP  \fBpcreapi\fP
1389  .\"  .\"
1390  documentation.  documentation.
1391    .P
1392    \fBWarning:\fP You cannot use different names to distinguish between two
1393    subpatterns with the same number because PCRE uses only the numbers when
1394    matching. For this reason, an error is given at compile time if different names
1395    are given to subpatterns with the same number. However, you can give the same
1396    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1397  .  .
1398  .  .
1399  .SH REPETITION  .SH REPETITION
# Line 1124  items: Line 1407  items:
1407    the \eC escape sequence    the \eC escape sequence
1408    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1409    the \eR escape sequence    the \eR escape sequence
1410    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1411    a character class    a character class
1412    a back reference (see next section)    a back reference (see next section)
1413    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1414      a recursive or "subroutine" call to a subpattern
1415  .sp  .sp
1416  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1417  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1159  support is available, \eX{3} matches thr Line 1443  support is available, \eX{3} matches thr
1443  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1444  .P  .P
1445  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1446  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1447    subpatterns that are referenced as
1448    .\" HTML <a href="#subpatternsassubroutines">
1449    .\" </a>
1450    subroutines
1451    .\"
1452    from elsewhere in the pattern (but see also the section entitled
1453    .\" HTML <a href="#subdefine">
1454    .\" </a>
1455    "Defining subpatterns for use by reference only"
1456    .\"
1457    below). Items other than subpatterns that have a {0} quantifier are omitted
1458    from the compiled pattern.
1459  .P  .P
1460  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1461  abbreviations:  abbreviations:
# Line 1231  worth setting PCRE_DOTALL in order to ob Line 1527  worth setting PCRE_DOTALL in order to ob
1527  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1528  .P  .P
1529  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1530  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1531  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
1532  succeeds. Consider, for example:  succeeds. Consider, for example:
1533  .sp  .sp
# Line 1384  no such problem when named parentheses a Line 1680  no such problem when named parentheses a
1680  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1681  .P  .P
1682  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
1683  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
1684  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
1685  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1686  .sp  .sp
1687    (ring), \e1    (ring), \e1
1688    (ring), \eg1    (ring), \eg1
# Line 1400  example: Line 1696  example:
1696    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1697  .sp  .sp
1698  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
1699  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.
1700  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
1701  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
1702  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1703  .P  .P
1704  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1705  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1442  after the reference. Line 1738  after the reference.
1738  .P  .P
1739  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1740  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1741  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1742  .sp  .sp
1743    (a|(bc))\e2    (a|(bc))\e2
1744  .sp  .sp
1745  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1746  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1747  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1748  with a digit character, some delimiter must be used to terminate the back  .P
1749  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1750  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1751    If the pattern continues with a digit character, some delimiter must be used to
1752    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1753    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1754  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1755  .\" </a>  .\" </a>
1756  "Comments"  "Comments"
1757  .\"  .\"
1758  below) can be used.  below) can be used.
1759  .P  .
1760    .SS "Recursive back references"
1761    .rs
1762    .sp
1763  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
1764  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.
1765  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1471  to the previous iteration. In order for Line 1773  to the previous iteration. In order for
1773  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
1774  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
1775  minimum of zero.  minimum of zero.
1776    .P
1777    Back references of this type cause the group that they reference to be treated
1778    as an
1779    .\" HTML <a href="#atomicgroup">
1780    .\" </a>
1781    atomic group.
1782    .\"
1783    Once the whole group has been matched, a subsequent matching failure cannot
1784    cause backtracking into the middle of the group.
1785  .  .
1786  .  .
1787  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1524  lookbehind assertion is needed to achiev Line 1835  lookbehind assertion is needed to achiev
1835  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
1836  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
1837  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.
1838    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1839  .  .
1840  .  .
1841  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1548  is permitted, but Line 1860  is permitted, but
1860  .sp  .sp
1861  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1862  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
1863  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1864  match the same length of string. An assertion such as  length of string. An assertion such as
1865  .sp  .sp
1866    (?<=ab(c|de))    (?<=ab(c|de))
1867  .sp  .sp
1868  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1869  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1870    branches:
1871  .sp  .sp
1872    (?<=abc|abde)    (?<=abc|abde)
1873  .sp  .sp
1874  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1875  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1876  .\" </a>  .\" </a>
1877  (see above)  (see above)
1878  .\"  .\"
1879  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1880  fixed-length.  restriction.
1881  .P  .P
1882  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1883  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1576  to appear in lookbehind assertions, beca Line 1889  to appear in lookbehind assertions, beca
1889  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1890  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1891  .P  .P
1892    .\" HTML <a href="#subpatternsassubroutines">
1893    .\" </a>
1894    "Subroutine"
1895    .\"
1896    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1897    as the subpattern matches a fixed-length string.
1898    .\" HTML <a href="#recursion">
1899    .\" </a>
1900    Recursion,
1901    .\"
1902    however, is not supported.
1903    .P
1904  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1905  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1906  pattern such as  strings. Consider a simple pattern such as
1907  .sp  .sp
1908    abcd$    abcd$
1909  .sp  .sp
# Line 1642  characters that are not "999". Line 1967  characters that are not "999".
1967  .sp  .sp
1968  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1969  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1970  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1971  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1972  .sp  .sp
1973    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1974    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1975  .sp  .sp
1976  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1977  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
1978  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1979    itself contain nested subpatterns of any form, including conditional
1980    subpatterns; the restriction to two alternatives applies only at the level of
1981    the condition. This pattern fragment is an example where the alternatives are
1982    complex:
1983    .sp
1984      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1985    .sp
1986  .P  .P
1987  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
1988  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1659  recursion, a pseudo-condition called DEF Line 1991  recursion, a pseudo-condition called DEF
1991  .rs  .rs
1992  .sp  .sp
1993  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
1994  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1995  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
1996  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1997  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1998  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1999  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2000    section about duplicate subpattern numbers),
2001    .\"
2002    the condition is true if any of them have matched. An alternative notation is
2003    to precede the digits with a plus or minus sign. In this case, the subpattern
2004    number is relative rather than absolute. The most recently opened parentheses
2005    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2006    loops it can also make sense to refer to subsequent groups. The next
2007    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2008    zero in any of these forms is not used; it provokes a compile-time error.)
2009  .P  .P
2010  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2011  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 1675  three parts for ease of discussion: Line 2016  three parts for ease of discussion:
2016  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2017  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
2018  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
2019  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2020  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,
2021  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
2022  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2023  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 1705  Rewriting the above example to use a nam Line 2046  Rewriting the above example to use a nam
2046  .sp  .sp
2047    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2048  .sp  .sp
2049    If the name used in a condition of this kind is a duplicate, the test is
2050    applied to all subpatterns of the same name, and is true if any one of them has
2051    matched.
2052  .  .
2053  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2054  .rs  .rs
# Line 1716  letter R, for example: Line 2060  letter R, for example:
2060  .sp  .sp
2061    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2062  .sp  .sp
2063  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2064  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
2065  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2066    applied to all subpatterns of the same name, and is true if any one of them is
2067    the most recent recursion.
2068  .P  .P
2069  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2070  patterns are described below.  .\" HTML <a href="#recursion">
2071    .\" </a>
2072    The syntax for recursive patterns
2073    .\"
2074    is described below.
2075  .  .
2076    .\" HTML <a name="subdefine"></a>
2077  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2078  .rs  .rs
2079  .sp  .sp
# Line 1730  If the condition is the string (DEFINE), Line 2081  If the condition is the string (DEFINE),
2081  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
2082  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2083  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
2084  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
2085  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2086  written like this (ignore whitespace and line breaks):  .\" </a>
2087    "subroutines"
2088    .\"
2089    is described below.) For example, a pattern to match an IPv4 address such as
2090    "192.168.23.245" could be written like this (ignore whitespace and line
2091    breaks):
2092  .sp  .sp
2093    (?(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) )
2094    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1740  written like this (ignore whitespace and Line 2096  written like this (ignore whitespace and
2096  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2097  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2098  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2099  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2100  .P  pattern uses references to the named group to match the four dot-separated
2101  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2102  .  .
2103  .SS "Assertion conditions"  .SS "Assertion conditions"
2104  .rs  .rs
# Line 1769  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2123  dd-aaa-dd or dd-dd-dd, where aaa are let
2123  .SH COMMENTS  .SH COMMENTS
2124  .rs  .rs
2125  .sp  .sp
2126  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
2127  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,
2128  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
2129    subpattern name or number. The characters that make up a comment play no part
2130    in the pattern matching.
2131  .P  .P
2132  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
2133  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2134  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2135    this case continues to immediately after the next newline character or
2136    character sequence in the pattern. Which characters are interpreted as newlines
2137    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2138    sequence at the start of the pattern, as described in the section entitled
2139    .\" HTML <a href="#newlines">
2140    .\" </a>
2141    "Newline conventions"
2142    .\"
2143    above. Note that the end of this type of comment is a literal newline sequence
2144    in the pattern; escape sequences that happen to represent a newline do not
2145    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2146    default newline convention is in force:
2147    .sp
2148      abc #comment \en still comment
2149    .sp
2150    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2151    a newline in the pattern. The sequence \en is still literal at this stage, so
2152    it does not terminate the comment. Only an actual character with the code value
2153    0x0a (the default newline) does so.
2154  .  .
2155  .  .
2156  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1801  recursively to the pattern in which it a Line 2176  recursively to the pattern in which it a
2176  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2177  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2178  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2179  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2180  .P  .P
2181  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
2182  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,
2183  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
2184    .\" HTML <a href="#subpatternsassubroutines">
2185    .\" </a>
2186    "subroutine"
2187    .\"
2188  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2189  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2190  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2191  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2192  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2193  .sp  .sp
2194    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2195  .sp  .sp
2196  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2197  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
2198  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2199  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2200    to avoid backtracking into sequences of non-parentheses.
2201  .P  .P
2202  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
2203  pattern, so instead you could use this:  pattern, so instead you could use this:
2204  .sp  .sp
2205    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2206  .sp  .sp
2207  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2208  them instead of the whole pattern.  them instead of the whole pattern.
2209  .P  .P
2210  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
2211  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
2212  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
2213  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2214  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.  
2215  .P  .P
2216  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2217  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2218  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2219  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2220    .\" </a>
2221    "subroutine"
2222    .\"
2223    calls, as described in the next section.
2224  .P  .P
2225  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2226  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2227  could rewrite the above example as follows:  could rewrite the above example as follows:
2228  .sp  .sp
2229    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2230  .sp  .sp
2231  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2232  used.  used.
2233  .P  .P
2234  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2235  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2236  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2237  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2238  .sp  .sp
2239    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2240  .sp  .sp
2241  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2242  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2243  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
2244  before failure can be reported.  before failure can be reported.
2245  .P  .P
2246  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
2247  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
2248  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  
2249  .\" HREF  .\" HREF
2250  \fBpcrecallout\fP  \fBpcrecallout\fP
2251  .\"  .\"
# Line 1876  documentation). If the pattern above is Line 2253  documentation). If the pattern above is
2253  .sp  .sp
2254    (ab(cd)ef)    (ab(cd)ef)
2255  .sp  .sp
2256  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
2257  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
2258  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2259    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2260       ^                        ^  .P
2261       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2262  .sp  obtain extra memory to store data during a recursion, which it does by using
2263  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
2264  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.  
2265  .P  .P
2266  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.
2267  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1901  different alternatives for the recursive Line 2275  different alternatives for the recursive
2275  is the actual recursive call.  is the actual recursive call.
2276  .  .
2277  .  .
2278    .\" HTML <a name="recursiondifference"></a>
2279    .SS "Recursion difference from Perl"
2280    .rs
2281    .sp
2282    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2283    treated as an atomic group. That is, once it has matched some of the subject
2284    string, it is never re-entered, even if it contains untried alternatives and
2285    there is a subsequent matching failure. This can be illustrated by the
2286    following pattern, which purports to match a palindromic string that contains
2287    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2288    .sp
2289      ^(.|(.)(?1)\e2)$
2290    .sp
2291    The idea is that it either matches a single character, or two identical
2292    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2293    it does not if the pattern is longer than three characters. Consider the
2294    subject string "abcba":
2295    .P
2296    At the top level, the first character is matched, but as it is not at the end
2297    of the string, the first alternative fails; the second alternative is taken
2298    and the recursion kicks in. The recursive call to subpattern 1 successfully
2299    matches the next character ("b"). (Note that the beginning and end of line
2300    tests are not part of the recursion).
2301    .P
2302    Back at the top level, the next character ("c") is compared with what
2303    subpattern 2 matched, which was "a". This fails. Because the recursion is
2304    treated as an atomic group, there are now no backtracking points, and so the
2305    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2306    try the second alternative.) However, if the pattern is written with the
2307    alternatives in the other order, things are different:
2308    .sp
2309      ^((.)(?1)\e2|.)$
2310    .sp
2311    This time, the recursing alternative is tried first, and continues to recurse
2312    until it runs out of characters, at which point the recursion fails. But this
2313    time we do have another alternative to try at the higher level. That is the big
2314    difference: in the previous case the remaining alternative is at a deeper
2315    recursion level, which PCRE cannot use.
2316    .P
2317    To change the pattern so that it matches all palindromic strings, not just
2318    those with an odd number of characters, it is tempting to change the pattern to
2319    this:
2320    .sp
2321      ^((.)(?1)\e2|.?)$
2322    .sp
2323    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2324    deeper recursion has matched a single character, it cannot be entered again in
2325    order to match an empty string. The solution is to separate the two cases, and
2326    write out the odd and even cases as alternatives at the higher level:
2327    .sp
2328      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2329    .sp
2330    If you want to match typical palindromic phrases, the pattern has to ignore all
2331    non-word characters, which can be done like this:
2332    .sp
2333      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2334    .sp
2335    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2336    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2337    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2338    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2339    more) to match typical phrases, and Perl takes so long that you think it has
2340    gone into a loop.
2341    .P
2342    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2343    string does not start with a palindrome that is shorter than the entire string.
2344    For example, although "abcba" is correctly matched, if the subject is "ababa",
2345    PCRE finds the palindrome "aba" at the start, then fails at top level because
2346    the end of the string does not follow. Once again, it cannot jump back into the
2347    recursion to try other alternatives, so the entire match fails.
2348    .
2349    .
2350  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2351  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2352  .rs  .rs
# Line 1927  matches "sense and sensibility" and "res Line 2373  matches "sense and sensibility" and "res
2373  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
2374  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2375  .P  .P
2376  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2377  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
2378  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
2379  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2380    call revert to their previous values afterwards.
2381  .P  .P
2382  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2383  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 1942  It matches "abcabc". It does not match " Line 2389  It matches "abcabc". It does not match "
2389  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2390  .  .
2391  .  .
2392    .\" HTML <a name="onigurumasubroutines"></a>
2393    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2394    .rs
2395    .sp
2396    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2397    a number enclosed either in angle brackets or single quotes, is an alternative
2398    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2399    are two of the examples used above, rewritten using this syntax:
2400    .sp
2401      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2402      (sens|respons)e and \eg'1'ibility
2403    .sp
2404    PCRE supports an extension to Oniguruma: if a number is preceded by a
2405    plus or a minus sign it is taken as a relative reference. For example:
2406    .sp
2407      (abc)(?i:\eg<-1>)
2408    .sp
2409    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2410    synonymous. The former is a back reference; the latter is a subroutine call.
2411    .
2412    .
2413  .SH CALLOUTS  .SH CALLOUTS
2414  .rs  .rs
2415  .sp  .sp
# Line 1978  description of the interface to the call Line 2446  description of the interface to the call
2446  documentation.  documentation.
2447  .  .
2448  .  .
2449  .SH "BACTRACKING CONTROL"  .\" HTML <a name="backtrackcontrol"></a>
2450    .SH "BACKTRACKING CONTROL"
2451  .rs  .rs
2452  .sp  .sp
2453  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
# Line 1987  or removal in a future version of Perl". Line 2456  or removal in a future version of Perl".
2456  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2457  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2458  .P  .P
2459  Since these verbs are specifically related to backtracking, they can be used  Since these verbs are specifically related to backtracking, most of them can be
2460  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2461  backtracking algorithm. They cause an error if encountered by  a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2462    failing negative assertion, they cause an error if encountered by
2463  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2464  .P  .P
2465    If any of these verbs are used in an assertion or subroutine subpattern
2466    (including recursive subpatterns), their effect is confined to that subpattern;
2467    it does not extend to the surrounding pattern. Note that such subpatterns are
2468    processed as anchored at the point where they are tested.
2469    .P
2470  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
2471  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2472  (*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,
2473  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
2474  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2475    parenthesis immediately follows the colon, the effect is as if the colon were
2476    not there. Any number of these verbs may occur in a pattern.
2477    .P
2478    PCRE contains some optimizations that are used to speed up matching by running
2479    some checks at the start of each match attempt. For example, it may know the
2480    minimum length of matching subject, or that a particular character must be
2481    present. When one of these optimizations suppresses the running of a match, any
2482    included backtracking verbs will not, of course, be processed. You can suppress
2483    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2484    when calling \fBpcre_exec()\fP.
2485    .
2486  .  .
2487  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2488  .rs  .rs
2489  .sp  .sp
2490  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2491    followed by a name.
2492  .sp  .sp
2493     (*ACCEPT)     (*ACCEPT)
2494  .sp  .sp
2495  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2496  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2497  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2498  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  captured. (This feature was added to PCRE at release 8.00.) For example:
 captured. For example:  
2499  .sp  .sp
2500    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2501  .sp  .sp
2502  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2503  captured.  the outer parentheses.
2504  .sp  .sp
2505    (*FAIL) or (*F)    (*FAIL) or (*F)
2506  .sp  .sp
# Line 2029  callout feature, as for example in this Line 2515  callout feature, as for example in this
2515  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
2516  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2517  .  .
2518    .
2519    .SS "Recording which path was taken"
2520    .rs
2521    .sp
2522    There is one verb whose main purpose is to track how a match was arrived at,
2523    though it also has a secondary use in conjunction with advancing the match
2524    starting point (see (*SKIP) below).
2525    .sp
2526      (*MARK:NAME) or (*:NAME)
2527    .sp
2528    A name is always required with this verb. There may be as many instances of
2529    (*MARK) as you like in a pattern, and their names do not have to be unique.
2530    .P
2531    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2532    to the caller via the \fIpcre_extra\fP data structure, as described in the
2533    .\" HTML <a href="pcreapi.html#extradata">
2534    .\" </a>
2535    section on \fIpcre_extra\fP
2536    .\"
2537    in the
2538    .\" HREF
2539    \fBpcreapi\fP
2540    .\"
2541    documentation. No data is returned for a partial match. Here is an example of
2542    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2543    outputting of (*MARK) data:
2544    .sp
2545      /X(*MARK:A)Y|X(*MARK:B)Z/K
2546      XY
2547       0: XY
2548      MK: A
2549      XZ
2550       0: XZ
2551      MK: B
2552    .sp
2553    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2554    indicates which of the two alternatives matched. This is a more efficient way
2555    of obtaining this information than putting each alternative in its own
2556    capturing parentheses.
2557    .P
2558    A name may also be returned after a failed match if the final path through the
2559    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2560    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2561    starting point for matching is advanced, the final check is often with an empty
2562    string, causing a failure before (*MARK) is reached. For example:
2563    .sp
2564      /X(*MARK:A)Y|X(*MARK:B)Z/K
2565      XP
2566      No match
2567    .sp
2568    There are three potential starting points for this match (starting with X,
2569    starting with P, and with an empty string). If the pattern is anchored, the
2570    result is different:
2571    .sp
2572      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2573      XP
2574      No match, mark = B
2575    .sp
2576    PCRE's start-of-match optimizations can also interfere with this. For example,
2577    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2578    subject length for a match, a shorter subject will not be scanned at all.
2579    .P
2580    Note that similar anomalies (though different in detail) exist in Perl, no
2581    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2582    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2583    .
2584    .
2585  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2586  .rs  .rs
2587  .sp  .sp
2588  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2589  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
2590  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
2591    the verb. However, when one of these verbs appears inside an atomic group, its
2592    effect is confined to that group, because once the group has been matched,
2593    there is never any backtracking into it. In this situation, backtracking can
2594    "jump back" to the left of the entire atomic group. (Remember also, as stated
2595    above, that this localization also applies in subroutine calls and assertions.)
2596    .P
2597    These verbs differ in exactly what kind of failure occurs when backtracking
2598    reaches them.
2599  .sp  .sp
2600    (*COMMIT)    (*COMMIT)
2601  .sp  .sp
2602  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
2603  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
2604  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2605  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
2606  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2607  .sp  .sp
2608    a+(*COMMIT)b    a+(*COMMIT)b
2609  .sp  .sp
2610  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
2611  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
2612  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2613    (*PRUNE)  match failure.
2614  .sp  .P
2615  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,
2616  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
2617  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2618  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2619  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2620  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2621  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2622  be expressed in any other way.    xyzabc\eY
2623      No match
2624    .sp
2625    PCRE knows that any match must start with "a", so the optimization skips along
2626    the subject to "a" before running the first match attempt, which succeeds. When
2627    the optimization is disabled by the \eY escape in the second subject, the match
2628    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2629    starting points.
2630    .sp
2631      (*PRUNE) or (*PRUNE:NAME)
2632    .sp
2633    This verb causes the match to fail at the current starting position in the
2634    subject if the rest of the pattern does not match. If the pattern is
2635    unanchored, the normal "bumpalong" advance to the next starting character then
2636    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2637    reached, or when matching to the right of (*PRUNE), but if there is no match to
2638    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2639    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2640    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2641    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2642    match fails completely; the name is passed back if this is the final attempt.
2643    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2644    pattern (*PRUNE) has the same effect as (*COMMIT).
2645  .sp  .sp
2646    (*SKIP)    (*SKIP)
2647  .sp  .sp
2648  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
2649  "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,
2650  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2651  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
2652    successful match. Consider:
2653  .sp  .sp
2654    a+(*SKIP)b    a+(*SKIP)b
2655  .sp  .sp
2656  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2657  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2658  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2659  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2660  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2661  instead of skipping on to "c".  instead of skipping on to "c".
2662  .sp  .sp
2663    (*THEN)    (*SKIP:NAME)
2664  .sp  .sp
2665  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
2666  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2667  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,
2668  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2669    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2670    matching name is found, normal "bumpalong" of one character happens (the
2671    (*SKIP) is ignored).
2672    .sp
2673      (*THEN) or (*THEN:NAME)
2674    .sp
2675    This verb causes a skip to the next alternation in the innermost enclosing
2676    group if the rest of the pattern does not match. That is, it cancels pending
2677    backtracking, but only within the current alternation. Its name comes from the
2678    observation that it can be used for a pattern-based if-then-else block:
2679  .sp  .sp
2680    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2681  .sp  .sp
2682  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
2683  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
2684  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2685  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
2686    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2687    like (*PRUNE).
2688    .
2689    .P
2690    The above verbs provide four different "strengths" of control when subsequent
2691    matching fails. (*THEN) is the weakest, carrying on the match at the next
2692    alternation. (*PRUNE) comes next, failing the match at the current starting
2693    position, but allowing an advance to the next character (for an unanchored
2694    pattern). (*SKIP) is similar, except that the advance may be more than one
2695    character. (*COMMIT) is the strongest, causing the entire match to fail.
2696    .P
2697    If more than one is present in a pattern, the "stongest" one wins. For example,
2698    consider this pattern, where A, B, etc. are complex pattern fragments:
2699    .sp
2700      (A(*COMMIT)B(*THEN)C|D)
2701    .sp
2702    Once A has matched, PCRE is committed to this match, at the current starting
2703    position. If subsequently B matches, but C does not, the normal (*THEN) action
2704    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2705    overrides.
2706  .  .
2707  .  .
2708  .SH "SEE ALSO"  .SH "SEE ALSO"
2709  .rs  .rs
2710  .sp  .sp
2711  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2712    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2713  .  .
2714  .  .
2715  .SH AUTHOR  .SH AUTHOR
# Line 2111  Cambridge CB2 3QH, England. Line 2726  Cambridge CB2 3QH, England.
2726  .rs  .rs
2727  .sp  .sp
2728  .nf  .nf
2729  Last updated: 09 August 2007  Last updated: 20 November 2010
2730  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2731  .fi  .fi

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