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# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  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
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    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 26  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 40  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 132  Perl, $ and @ cause variable interpolati Line 211  Perl, $ and @ cause variable interpolati
211    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
212  .sp  .sp
213  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
214    An isolated \eE that is not preceded by \eQ is ignored.
215  .  .
216  .  .
217  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 141  The \eQ...\eE sequence is recognized bot Line 221  The \eQ...\eE sequence is recognized bot
221  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
222  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
223  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
224  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
225  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:  
226  .sp  .sp
227    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
228    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
229    \ee        escape (hex 1B)    \ee        escape (hex 1B)
230    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
231    \en        newline (hex 0A)    \en        linefeed (hex 0A)
232    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
233    \et        tab (hex 09)    \et        tab (hex 09)
234    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
235    \exhh      character with hex code hh    \exhh      character with hex code hh
236    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
237  .sp  .sp
# Line 164  Thus \ecz becomes hex 1A, but \ec{ becom Line 243  Thus \ecz becomes hex 1A, but \ec{ becom
243  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
244  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{
245  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
246  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
247  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
248  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
249  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
250  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
251    there is no terminating }, this form of escape is not recognized. Instead, the
252    initial \ex will be interpreted as a basic hexadecimal escape, with no
253    following digits, giving a character whose value is zero.
254  .P  .P
255  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
256  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 227  zero, because no more than three octal d Line 309  zero, because no more than three octal d
309  .P  .P
310  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
311  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
312  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
313  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
314  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
315  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
316  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
317  .  .
318  .  .
319  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
320  .rs  .rs
321  .sp  .sp
322  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
323  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
324  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
325  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
326  .\" </a>  .\" </a>
327  later,  later,
# Line 254  parenthesized subpatterns. Line 333  parenthesized subpatterns.
333  .\"  .\"
334  .  .
335  .  .
336    .SS "Absolute and relative subroutine calls"
337    .rs
338    .sp
339    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
340    a number enclosed either in angle brackets or single quotes, is an alternative
341    syntax for referencing a subpattern as a "subroutine". Details are discussed
342    .\" HTML <a href="#onigurumasubroutines">
343    .\" </a>
344    later.
345    .\"
346    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
347    synonymous. The former is a back reference; the latter is a
348    .\" HTML <a href="#subpatternsassubroutines">
349    .\" </a>
350    subroutine
351    .\"
352    call.
353    .
354    .
355    .\" HTML <a name="genericchartypes"></a>
356  .SS "Generic character types"  .SS "Generic character types"
357  .rs  .rs
358  .sp  .sp
359  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:  
360  .sp  .sp
361    \ed     any decimal digit    \ed     any decimal digit
362    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
363    \eh     any horizontal whitespace character    \eh     any horizontal whitespace character
364    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal whitespace character
365    \es     any whitespace character    \es     any whitespace character
366    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
367    \ev     any vertical whitespace character    \ev     any vertical whitespace character
368    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical whitespace character
369    \ew     any "word" character    \ew     any "word" character
370    \eW     any "non-word" character    \eW     any "non-word" character
371  .sp  .sp
372  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.
373  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
374    .\" HTML <a href="#fullstopdot">
375    .\" </a>
376    the "." metacharacter
377    .\"
378    when PCRE_DOTALL is not set.
379  .P  .P
380  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
381    of characters into two disjoint sets. Any given character matches one, and only
382    one, of each pair. The sequences can appear both inside and outside character
383  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
384  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
385  there is no character to match.  there is no character to match.
386  .P  .P
387  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 285  are HT (9), LF (10), FF (12), CR (13), a Line 390  are HT (9), LF (10), FF (12), CR (13), a
390  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
391  does.  does.
392  .P  .P
393  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.
394  \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
395  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
396  meanings from before UTF-8 support was available, mainly for efficiency  place (see
397  reasons.  .\" HTML <a href="pcreapi.html#localesupport">
398  .P  .\" </a>
399  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  "Locale support"
400  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\"
401  The horizontal space characters are:  in the
402    .\" HREF
403    \fBpcreapi\fP
404    .\"
405    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
406    or "french" in Windows, some character codes greater than 128 are used for
407    accented letters, and these are then matched by \ew. The use of locales with
408    Unicode is discouraged.
409    .P
410    By default, in UTF-8 mode, characters with values greater than 128 never match
411    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
412    their original meanings from before UTF-8 support was available, mainly for
413    efficiency reasons. However, if PCRE is compiled with Unicode property support,
414    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
415    properties are used to determine character types, as follows:
416    .sp
417      \ed  any character that \ep{Nd} matches (decimal digit)
418      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
419      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
420    .sp
421    The upper case escapes match the inverse sets of characters. Note that \ed
422    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
423    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
424    \eB because they are defined in terms of \ew and \eW. Matching these sequences
425    is noticeably slower when PCRE_UCP is set.
426    .P
427    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
428    release 5.10. In contrast to the other sequences, which match only ASCII
429    characters by default, these always match certain high-valued codepoints in
430    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
431    are:
432  .sp  .sp
433    U+0009     Horizontal tab    U+0009     Horizontal tab
434    U+0020     Space    U+0020     Space
# Line 324  The vertical space characters are: Line 459  The vertical space characters are:
459    U+0085     Next line    U+0085     Next line
460    U+2028     Line separator    U+2028     Line separator
461    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.  
462  .  .
463  .  .
464    .\" HTML <a name="newlineseq"></a>
465  .SS "Newline sequences"  .SS "Newline sequences"
466  .rs  .rs
467  .sp  .sp
468  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
469  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:  
470  .sp  .sp
471    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
472  .sp  .sp
# Line 368  are added: LS (line separator, U+2028) a Line 486  are added: LS (line separator, U+2028) a
486  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
487  recognized.  recognized.
488  .P  .P
489  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
490    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
491    either at compile time or when the pattern is matched. (BSR is an abbrevation
492    for "backslash R".) This can be made the default when PCRE is built; if this is
493    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
494    It is also possible to specify these settings by starting a pattern string with
495    one of the following sequences:
496    .sp
497      (*BSR_ANYCRLF)   CR, LF, or CRLF only
498      (*BSR_UNICODE)   any Unicode newline sequence
499    .sp
500    These override the default and the options given to \fBpcre_compile()\fP or
501    \fBpcre_compile2()\fP, but they can be overridden by options given to
502    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
503    which are not Perl-compatible, are recognized only at the very start of a
504    pattern, and that they must be in upper case. If more than one of them is
505    present, the last one is used. They can be combined with a change of newline
506    convention; for example, a pattern can start with:
507    .sp
508      (*ANY)(*BSR_ANYCRLF)
509    .sp
510    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
511    a character class, \eR is treated as an unrecognized escape sequence, and so
512    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
513  .  .
514  .  .
515  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 376  Inside a character class, \eR matches th Line 517  Inside a character class, \eR matches th
517  .rs  .rs
518  .sp  .sp
519  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
520  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
521  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
522    characters whose codepoints are less than 256, but they do work in this mode.
523    The extra escape sequences are:
524  .sp  .sp
525    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
526    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
527    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
528  .sp  .sp
529  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
530  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
531  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
532  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
533  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
534    .\" </a>
535    next section).
536    .\"
537    Other Perl properties such as "InMusicalSymbols" are not currently supported by
538    PCRE. Note that \eP{Any} does not match any characters, so always causes a
539    match failure.
540  .P  .P
541  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
542  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 401  Those that are not part of an identified Line 550  Those that are not part of an identified
550  .P  .P
551  Arabic,  Arabic,
552  Armenian,  Armenian,
553    Avestan,
554  Balinese,  Balinese,
555    Bamum,
556  Bengali,  Bengali,
557  Bopomofo,  Bopomofo,
558  Braille,  Braille,
559  Buginese,  Buginese,
560  Buhid,  Buhid,
561  Canadian_Aboriginal,  Canadian_Aboriginal,
562    Carian,
563    Cham,
564  Cherokee,  Cherokee,
565  Common,  Common,
566  Coptic,  Coptic,
# Line 416  Cypriot, Line 569  Cypriot,
569  Cyrillic,  Cyrillic,
570  Deseret,  Deseret,
571  Devanagari,  Devanagari,
572    Egyptian_Hieroglyphs,
573  Ethiopic,  Ethiopic,
574  Georgian,  Georgian,
575  Glagolitic,  Glagolitic,
# Line 428  Hangul, Line 582  Hangul,
582  Hanunoo,  Hanunoo,
583  Hebrew,  Hebrew,
584  Hiragana,  Hiragana,
585    Imperial_Aramaic,
586  Inherited,  Inherited,
587    Inscriptional_Pahlavi,
588    Inscriptional_Parthian,
589    Javanese,
590    Kaithi,
591  Kannada,  Kannada,
592  Katakana,  Katakana,
593    Kayah_Li,
594  Kharoshthi,  Kharoshthi,
595  Khmer,  Khmer,
596  Lao,  Lao,
597  Latin,  Latin,
598    Lepcha,
599  Limbu,  Limbu,
600  Linear_B,  Linear_B,
601    Lisu,
602    Lycian,
603    Lydian,
604  Malayalam,  Malayalam,
605    Meetei_Mayek,
606  Mongolian,  Mongolian,
607  Myanmar,  Myanmar,
608  New_Tai_Lue,  New_Tai_Lue,
# Line 445  Nko, Line 610  Nko,
610  Ogham,  Ogham,
611  Old_Italic,  Old_Italic,
612  Old_Persian,  Old_Persian,
613    Old_South_Arabian,
614    Old_Turkic,
615    Ol_Chiki,
616  Oriya,  Oriya,
617  Osmanya,  Osmanya,
618  Phags_Pa,  Phags_Pa,
619  Phoenician,  Phoenician,
620    Rejang,
621  Runic,  Runic,
622    Samaritan,
623    Saurashtra,
624  Shavian,  Shavian,
625  Sinhala,  Sinhala,
626    Sundanese,
627  Syloti_Nagri,  Syloti_Nagri,
628  Syriac,  Syriac,
629  Tagalog,  Tagalog,
630  Tagbanwa,  Tagbanwa,
631  Tai_Le,  Tai_Le,
632    Tai_Tham,
633    Tai_Viet,
634  Tamil,  Tamil,
635  Telugu,  Telugu,
636  Thaana,  Thaana,
# Line 464  Thai, Line 638  Thai,
638  Tibetan,  Tibetan,
639  Tifinagh,  Tifinagh,
640  Ugaritic,  Ugaritic,
641    Vai,
642  Yi.  Yi.
643  .P  .P
644  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
645  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
646  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
647  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
648  .P  .P
649  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
650  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 529  The special property L& is also supporte Line 704  The special property L& is also supporte
704  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
705  a modifier or "other".  a modifier or "other".
706  .P  .P
707  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
708    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
709    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
710    (see the discussion of PCRE_NO_UTF8_CHECK in the
711    .\" HREF
712    \fBpcreapi\fP
713    .\"
714    page). Perl does not support the Cs property.
715    .P
716    The long synonyms for property names that Perl supports (such as \ep{Letter})
717  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
718  properties with "Is".  properties with "Is".
719  .P  .P
# Line 553  atomic group Line 737  atomic group
737  (see below).  (see below).
738  .\"  .\"
739  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
740  preceding character.  preceding character. None of them have codepoints less than 256, so in
741    non-UTF-8 mode \eX matches any one character.
742  .P  .P
743  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
744  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
745  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
746  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
747    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
748    (*UCP).
749    .
750    .
751    .\" HTML <a name="extraprops"></a>
752    .SS PCRE's additional properties
753    .rs
754    .sp
755    As well as the standard Unicode properties described in the previous
756    section, PCRE supports four more that make it possible to convert traditional
757    escape sequences such as \ew and \es and POSIX character classes to use Unicode
758    properties. PCRE uses these non-standard, non-Perl properties internally when
759    PCRE_UCP is set. They are:
760    .sp
761      Xan   Any alphanumeric character
762      Xps   Any POSIX space character
763      Xsp   Any Perl space character
764      Xwd   Any Perl "word" character
765    .sp
766    Xan matches characters that have either the L (letter) or the N (number)
767    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
768    carriage return, and any other character that has the Z (separator) property.
769    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
770    same characters as Xan, plus underscore.
771  .  .
772  .  .
773  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
774  .SS "Resetting the match start"  .SS "Resetting the match start"
775  .rs  .rs
776  .sp  .sp
777  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
778  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:  
779  .sp  .sp
780    foo\eKbar    foo\eKbar
781  .sp  .sp
# Line 589  For example, when the pattern Line 797  For example, when the pattern
797    (foo)\eKbar    (foo)\eKbar
798  .sp  .sp
799  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
800    .P
801    Perl documents that the use of \eK within assertions is "not well defined". In
802    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
803    ignored in negative assertions.
804  .  .
805  .  .
806  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 613  The backslashed assertions are: Line 825  The backslashed assertions are:
825    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
826    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
827  .sp  .sp
828  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
829  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
830    default it matches the corresponding literal character (for example, \eB
831    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
832    escape sequence" error is generated instead.
833  .P  .P
834  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
835  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
836  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
837  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
838    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
839    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
840    of word" or "end of word" metasequence. However, whatever follows \eb normally
841    determines which it is. For example, the fragment \eba matches "a" at the start
842    of a word.
843  .P  .P
844  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
845  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 703  end of the subject in both modes, and if Line 923  end of the subject in both modes, and if
923  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
924  .  .
925  .  .
926  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
927    .SH "FULL STOP (PERIOD, DOT) AND \eN"
928  .rs  .rs
929  .sp  .sp
930  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 725  to match it. Line 946  to match it.
946  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
947  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
948  special meaning in a character class.  special meaning in a character class.
949    .P
950    The escape sequence \eN behaves like a dot, except that it is not affected by
951    the PCRE_DOTALL option. In other words, it matches any character except one
952    that signifies the end of a line.
953  .  .
954  .  .
955  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 733  special meaning in a character class. Line 958  special meaning in a character class.
958  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
959  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
960  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
961  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
962  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,
963  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
964  .P  .P
965  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 751  the lookbehind. Line 976  the lookbehind.
976  .rs  .rs
977  .sp  .sp
978  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
979  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
980  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
981  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
982  escaped with a backslash.  a member of the class, it should be the first data character in the class
983    (after an initial circumflex, if present) or escaped with a backslash.
984  .P  .P
985  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
986  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
987  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
988  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
989  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
990  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 768  For example, the character class [aeiou] Line 994  For example, the character class [aeiou]
994  [^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
995  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
996  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
997  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
998  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
999  string.  string.
1000  .P  .P
# Line 782  caseful version would. In UTF-8 mode, PC Line 1008  caseful version would. In UTF-8 mode, PC
1008  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
1009  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1010  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1011  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,
1012  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
1013  UTF-8 support.  with UTF-8 support.
1014  .P  .P
1015  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
1016  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 818  characters in both cases. In UTF-8 mode, Line 1044  characters in both cases. In UTF-8 mode,
1044  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1045  property support.  property support.
1046  .P  .P
1047  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1048  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1049  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1050  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1051  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1052  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1053  .P  .P
1054  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1055  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 843  this notation. For example, Line 1069  this notation. For example,
1069    [01[:alpha:]%]    [01[:alpha:]%]
1070  .sp  .sp
1071  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1072  are  are:
1073  .sp  .sp
1074    alnum    letters and digits    alnum    letters and digits
1075    alpha    letters    alpha    letters
# Line 854  are Line 1080  are
1080    graph    printing characters, excluding space    graph    printing characters, excluding space
1081    lower    lower case letters    lower    lower case letters
1082    print    printing characters, including space    print    printing characters, including space
1083    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1084    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1085    upper    upper case letters    upper    upper case letters
1086    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 875  matches "1", "2", or any non-digit. PCRE Line 1101  matches "1", "2", or any non-digit. PCRE
1101  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1102  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1103  .P  .P
1104  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1105  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1106    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1107    character properties are used. This is achieved by replacing the POSIX classes
1108    by other sequences, as follows:
1109    .sp
1110      [:alnum:]  becomes  \ep{Xan}
1111      [:alpha:]  becomes  \ep{L}
1112      [:blank:]  becomes  \eh
1113      [:digit:]  becomes  \ep{Nd}
1114      [:lower:]  becomes  \ep{Ll}
1115      [:space:]  becomes  \ep{Xps}
1116      [:upper:]  becomes  \ep{Lu}
1117      [:word:]   becomes  \ep{Xwd}
1118    .sp
1119    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1120    classes are unchanged, and match only characters with code points less than
1121    128.
1122  .  .
1123  .  .
1124  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 903  alternative in the subpattern. Line 1145  alternative in the subpattern.
1145  .rs  .rs
1146  .sp  .sp
1147  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1148  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
1149  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1150    The option letters are
1151  .sp  .sp
1152    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1153    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 918  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1161  PCRE_MULTILINE while unsetting PCRE_DOTA
1161  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
1162  unset.  unset.
1163  .P  .P
1164  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
1165  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
1166  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1167  the global options (and it will therefore show up in data extracted by the  .P
1168  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1169    subpattern parentheses), the change applies to the remainder of the pattern
1170    that follows. If the change is placed right at the start of a pattern, PCRE
1171    extracts it into the global options (and it will therefore show up in data
1172    extracted by the \fBpcre_fullinfo()\fP function).
1173  .P  .P
1174  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1175  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
1176  .sp  .sp
1177    (a(?i)b)c    (a(?i)b)c
1178  .sp  .sp
# Line 941  branch is abandoned before the option se Line 1188  branch is abandoned before the option se
1188  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1189  behaviour otherwise.  behaviour otherwise.
1190  .P  .P
1191  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
1192  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
1193  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1194    the application has set or what has been defaulted. Details are given in the
1195    section entitled
1196    .\" HTML <a href="#newlineseq">
1197    .\" </a>
1198    "Newline sequences"
1199    .\"
1200    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1201    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1202    PCRE_UTF8 and the PCRE_UCP options, respectively.
1203  .  .
1204  .  .
1205  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 957  Turning part of a pattern into a subpatt Line 1213  Turning part of a pattern into a subpatt
1213  .sp  .sp
1214    cat(aract|erpillar|)    cat(aract|erpillar|)
1215  .sp  .sp
1216  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1217  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1218  .sp  .sp
1219  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
1220  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
1221  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
1222  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1223  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1224  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1225  .sp  .sp
1226    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1227  .sp  .sp
# Line 998  is reached, an option setting in one bra Line 1253  is reached, an option setting in one bra
1253  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1254  .  .
1255  .  .
1256    .\" HTML <a name="dupsubpatternnumber"></a>
1257  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1258  .rs  .rs
1259  .sp  .sp
1260  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1261  the same numbers for its capturing parentheses. Such a subpattern starts with  the same numbers for its capturing parentheses. Such a subpattern starts with
1262  (?| and is itself a non-capturing subpattern. For example, consider this  (?| and is itself a non-capturing subpattern. For example, consider this
1263  pattern:  pattern:
1264  .sp  .sp
1265    (?|(Sat)ur|(Sun))day    (?|(Sat)ur|(Sun))day
1266  .sp  .sp
1267  Because the two alternatives are inside a (?| group, both sets of capturing  Because the two alternatives are inside a (?| group, both sets of capturing
1268  parentheses are numbered one. Thus, when the pattern matches, you can look  parentheses are numbered one. Thus, when the pattern matches, you can look
1269  at captured substring number one, whichever alternative matched. This construct  at captured substring number one, whichever alternative matched. This construct
1270  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
1271  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1272  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
1273  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
1274  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1275  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.  
1276  .sp  .sp
1277    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1278    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1279    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1280  .sp  .sp
1281  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
1282  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1283    or "defdef":
1284    .sp
1285      /(?|(abc)|(def))\e1/
1286    .sp
1287    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1288    refers to the first one in the pattern with the given number. The following
1289    pattern matches "abcabc" or "defabc":
1290    .sp
1291      /(?|(abc)|(def))(?1)/
1292    .sp
1293    If a
1294    .\" HTML <a href="#conditions">
1295    .\" </a>
1296    condition test
1297    .\"
1298    for a subpattern's having matched refers to a non-unique number, the test is
1299    true if any of the subpatterns of that number have matched.
1300  .P  .P
1301  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1302  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1039  if an expression is modified, the number Line 1311  if an expression is modified, the number
1311  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1312  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
1313  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
1314  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1315    have different names, but PCRE does not.
1316  .P  .P
1317  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
1318  (?'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
1319  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1320  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1321  .\" </a>  .\" </a>
1322  backreferences,  back references,
1323  .\"  .\"
1324  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1325  .\" </a>  .\" </a>
# Line 1066  extracting the name-to-number translatio Line 1339  extracting the name-to-number translatio
1339  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1340  .P  .P
1341  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
1342  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
1343  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
1344  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
1345  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
1346  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
1347    name, and in both cases you want to extract the abbreviation. This pattern
1348    (ignoring the line breaks) does the job:
1349  .sp  .sp
1350    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1351    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1079  abbreviation. This pattern (ignoring the Line 1354  abbreviation. This pattern (ignoring the
1354    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1355  .sp  .sp
1356  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1357  (An alternative way of solving this problem is to use a "branch reset"  (An alternative way of solving this problem is to use a "branch reset"
1358  subpattern, as described in the previous section.)  subpattern, as described in the previous section.)
1359  .P  .P
1360  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1361  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
1362  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1363  make a reference to a non-unique named subpattern from elsewhere in the  .P
1364  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
1365  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1366    used. In the absence of duplicate numbers (see the previous section) this is
1367    the one with the lowest number. If you use a named reference in a condition
1368    test (see the
1369    .\"
1370    .\" HTML <a href="#conditions">
1371    .\" </a>
1372    section about conditions
1373    .\"
1374    below), either to check whether a subpattern has matched, or to check for
1375    recursion, all subpatterns with the same name are tested. If the condition is
1376    true for any one of them, the overall condition is true. This is the same
1377    behaviour as testing by number. For further details of the interfaces for
1378    handling named subpatterns, see the
1379  .\" HREF  .\" HREF
1380  \fBpcreapi\fP  \fBpcreapi\fP
1381  .\"  .\"
1382  documentation.  documentation.
1383    .P
1384    \fBWarning:\fP You cannot use different names to distinguish between two
1385    subpatterns with the same number because PCRE uses only the numbers when
1386    matching. For this reason, an error is given at compile time if different names
1387    are given to subpatterns with the same number. However, you can give the same
1388    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1389  .  .
1390  .  .
1391  .SH REPETITION  .SH REPETITION
# Line 1105  items: Line 1399  items:
1399    the \eC escape sequence    the \eC escape sequence
1400    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1401    the \eR escape sequence    the \eR escape sequence
1402    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1403    a character class    a character class
1404    a back reference (see next section)    a back reference (see next section)
1405    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1406      a recursive or "subroutine" call to a subpattern
1407  .sp  .sp
1408  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1409  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1140  support is available, \eX{3} matches thr Line 1435  support is available, \eX{3} matches thr
1435  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).
1436  .P  .P
1437  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
1438  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1439    subpatterns that are referenced as
1440    .\" HTML <a href="#subpatternsassubroutines">
1441    .\" </a>
1442    subroutines
1443    .\"
1444    from elsewhere in the pattern (but see also the section entitled
1445    .\" HTML <a href="#subdefine">
1446    .\" </a>
1447    "Defining subpatterns for use by reference only"
1448    .\"
1449    below). Items other than subpatterns that have a {0} quantifier are omitted
1450    from the compiled pattern.
1451  .P  .P
1452  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1453  abbreviations:  abbreviations:
# Line 1212  worth setting PCRE_DOTALL in order to ob Line 1519  worth setting PCRE_DOTALL in order to ob
1519  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1520  .P  .P
1521  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1522  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1523  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
1524  succeeds. Consider, for example:  succeeds. Consider, for example:
1525  .sp  .sp
# Line 1287  previous example can be rewritten as Line 1594  previous example can be rewritten as
1594  .sp  .sp
1595    \ed++foo    \ed++foo
1596  .sp  .sp
1597    Note that a possessive quantifier can be used with an entire group, for
1598    example:
1599    .sp
1600      (abc|xyz){2,3}+
1601    .sp
1602  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1603  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1604  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1360  no such problem when named parentheses a Line 1672  no such problem when named parentheses a
1672  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1673  .P  .P
1674  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
1675  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
1676  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1677  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1678  .sp  .sp
1679    (ring), \e1    (ring), \e1
1680    (ring), \eg1    (ring), \eg1
1681    (ring), \eg{1}    (ring), \eg{1}
1682  .sp  .sp
1683  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1684  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1685  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1686    example:
1687  .sp  .sp
1688    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1689  .sp  .sp
1690  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
1691  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.
1692  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
1693  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
1694  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1695  .P  .P
1696  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1697  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1417  after the reference. Line 1730  after the reference.
1730  .P  .P
1731  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
1732  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1733  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1734  .sp  .sp
1735    (a|(bc))\e2    (a|(bc))\e2
1736  .sp  .sp
1737  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
1738  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
1739  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1740  with a digit character, some delimiter must be used to terminate the back  .P
1741  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1742  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1743    If the pattern continues with a digit character, some delimiter must be used to
1744    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1745    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1746  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1747  .\" </a>  .\" </a>
1748  "Comments"  "Comments"
1749  .\"  .\"
1750  below) can be used.  below) can be used.
1751  .P  .
1752    .SS "Recursive back references"
1753    .rs
1754    .sp
1755  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
1756  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.
1757  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1446  to the previous iteration. In order for Line 1765  to the previous iteration. In order for
1765  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
1766  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
1767  minimum of zero.  minimum of zero.
1768    .P
1769    Back references of this type cause the group that they reference to be treated
1770    as an
1771    .\" HTML <a href="#atomicgroup">
1772    .\" </a>
1773    atomic group.
1774    .\"
1775    Once the whole group has been matched, a subsequent matching failure cannot
1776    cause backtracking into the middle of the group.
1777  .  .
1778  .  .
1779  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1499  lookbehind assertion is needed to achiev Line 1827  lookbehind assertion is needed to achiev
1827  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
1828  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
1829  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.
1830    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1831  .  .
1832  .  .
1833  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1523  is permitted, but Line 1852  is permitted, but
1852  .sp  .sp
1853  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1854  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
1855  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
1856  match the same length of string. An assertion such as  length of string. An assertion such as
1857  .sp  .sp
1858    (?<=ab(c|de))    (?<=ab(c|de))
1859  .sp  .sp
1860  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
1861  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
1862    branches:
1863  .sp  .sp
1864    (?<=abc|abde)    (?<=abc|abde)
1865  .sp  .sp
1866  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1867  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1868  .\" </a>  .\" </a>
1869  (see above)  (see above)
1870  .\"  .\"
1871  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
1872  fixed-length.  restriction.
1873  .P  .P
1874  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1875  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 1551  to appear in lookbehind assertions, beca Line 1881  to appear in lookbehind assertions, beca
1881  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
1882  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1883  .P  .P
1884    .\" HTML <a href="#subpatternsassubroutines">
1885    .\" </a>
1886    "Subroutine"
1887    .\"
1888    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1889    as the subpattern matches a fixed-length string.
1890    .\" HTML <a href="#recursion">
1891    .\" </a>
1892    Recursion,
1893    .\"
1894    however, is not supported.
1895    .P
1896  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1897  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
1898  pattern such as  strings. Consider a simple pattern such as
1899  .sp  .sp
1900    abcd$    abcd$
1901  .sp  .sp
# Line 1617  characters that are not "999". Line 1959  characters that are not "999".
1959  .sp  .sp
1960  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1961  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1962  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1963  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1964  .sp  .sp
1965    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1966    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1967  .sp  .sp
1968  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1969  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
1970  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1971    itself contain nested subpatterns of any form, including conditional
1972    subpatterns; the restriction to two alternatives applies only at the level of
1973    the condition. This pattern fragment is an example where the alternatives are
1974    complex:
1975    .sp
1976      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1977    .sp
1978  .P  .P
1979  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
1980  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1634  recursion, a pseudo-condition called DEF Line 1983  recursion, a pseudo-condition called DEF
1983  .rs  .rs
1984  .sp  .sp
1985  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
1986  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1987  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
1988  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1989  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1990  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1991  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1992    section about duplicate subpattern numbers),
1993    .\"
1994    the condition is true if any of them have matched. An alternative notation is
1995    to precede the digits with a plus or minus sign. In this case, the subpattern
1996    number is relative rather than absolute. The most recently opened parentheses
1997    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
1998    loops it can also make sense to refer to subsequent groups. The next
1999    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2000    zero in any of these forms is not used; it provokes a compile-time error.)
2001  .P  .P
2002  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2003  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 1650  three parts for ease of discussion: Line 2008  three parts for ease of discussion:
2008  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2009  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
2010  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
2011  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2012  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,
2013  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
2014  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2015  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 1680  Rewriting the above example to use a nam Line 2038  Rewriting the above example to use a nam
2038  .sp  .sp
2039    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2040  .sp  .sp
2041    If the name used in a condition of this kind is a duplicate, the test is
2042    applied to all subpatterns of the same name, and is true if any one of them has
2043    matched.
2044  .  .
2045  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2046  .rs  .rs
# Line 1691  letter R, for example: Line 2052  letter R, for example:
2052  .sp  .sp
2053    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2054  .sp  .sp
2055  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
2056  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
2057  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2058    applied to all subpatterns of the same name, and is true if any one of them is
2059    the most recent recursion.
2060  .P  .P
2061  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2062  patterns are described below.  .\" HTML <a href="#recursion">
2063    .\" </a>
2064    The syntax for recursive patterns
2065    .\"
2066    is described below.
2067  .  .
2068    .\" HTML <a name="subdefine"></a>
2069  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2070  .rs  .rs
2071  .sp  .sp
# Line 1705  If the condition is the string (DEFINE), Line 2073  If the condition is the string (DEFINE),
2073  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
2074  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2075  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
2076  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
2077  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2078  written like this (ignore whitespace and line breaks):  .\" </a>
2079    "subroutines"
2080    .\"
2081    is described below.) For example, a pattern to match an IPv4 address such as
2082    "192.168.23.245" could be written like this (ignore whitespace and line
2083    breaks):
2084  .sp  .sp
2085    (?(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) )
2086    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1715  written like this (ignore whitespace and Line 2088  written like this (ignore whitespace and
2088  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
2089  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2090  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
2091  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2092  .P  pattern uses references to the named group to match the four dot-separated
2093  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.  
2094  .  .
2095  .SS "Assertion conditions"  .SS "Assertion conditions"
2096  .rs  .rs
# Line 1744  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2115  dd-aaa-dd or dd-dd-dd, where aaa are let
2115  .SH COMMENTS  .SH COMMENTS
2116  .rs  .rs
2117  .sp  .sp
2118  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
2119  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,
2120  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
2121    subpattern name or number. The characters that make up a comment play no part
2122    in the pattern matching.
2123  .P  .P
2124  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
2125  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2126  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2127    this case continues to immediately after the next newline character or
2128    character sequence in the pattern. Which characters are interpreted as newlines
2129    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2130    sequence at the start of the pattern, as described in the section entitled
2131    .\" HTML <a href="#newlines">
2132    .\" </a>
2133    "Newline conventions"
2134    .\"
2135    above. Note that the end of this type of comment is a literal newline sequence
2136    in the pattern; escape sequences that happen to represent a newline do not
2137    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2138    default newline convention is in force:
2139    .sp
2140      abc #comment \en still comment
2141    .sp
2142    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2143    a newline in the pattern. The sequence \en is still literal at this stage, so
2144    it does not terminate the comment. Only an actual character with the code value
2145    0x0a (the default newline) does so.
2146  .  .
2147  .  .
2148  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1776  recursively to the pattern in which it a Line 2168  recursively to the pattern in which it a
2168  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2169  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2170  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2171  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.
2172  .P  .P
2173  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
2174  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,
2175  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
2176    .\" HTML <a href="#subpatternsassubroutines">
2177    .\" </a>
2178    "subroutine"
2179    .\"
2180  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
2181  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2182  .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  
2183  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2184  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2185  .sp  .sp
2186    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2187  .sp  .sp
2188  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2189  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
2190  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2191  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2192    to avoid backtracking into sequences of non-parentheses.
2193  .P  .P
2194  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
2195  pattern, so instead you could use this:  pattern, so instead you could use this:
2196  .sp  .sp
2197    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2198  .sp  .sp
2199  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
2200  them instead of the whole pattern.  them instead of the whole pattern.
2201  .P  .P
2202  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
2203  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
2204  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
2205  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2206  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.  
2207  .P  .P
2208  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2209  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2210  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2211  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2212    .\" </a>
2213    "subroutine"
2214    .\"
2215    calls, as described in the next section.
2216  .P  .P
2217  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2218  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
2219  could rewrite the above example as follows:  could rewrite the above example as follows:
2220  .sp  .sp
2221    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2222  .sp  .sp
2223  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
2224  used.  used.
2225  .P  .P
2226  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2227  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
2228  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
2229  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2230  .sp  .sp
2231    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2232  .sp  .sp
2233  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,
2234  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
2235  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
2236  before failure can be reported.  before failure can be reported.
2237  .P  .P
2238  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
2239  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
2240  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  
2241  .\" HREF  .\" HREF
2242  \fBpcrecallout\fP  \fBpcrecallout\fP
2243  .\"  .\"
# Line 1851  documentation). If the pattern above is Line 2245  documentation). If the pattern above is
2245  .sp  .sp
2246    (ab(cd)ef)    (ab(cd)ef)
2247  .sp  .sp
2248  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
2249  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
2250  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2251    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2252       ^                        ^  .P
2253       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2254  .sp  obtain extra memory to store data during a recursion, which it does by using
2255  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
2256  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.  
2257  .P  .P
2258  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.
2259  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1876  different alternatives for the recursive Line 2267  different alternatives for the recursive
2267  is the actual recursive call.  is the actual recursive call.
2268  .  .
2269  .  .
2270    .\" HTML <a name="recursiondifference"></a>
2271    .SS "Recursion difference from Perl"
2272    .rs
2273    .sp
2274    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2275    treated as an atomic group. That is, once it has matched some of the subject
2276    string, it is never re-entered, even if it contains untried alternatives and
2277    there is a subsequent matching failure. This can be illustrated by the
2278    following pattern, which purports to match a palindromic string that contains
2279    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2280    .sp
2281      ^(.|(.)(?1)\e2)$
2282    .sp
2283    The idea is that it either matches a single character, or two identical
2284    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2285    it does not if the pattern is longer than three characters. Consider the
2286    subject string "abcba":
2287    .P
2288    At the top level, the first character is matched, but as it is not at the end
2289    of the string, the first alternative fails; the second alternative is taken
2290    and the recursion kicks in. The recursive call to subpattern 1 successfully
2291    matches the next character ("b"). (Note that the beginning and end of line
2292    tests are not part of the recursion).
2293    .P
2294    Back at the top level, the next character ("c") is compared with what
2295    subpattern 2 matched, which was "a". This fails. Because the recursion is
2296    treated as an atomic group, there are now no backtracking points, and so the
2297    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2298    try the second alternative.) However, if the pattern is written with the
2299    alternatives in the other order, things are different:
2300    .sp
2301      ^((.)(?1)\e2|.)$
2302    .sp
2303    This time, the recursing alternative is tried first, and continues to recurse
2304    until it runs out of characters, at which point the recursion fails. But this
2305    time we do have another alternative to try at the higher level. That is the big
2306    difference: in the previous case the remaining alternative is at a deeper
2307    recursion level, which PCRE cannot use.
2308    .P
2309    To change the pattern so that it matches all palindromic strings, not just
2310    those with an odd number of characters, it is tempting to change the pattern to
2311    this:
2312    .sp
2313      ^((.)(?1)\e2|.?)$
2314    .sp
2315    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2316    deeper recursion has matched a single character, it cannot be entered again in
2317    order to match an empty string. The solution is to separate the two cases, and
2318    write out the odd and even cases as alternatives at the higher level:
2319    .sp
2320      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2321    .sp
2322    If you want to match typical palindromic phrases, the pattern has to ignore all
2323    non-word characters, which can be done like this:
2324    .sp
2325      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2326    .sp
2327    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2328    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2329    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2330    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2331    more) to match typical phrases, and Perl takes so long that you think it has
2332    gone into a loop.
2333    .P
2334    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2335    string does not start with a palindrome that is shorter than the entire string.
2336    For example, although "abcba" is correctly matched, if the subject is "ababa",
2337    PCRE finds the palindrome "aba" at the start, then fails at top level because
2338    the end of the string does not follow. Once again, it cannot jump back into the
2339    recursion to try other alternatives, so the entire match fails.
2340    .
2341    .
2342  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2343  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2344  .rs  .rs
# Line 1902  matches "sense and sensibility" and "res Line 2365  matches "sense and sensibility" and "res
2365  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
2366  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2367  .P  .P
2368  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2369  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
2370  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
2371  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2372    call revert to their previous values afterwards.
2373  .P  .P
2374  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2375  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 1917  It matches "abcabc". It does not match " Line 2381  It matches "abcabc". It does not match "
2381  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2382  .  .
2383  .  .
2384    .\" HTML <a name="onigurumasubroutines"></a>
2385    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2386    .rs
2387    .sp
2388    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2389    a number enclosed either in angle brackets or single quotes, is an alternative
2390    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2391    are two of the examples used above, rewritten using this syntax:
2392    .sp
2393      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2394      (sens|respons)e and \eg'1'ibility
2395    .sp
2396    PCRE supports an extension to Oniguruma: if a number is preceded by a
2397    plus or a minus sign it is taken as a relative reference. For example:
2398    .sp
2399      (abc)(?i:\eg<-1>)
2400    .sp
2401    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2402    synonymous. The former is a back reference; the latter is a subroutine call.
2403    .
2404    .
2405  .SH CALLOUTS  .SH CALLOUTS
2406  .rs  .rs
2407  .sp  .sp
# Line 1953  description of the interface to the call Line 2438  description of the interface to the call
2438  documentation.  documentation.
2439  .  .
2440  .  .
2441    .\" HTML <a name="backtrackcontrol"></a>
2442    .SH "BACKTRACKING CONTROL"
2443    .rs
2444    .sp
2445    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2446    are described in the Perl documentation as "experimental and subject to change
2447    or removal in a future version of Perl". It goes on to say: "Their usage in
2448    production code should be noted to avoid problems during upgrades." The same
2449    remarks apply to the PCRE features described in this section.
2450    .P
2451    Since these verbs are specifically related to backtracking, most of them can be
2452    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2453    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2454    failing negative assertion, they cause an error if encountered by
2455    \fBpcre_dfa_exec()\fP.
2456    .P
2457    If any of these verbs are used in an assertion or subroutine subpattern
2458    (including recursive subpatterns), their effect is confined to that subpattern;
2459    it does not extend to the surrounding pattern. Note that such subpatterns are
2460    processed as anchored at the point where they are tested.
2461    .P
2462    The new verbs make use of what was previously invalid syntax: an opening
2463    parenthesis followed by an asterisk. They are generally of the form
2464    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2465    depending on whether or not an argument is present. An name is a sequence of
2466    letters, digits, and underscores. If the name is empty, that is, if the closing
2467    parenthesis immediately follows the colon, the effect is as if the colon were
2468    not there. Any number of these verbs may occur in a pattern.
2469    .P
2470    PCRE contains some optimizations that are used to speed up matching by running
2471    some checks at the start of each match attempt. For example, it may know the
2472    minimum length of matching subject, or that a particular character must be
2473    present. When one of these optimizations suppresses the running of a match, any
2474    included backtracking verbs will not, of course, be processed. You can suppress
2475    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2476    when calling \fBpcre_exec()\fP.
2477    .
2478    .
2479    .SS "Verbs that act immediately"
2480    .rs
2481    .sp
2482    The following verbs act as soon as they are encountered. They may not be
2483    followed by a name.
2484    .sp
2485       (*ACCEPT)
2486    .sp
2487    This verb causes the match to end successfully, skipping the remainder of the
2488    pattern. When inside a recursion, only the innermost pattern is ended
2489    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2490    captured. (This feature was added to PCRE at release 8.00.) For example:
2491    .sp
2492      A((?:A|B(*ACCEPT)|C)D)
2493    .sp
2494    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2495    the outer parentheses.
2496    .sp
2497      (*FAIL) or (*F)
2498    .sp
2499    This verb causes the match to fail, forcing backtracking to occur. It is
2500    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2501    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2502    Perl features that are not present in PCRE. The nearest equivalent is the
2503    callout feature, as for example in this pattern:
2504    .sp
2505      a+(?C)(*FAIL)
2506    .sp
2507    A match with the string "aaaa" always fails, but the callout is taken before
2508    each backtrack happens (in this example, 10 times).
2509    .
2510    .
2511    .SS "Recording which path was taken"
2512    .rs
2513    .sp
2514    There is one verb whose main purpose is to track how a match was arrived at,
2515    though it also has a secondary use in conjunction with advancing the match
2516    starting point (see (*SKIP) below).
2517    .sp
2518      (*MARK:NAME) or (*:NAME)
2519    .sp
2520    A name is always required with this verb. There may be as many instances of
2521    (*MARK) as you like in a pattern, and their names do not have to be unique.
2522    .P
2523    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2524    to the caller via the \fIpcre_extra\fP data structure, as described in the
2525    .\" HTML <a href="pcreapi.html#extradata">
2526    .\" </a>
2527    section on \fIpcre_extra\fP
2528    .\"
2529    in the
2530    .\" HREF
2531    \fBpcreapi\fP
2532    .\"
2533    documentation. No data is returned for a partial match. Here is an example of
2534    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2535    outputting of (*MARK) data:
2536    .sp
2537      /X(*MARK:A)Y|X(*MARK:B)Z/K
2538      XY
2539       0: XY
2540      MK: A
2541      XZ
2542       0: XZ
2543      MK: B
2544    .sp
2545    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2546    indicates which of the two alternatives matched. This is a more efficient way
2547    of obtaining this information than putting each alternative in its own
2548    capturing parentheses.
2549    .P
2550    A name may also be returned after a failed match if the final path through the
2551    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2552    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2553    starting point for matching is advanced, the final check is often with an empty
2554    string, causing a failure before (*MARK) is reached. For example:
2555    .sp
2556      /X(*MARK:A)Y|X(*MARK:B)Z/K
2557      XP
2558      No match
2559    .sp
2560    There are three potential starting points for this match (starting with X,
2561    starting with P, and with an empty string). If the pattern is anchored, the
2562    result is different:
2563    .sp
2564      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2565      XP
2566      No match, mark = B
2567    .sp
2568    PCRE's start-of-match optimizations can also interfere with this. For example,
2569    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2570    subject length for a match, a shorter subject will not be scanned at all.
2571    .P
2572    Note that similar anomalies (though different in detail) exist in Perl, no
2573    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2574    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2575    .
2576    .
2577    .SS "Verbs that act after backtracking"
2578    .rs
2579    .sp
2580    The following verbs do nothing when they are encountered. Matching continues
2581    with what follows, but if there is no subsequent match, causing a backtrack to
2582    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2583    the verb. However, when one of these verbs appears inside an atomic group, its
2584    effect is confined to that group, because once the group has been matched,
2585    there is never any backtracking into it. In this situation, backtracking can
2586    "jump back" to the left of the entire atomic group. (Remember also, as stated
2587    above, that this localization also applies in subroutine calls and assertions.)
2588    .P
2589    These verbs differ in exactly what kind of failure occurs when backtracking
2590    reaches them.
2591    .sp
2592      (*COMMIT)
2593    .sp
2594    This verb, which may not be followed by a name, causes the whole match to fail
2595    outright if the rest of the pattern does not match. Even if the pattern is
2596    unanchored, no further attempts to find a match by advancing the starting point
2597    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2598    finding a match at the current starting point, or not at all. For example:
2599    .sp
2600      a+(*COMMIT)b
2601    .sp
2602    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2603    dynamic anchor, or "I've started, so I must finish." The name of the most
2604    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2605    match failure.
2606    .P
2607    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2608    unless PCRE's start-of-match optimizations are turned off, as shown in this
2609    \fBpcretest\fP example:
2610    .sp
2611      /(*COMMIT)abc/
2612      xyzabc
2613       0: abc
2614      xyzabc\eY
2615      No match
2616    .sp
2617    PCRE knows that any match must start with "a", so the optimization skips along
2618    the subject to "a" before running the first match attempt, which succeeds. When
2619    the optimization is disabled by the \eY escape in the second subject, the match
2620    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2621    starting points.
2622    .sp
2623      (*PRUNE) or (*PRUNE:NAME)
2624    .sp
2625    This verb causes the match to fail at the current starting position in the
2626    subject if the rest of the pattern does not match. If the pattern is
2627    unanchored, the normal "bumpalong" advance to the next starting character then
2628    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2629    reached, or when matching to the right of (*PRUNE), but if there is no match to
2630    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2631    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2632    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2633    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2634    match fails completely; the name is passed back if this is the final attempt.
2635    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2636    pattern (*PRUNE) has the same effect as (*COMMIT).
2637    .sp
2638      (*SKIP)
2639    .sp
2640    This verb, when given without a name, is like (*PRUNE), except that if the
2641    pattern is unanchored, the "bumpalong" advance is not to the next character,
2642    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2643    signifies that whatever text was matched leading up to it cannot be part of a
2644    successful match. Consider:
2645    .sp
2646      a+(*SKIP)b
2647    .sp
2648    If the subject is "aaaac...", after the first match attempt fails (starting at
2649    the first character in the string), the starting point skips on to start the
2650    next attempt at "c". Note that a possessive quantifer does not have the same
2651    effect as this example; although it would suppress backtracking during the
2652    first match attempt, the second attempt would start at the second character
2653    instead of skipping on to "c".
2654    .sp
2655      (*SKIP:NAME)
2656    .sp
2657    When (*SKIP) has an associated name, its behaviour is modified. If the
2658    following pattern fails to match, the previous path through the pattern is
2659    searched for the most recent (*MARK) that has the same name. If one is found,
2660    the "bumpalong" advance is to the subject position that corresponds to that
2661    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2662    matching name is found, normal "bumpalong" of one character happens (the
2663    (*SKIP) is ignored).
2664    .sp
2665      (*THEN) or (*THEN:NAME)
2666    .sp
2667    This verb causes a skip to the next alternation in the innermost enclosing
2668    group if the rest of the pattern does not match. That is, it cancels pending
2669    backtracking, but only within the current alternation. Its name comes from the
2670    observation that it can be used for a pattern-based if-then-else block:
2671    .sp
2672      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2673    .sp
2674    If the COND1 pattern matches, FOO is tried (and possibly further items after
2675    the end of the group if FOO succeeds); on failure the matcher skips to the
2676    second alternative and tries COND2, without backtracking into COND1. The
2677    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2678    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2679    like (*PRUNE).
2680    .
2681    .P
2682    The above verbs provide four different "strengths" of control when subsequent
2683    matching fails. (*THEN) is the weakest, carrying on the match at the next
2684    alternation. (*PRUNE) comes next, failing the match at the current starting
2685    position, but allowing an advance to the next character (for an unanchored
2686    pattern). (*SKIP) is similar, except that the advance may be more than one
2687    character. (*COMMIT) is the strongest, causing the entire match to fail.
2688    .P
2689    If more than one is present in a pattern, the "stongest" one wins. For example,
2690    consider this pattern, where A, B, etc. are complex pattern fragments:
2691    .sp
2692      (A(*COMMIT)B(*THEN)C|D)
2693    .sp
2694    Once A has matched, PCRE is committed to this match, at the current starting
2695    position. If subsequently B matches, but C does not, the normal (*THEN) action
2696    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2697    overrides.
2698    .
2699    .
2700  .SH "SEE ALSO"  .SH "SEE ALSO"
2701  .rs  .rs
2702  .sp  .sp
2703  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2704    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2705  .  .
2706  .  .
2707  .SH AUTHOR  .SH AUTHOR
# Line 1973  Cambridge CB2 3QH, England. Line 2718  Cambridge CB2 3QH, England.
2718  .rs  .rs
2719  .sp  .sp
2720  .nf  .nf
2721  Last updated: 13 June 2007  Last updated: 17 November 2010
2722  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2723  .fi  .fi

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