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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,
58  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59  Perl-compatible. Some of the features discussed below are not available when  Perl-compatible. Some of the features discussed below are not available when
60  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61  alternative function, and how it differs from the normal function, are  alternative function, and how it differs from the normal function, are
62  discussed in the  discussed in the
# 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 103  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 113  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 132  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored.
219  .  .
220  .  .
221  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 141  The \eQ...\eE sequence is recognized bot Line 225  The \eQ...\eE sequence is recognized bot
225  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
226  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
227  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
228  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
229  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
230  .sp  .sp
231    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
232    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
233    \ee        escape (hex 1B)    \ee        escape (hex 1B)
234    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
235    \en        newline (hex 0A)    \en        linefeed (hex 0A)
236    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
237    \et        tab (hex 09)    \et        tab (hex 09)
238    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
239    \exhh      character with hex code hh    \exhh      character with hex code hh
240    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
241  .sp  .sp
242  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
243  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
244  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
245  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
246    than 127, a compile-time error occurs. This locks out non-ASCII characters in
247    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
248    values are valid. A lower case letter is converted to upper case, and then the
249    0xc0 bits are flipped.)
250  .P  .P
251  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
252  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
253  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
254  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
255  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
256  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
257  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
258  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
259    there is no terminating }, this form of escape is not recognized. Instead, the
260    initial \ex will be interpreted as a basic hexadecimal escape, with no
261    following digits, giving a character whose value is zero.
262  .P  .P
263  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
264  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 317  zero, because no more than three octal d
317  .P  .P
318  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
319  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
320  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
321  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
322  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
323  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
324  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
325  .  .
326  .  .
327  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
328  .rs  .rs
329  .sp  .sp
330  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
331  in braces, is an absolute or relative back reference. Back references are  enclosed in braces, is an absolute or relative back reference. A named back
332  discussed  reference can be coded as \eg{name}. Back references are discussed
333  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
334  .\" </a>  .\" </a>
335  later,  later,
# Line 254  parenthesized subpatterns. Line 341  parenthesized subpatterns.
341  .\"  .\"
342  .  .
343  .  .
344    .SS "Absolute and relative subroutine calls"
345    .rs
346    .sp
347    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
348    a number enclosed either in angle brackets or single quotes, is an alternative
349    syntax for referencing a subpattern as a "subroutine". Details are discussed
350    .\" HTML <a href="#onigurumasubroutines">
351    .\" </a>
352    later.
353    .\"
354    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
355    synonymous. The former is a back reference; the latter is a
356    .\" HTML <a href="#subpatternsassubroutines">
357    .\" </a>
358    subroutine
359    .\"
360    call.
361    .
362    .
363    .\" HTML <a name="genericchartypes"></a>
364  .SS "Generic character types"  .SS "Generic character types"
365  .rs  .rs
366  .sp  .sp
367  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
368  .sp  .sp
369    \ed     any decimal digit    \ed     any decimal digit
370    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
371      \eh     any horizontal whitespace character
372      \eH     any character that is not a horizontal whitespace character
373    \es     any whitespace character    \es     any whitespace character
374    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
375      \ev     any vertical whitespace character
376      \eV     any character that is not a vertical whitespace character
377    \ew     any "word" character    \ew     any "word" character
378    \eW     any "non-word" character    \eW     any "non-word" character
379  .sp  .sp
380  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
381  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
382    .\" HTML <a href="#fullstopdot">
383    .\" </a>
384    the "." metacharacter
385    .\"
386    when PCRE_DOTALL is not set.
387  .P  .P
388  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
389    of characters into two disjoint sets. Any given character matches one, and only
390    one, of each pair. The sequences can appear both inside and outside character
391  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
392  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
393  there is no character to match.  there is no character to match.
394  .P  .P
395  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
396  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
397  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
398  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
399  does.)  does.
400  .P  .P
401  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
402  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
403  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
404  place (see  place (see
405  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 295  in the Line 412  in the
412  .\"  .\"
413  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
414  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
415  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
416    Unicode is discouraged.
417  .P  .P
418  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in UTF-8 mode, characters with values greater than 128 never match
419  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
420  character property support is available. The use of locales with Unicode is  their original meanings from before UTF-8 support was available, mainly for
421  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
422    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
423    properties are used to determine character types, as follows:
424    .sp
425      \ed  any character that \ep{Nd} matches (decimal digit)
426      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
427      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
428    .sp
429    The upper case escapes match the inverse sets of characters. Note that \ed
430    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
431    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
432    \eB because they are defined in terms of \ew and \eW. Matching these sequences
433    is noticeably slower when PCRE_UCP is set.
434    .P
435    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
436    release 5.10. In contrast to the other sequences, which match only ASCII
437    characters by default, these always match certain high-valued codepoints in
438    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
439    are:
440    .sp
441      U+0009     Horizontal tab
442      U+0020     Space
443      U+00A0     Non-break space
444      U+1680     Ogham space mark
445      U+180E     Mongolian vowel separator
446      U+2000     En quad
447      U+2001     Em quad
448      U+2002     En space
449      U+2003     Em space
450      U+2004     Three-per-em space
451      U+2005     Four-per-em space
452      U+2006     Six-per-em space
453      U+2007     Figure space
454      U+2008     Punctuation space
455      U+2009     Thin space
456      U+200A     Hair space
457      U+202F     Narrow no-break space
458      U+205F     Medium mathematical space
459      U+3000     Ideographic space
460    .sp
461    The vertical space characters are:
462    .sp
463      U+000A     Linefeed
464      U+000B     Vertical tab
465      U+000C     Formfeed
466      U+000D     Carriage return
467      U+0085     Next line
468      U+2028     Line separator
469      U+2029     Paragraph separator
470  .  .
471  .  .
472    .\" HTML <a name="newlineseq"></a>
473  .SS "Newline sequences"  .SS "Newline sequences"
474  .rs  .rs
475  .sp  .sp
476  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
477  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 the following:  
478  .sp  .sp
479    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
480  .sp  .sp
# Line 328  are added: LS (line separator, U+2028) a Line 494  are added: LS (line separator, U+2028) a
494  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
495  recognized.  recognized.
496  .P  .P
497  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
498    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
499    either at compile time or when the pattern is matched. (BSR is an abbrevation
500    for "backslash R".) This can be made the default when PCRE is built; if this is
501    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
502    It is also possible to specify these settings by starting a pattern string with
503    one of the following sequences:
504    .sp
505      (*BSR_ANYCRLF)   CR, LF, or CRLF only
506      (*BSR_UNICODE)   any Unicode newline sequence
507    .sp
508    These override the default and the options given to \fBpcre_compile()\fP or
509    \fBpcre_compile2()\fP, but they can be overridden by options given to
510    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
511    which are not Perl-compatible, are recognized only at the very start of a
512    pattern, and that they must be in upper case. If more than one of them is
513    present, the last one is used. They can be combined with a change of newline
514    convention; for example, a pattern can start with:
515    .sp
516      (*ANY)(*BSR_ANYCRLF)
517    .sp
518    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
519    a character class, \eR is treated as an unrecognized escape sequence, and so
520    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
521  .  .
522  .  .
523  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 336  Inside a character class, \eR matches th Line 525  Inside a character class, \eR matches th
525  .rs  .rs
526  .sp  .sp
527  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
528  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
529  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
530    characters whose codepoints are less than 256, but they do work in this mode.
531    The extra escape sequences are:
532  .sp  .sp
533    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
534    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
535    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
536  .sp  .sp
537  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
538  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
539  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
540  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
541  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
542    .\" </a>
543    next section).
544    .\"
545    Other Perl properties such as "InMusicalSymbols" are not currently supported by
546    PCRE. Note that \eP{Any} does not match any characters, so always causes a
547    match failure.
548  .P  .P
549  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
550  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 361  Those that are not part of an identified Line 558  Those that are not part of an identified
558  .P  .P
559  Arabic,  Arabic,
560  Armenian,  Armenian,
561    Avestan,
562  Balinese,  Balinese,
563    Bamum,
564  Bengali,  Bengali,
565  Bopomofo,  Bopomofo,
566  Braille,  Braille,
567  Buginese,  Buginese,
568  Buhid,  Buhid,
569  Canadian_Aboriginal,  Canadian_Aboriginal,
570    Carian,
571    Cham,
572  Cherokee,  Cherokee,
573  Common,  Common,
574  Coptic,  Coptic,
# Line 376  Cypriot, Line 577  Cypriot,
577  Cyrillic,  Cyrillic,
578  Deseret,  Deseret,
579  Devanagari,  Devanagari,
580    Egyptian_Hieroglyphs,
581  Ethiopic,  Ethiopic,
582  Georgian,  Georgian,
583  Glagolitic,  Glagolitic,
# Line 388  Hangul, Line 590  Hangul,
590  Hanunoo,  Hanunoo,
591  Hebrew,  Hebrew,
592  Hiragana,  Hiragana,
593    Imperial_Aramaic,
594  Inherited,  Inherited,
595    Inscriptional_Pahlavi,
596    Inscriptional_Parthian,
597    Javanese,
598    Kaithi,
599  Kannada,  Kannada,
600  Katakana,  Katakana,
601    Kayah_Li,
602  Kharoshthi,  Kharoshthi,
603  Khmer,  Khmer,
604  Lao,  Lao,
605  Latin,  Latin,
606    Lepcha,
607  Limbu,  Limbu,
608  Linear_B,  Linear_B,
609    Lisu,
610    Lycian,
611    Lydian,
612  Malayalam,  Malayalam,
613    Meetei_Mayek,
614  Mongolian,  Mongolian,
615  Myanmar,  Myanmar,
616  New_Tai_Lue,  New_Tai_Lue,
# Line 405  Nko, Line 618  Nko,
618  Ogham,  Ogham,
619  Old_Italic,  Old_Italic,
620  Old_Persian,  Old_Persian,
621    Old_South_Arabian,
622    Old_Turkic,
623    Ol_Chiki,
624  Oriya,  Oriya,
625  Osmanya,  Osmanya,
626  Phags_Pa,  Phags_Pa,
627  Phoenician,  Phoenician,
628    Rejang,
629  Runic,  Runic,
630    Samaritan,
631    Saurashtra,
632  Shavian,  Shavian,
633  Sinhala,  Sinhala,
634    Sundanese,
635  Syloti_Nagri,  Syloti_Nagri,
636  Syriac,  Syriac,
637  Tagalog,  Tagalog,
638  Tagbanwa,  Tagbanwa,
639  Tai_Le,  Tai_Le,
640    Tai_Tham,
641    Tai_Viet,
642  Tamil,  Tamil,
643  Telugu,  Telugu,
644  Thaana,  Thaana,
# Line 424  Thai, Line 646  Thai,
646  Tibetan,  Tibetan,
647  Tifinagh,  Tifinagh,
648  Ugaritic,  Ugaritic,
649    Vai,
650  Yi.  Yi.
651  .P  .P
652  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
653  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
654  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
655  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
656  .P  .P
657  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
658  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 489  The special property L& is also supporte Line 712  The special property L& is also supporte
712  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
713  a modifier or "other".  a modifier or "other".
714  .P  .P
715  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
716    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
717    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
718    (see the discussion of PCRE_NO_UTF8_CHECK in the
719    .\" HREF
720    \fBpcreapi\fP
721    .\"
722    page). Perl does not support the Cs property.
723    .P
724    The long synonyms for property names that Perl supports (such as \ep{Letter})
725  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
726  properties with "Is".  properties with "Is".
727  .P  .P
# Line 513  atomic group Line 745  atomic group
745  (see below).  (see below).
746  .\"  .\"
747  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
748  preceding character.  preceding character. None of them have codepoints less than 256, so in
749    non-UTF-8 mode \eX matches any one character.
750  .P  .P
751  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
752  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
753  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
754  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
755    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
756    (*UCP).
757    .
758    .
759    .\" HTML <a name="extraprops"></a>
760    .SS PCRE's additional properties
761    .rs
762    .sp
763    As well as the standard Unicode properties described in the previous
764    section, PCRE supports four more that make it possible to convert traditional
765    escape sequences such as \ew and \es and POSIX character classes to use Unicode
766    properties. PCRE uses these non-standard, non-Perl properties internally when
767    PCRE_UCP is set. They are:
768    .sp
769      Xan   Any alphanumeric character
770      Xps   Any POSIX space character
771      Xsp   Any Perl space character
772      Xwd   Any Perl "word" character
773    .sp
774    Xan matches characters that have either the L (letter) or the N (number)
775    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
776    carriage return, and any other character that has the Z (separator) property.
777    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
778    same characters as Xan, plus underscore.
779  .  .
780  .  .
781  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
782  .SS "Resetting the match start"  .SS "Resetting the match start"
783  .rs  .rs
784  .sp  .sp
785  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
786  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
787  .sp  .sp
788    foo\eKbar    foo\eKbar
789  .sp  .sp
790  matches "foobar", but reports that it has matched "bar". This feature is  matches "foobar", but reports that it has matched "bar". This feature is
791  similar to a lookbehind assertion  similar to a lookbehind assertion
792  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
793  .\" </a>  .\" </a>
794  (described below).  (described below).
795  .\"  .\"
796  However, in this case, the part of the subject before the real match does not  However, in this case, the part of the subject before the real match does not
797  have to be of fixed length, as lookbehind assertions do. The use of \eK does  have to be of fixed length, as lookbehind assertions do. The use of \eK does
798  not interfere with the setting of  not interfere with the setting of
799  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
800  .\" </a>  .\" </a>
801  captured substrings.  captured substrings.
802  .\"  .\"
803  For example, when the pattern  For example, when the pattern
804  .sp  .sp
805    (foo)\eKbar    (foo)\eKbar
806  .sp  .sp
807  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
808    .P
809    Perl documents that the use of \eK within assertions is "not well defined". In
810    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
811    ignored in negative assertions.
812  .  .
813  .  .
814  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 573  The backslashed assertions are: Line 833  The backslashed assertions are:
833    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
834    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
835  .sp  .sp
836  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
837  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
838    default it matches the corresponding literal character (for example, \eB
839    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
840    escape sequence" error is generated instead.
841  .P  .P
842  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
843  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
844  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
845  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
846    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
847    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
848    of word" or "end of word" metasequence. However, whatever follows \eb normally
849    determines which it is. For example, the fragment \eba matches "a" at the start
850    of a word.
851  .P  .P
852  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
853  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 663  end of the subject in both modes, and if Line 931  end of the subject in both modes, and if
931  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
932  .  .
933  .  .
934  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
935    .SH "FULL STOP (PERIOD, DOT) AND \eN"
936  .rs  .rs
937  .sp  .sp
938  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 685  to match it. Line 954  to match it.
954  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
955  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
956  special meaning in a character class.  special meaning in a character class.
957    .P
958    The escape sequence \eN behaves like a dot, except that it is not affected by
959    the PCRE_DOTALL option. In other words, it matches any character except one
960    that signifies the end of a line.
961  .  .
962  .  .
963  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 693  special meaning in a character class. Line 966  special meaning in a character class.
966  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
967  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
968  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
969  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
970  what remains in the string may be a malformed UTF-8 string. For this reason,  rest of the string may start with a malformed UTF-8 character. For this reason,
971  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
972  .P  .P
973  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 711  the lookbehind. Line 984  the lookbehind.
984  .rs  .rs
985  .sp  .sp
986  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
987  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
988  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
989  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
990  escaped with a backslash.  a member of the class, it should be the first data character in the class
991    (after an initial circumflex, if present) or escaped with a backslash.
992  .P  .P
993  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
994  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
995  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
996  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
997  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
998  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 728  For example, the character class [aeiou] Line 1002  For example, the character class [aeiou]
1002  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1003  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1004  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1005  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1006  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1007  string.  string.
1008  .P  .P
# Line 742  caseful version would. In UTF-8 mode, PC Line 1016  caseful version would. In UTF-8 mode, PC
1016  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1017  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1018  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1019  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1020  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
1021  UTF-8 support.  with UTF-8 support.
1022  .P  .P
1023  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1024  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 778  characters in both cases. In UTF-8 mode, Line 1052  characters in both cases. In UTF-8 mode,
1052  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1053  property support.  property support.
1054  .P  .P
1055  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1056  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1057  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1058  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1059  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1060  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1061    .\" HTML <a href="#genericchartypes">
1062    .\" </a>
1063    "Generic character types"
1064    .\"
1065    above. The escape sequence \eb has a different meaning inside a character
1066    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1067    are not special inside a character class. Like any other unrecognized escape
1068    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1069    default, but cause an error if the PCRE_EXTRA option is set.
1070    .P
1071    A circumflex can conveniently be used with the upper case character types to
1072    specify a more restricted set of characters than the matching lower case type.
1073    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1074    whereas [\ew] includes underscore. A positive character class should be read as
1075    "something OR something OR ..." and a negative class as "NOT something AND NOT
1076    something AND NOT ...".
1077  .P  .P
1078  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1079  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 803  this notation. For example, Line 1093  this notation. For example,
1093    [01[:alpha:]%]    [01[:alpha:]%]
1094  .sp  .sp
1095  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1096  are  are:
1097  .sp  .sp
1098    alnum    letters and digits    alnum    letters and digits
1099    alpha    letters    alpha    letters
# Line 814  are Line 1104  are
1104    graph    printing characters, excluding space    graph    printing characters, excluding space
1105    lower    lower case letters    lower    lower case letters
1106    print    printing characters, including space    print    printing characters, including space
1107    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1108    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1109    upper    upper case letters    upper    upper case letters
1110    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 835  matches "1", "2", or any non-digit. PCRE Line 1125  matches "1", "2", or any non-digit. PCRE
1125  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
1126  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1127  .P  .P
1128  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
1129  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1130    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1131    character properties are used. This is achieved by replacing the POSIX classes
1132    by other sequences, as follows:
1133    .sp
1134      [:alnum:]  becomes  \ep{Xan}
1135      [:alpha:]  becomes  \ep{L}
1136      [:blank:]  becomes  \eh
1137      [:digit:]  becomes  \ep{Nd}
1138      [:lower:]  becomes  \ep{Ll}
1139      [:space:]  becomes  \ep{Xps}
1140      [:upper:]  becomes  \ep{Lu}
1141      [:word:]   becomes  \ep{Xwd}
1142    .sp
1143    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1144    classes are unchanged, and match only characters with code points less than
1145    128.
1146  .  .
1147  .  .
1148  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 863  alternative in the subpattern. Line 1169  alternative in the subpattern.
1169  .rs  .rs
1170  .sp  .sp
1171  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1172  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
1173  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1174    The option letters are
1175  .sp  .sp
1176    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1177    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 878  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1185  PCRE_MULTILINE while unsetting PCRE_DOTA
1185  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
1186  unset.  unset.
1187  .P  .P
1188  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
1189  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
1190  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1191  the global options (and it will therefore show up in data extracted by the  .P
1192  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1193    subpattern parentheses), the change applies to the remainder of the pattern
1194    that follows. If the change is placed right at the start of a pattern, PCRE
1195    extracts it into the global options (and it will therefore show up in data
1196    extracted by the \fBpcre_fullinfo()\fP function).
1197  .P  .P
1198  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1199  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
1200  .sp  .sp
1201    (a(?i)b)c    (a(?i)b)c
1202  .sp  .sp
# Line 901  branch is abandoned before the option se Line 1212  branch is abandoned before the option se
1212  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1213  behaviour otherwise.  behaviour otherwise.
1214  .P  .P
1215  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
1216  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
1217  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1218    the application has set or what has been defaulted. Details are given in the
1219    section entitled
1220    .\" HTML <a href="#newlineseq">
1221    .\" </a>
1222    "Newline sequences"
1223    .\"
1224    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1225    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1226    PCRE_UTF8 and the PCRE_UCP options, respectively.
1227  .  .
1228  .  .
1229  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 917  Turning part of a pattern into a subpatt Line 1237  Turning part of a pattern into a subpatt
1237  .sp  .sp
1238    cat(aract|erpillar|)    cat(aract|erpillar|)
1239  .sp  .sp
1240  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1241  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1242  .sp  .sp
1243  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
1244  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
1245  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
1246  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1247  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1248  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1249  .sp  .sp
1250    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1251  .sp  .sp
# Line 958  is reached, an option setting in one bra Line 1277  is reached, an option setting in one bra
1277  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1278  .  .
1279  .  .
1280    .\" HTML <a name="dupsubpatternnumber"></a>
1281    .SH "DUPLICATE SUBPATTERN NUMBERS"
1282    .rs
1283    .sp
1284    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1285    the same numbers for its capturing parentheses. Such a subpattern starts with
1286    (?| and is itself a non-capturing subpattern. For example, consider this
1287    pattern:
1288    .sp
1289      (?|(Sat)ur|(Sun))day
1290    .sp
1291    Because the two alternatives are inside a (?| group, both sets of capturing
1292    parentheses are numbered one. Thus, when the pattern matches, you can look
1293    at captured substring number one, whichever alternative matched. This construct
1294    is useful when you want to capture part, but not all, of one of a number of
1295    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1296    number is reset at the start of each branch. The numbers of any capturing
1297    parentheses that follow the subpattern start after the highest number used in
1298    any branch. The following example is taken from the Perl documentation. The
1299    numbers underneath show in which buffer the captured content will be stored.
1300    .sp
1301      # before  ---------------branch-reset----------- after
1302      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1303      # 1            2         2  3        2     3     4
1304    .sp
1305    A back reference to a numbered subpattern uses the most recent value that is
1306    set for that number by any subpattern. The following pattern matches "abcabc"
1307    or "defdef":
1308    .sp
1309      /(?|(abc)|(def))\e1/
1310    .sp
1311    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1312    refers to the first one in the pattern with the given number. The following
1313    pattern matches "abcabc" or "defabc":
1314    .sp
1315      /(?|(abc)|(def))(?1)/
1316    .sp
1317    If a
1318    .\" HTML <a href="#conditions">
1319    .\" </a>
1320    condition test
1321    .\"
1322    for a subpattern's having matched refers to a non-unique number, the test is
1323    true if any of the subpatterns of that number have matched.
1324    .P
1325    An alternative approach to using this "branch reset" feature is to use
1326    duplicate named subpatterns, as described in the next section.
1327    .
1328    .
1329  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1330  .rs  .rs
1331  .sp  .sp
# Line 967  if an expression is modified, the number Line 1335  if an expression is modified, the number
1335  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1336  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
1337  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
1338  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1339    have different names, but PCRE does not.
1340  .P  .P
1341  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
1342  (?'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
1343  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1344  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1345  .\" </a>  .\" </a>
1346  backreferences,  back references,
1347  .\"  .\"
1348  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1349  .\" </a>  .\" </a>
# Line 994  extracting the name-to-number translatio Line 1363  extracting the name-to-number translatio
1363  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1364  .P  .P
1365  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
1366  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
1367  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
1368  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
1369  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
1370  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
1371    name, and in both cases you want to extract the abbreviation. This pattern
1372    (ignoring the line breaks) does the job:
1373  .sp  .sp
1374    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1375    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1007  abbreviation. This pattern (ignoring the Line 1378  abbreviation. This pattern (ignoring the
1378    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1379  .sp  .sp
1380  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.
1381    (An alternative way of solving this problem is to use a "branch reset"
1382    subpattern, as described in the previous section.)
1383    .P
1384  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1385  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
1386  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1387  make a reference to a non-unique named subpattern from elsewhere in the  .P
1388  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
1389  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1390    used. In the absence of duplicate numbers (see the previous section) this is
1391    the one with the lowest number. If you use a named reference in a condition
1392    test (see the
1393    .\"
1394    .\" HTML <a href="#conditions">
1395    .\" </a>
1396    section about conditions
1397    .\"
1398    below), either to check whether a subpattern has matched, or to check for
1399    recursion, all subpatterns with the same name are tested. If the condition is
1400    true for any one of them, the overall condition is true. This is the same
1401    behaviour as testing by number. For further details of the interfaces for
1402    handling named subpatterns, see the
1403  .\" HREF  .\" HREF
1404  \fBpcreapi\fP  \fBpcreapi\fP
1405  .\"  .\"
1406  documentation.  documentation.
1407    .P
1408    \fBWarning:\fP You cannot use different names to distinguish between two
1409    subpatterns with the same number because PCRE uses only the numbers when
1410    matching. For this reason, an error is given at compile time if different names
1411    are given to subpatterns with the same number. However, you can give the same
1412    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1413  .  .
1414  .  .
1415  .SH REPETITION  .SH REPETITION
# Line 1030  items: Line 1423  items:
1423    the \eC escape sequence    the \eC escape sequence
1424    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1425    the \eR escape sequence    the \eR escape sequence
1426    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1427    a character class    a character class
1428    a back reference (see next section)    a back reference (see next section)
1429    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1430      a recursive or "subroutine" call to a subpattern
1431  .sp  .sp
1432  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1433  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1065  support is available, \eX{3} matches thr Line 1459  support is available, \eX{3} matches thr
1459  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).
1460  .P  .P
1461  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
1462  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1463    subpatterns that are referenced as
1464    .\" HTML <a href="#subpatternsassubroutines">
1465    .\" </a>
1466    subroutines
1467    .\"
1468    from elsewhere in the pattern (but see also the section entitled
1469    .\" HTML <a href="#subdefine">
1470    .\" </a>
1471    "Defining subpatterns for use by reference only"
1472    .\"
1473    below). Items other than subpatterns that have a {0} quantifier are omitted
1474    from the compiled pattern.
1475  .P  .P
1476  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1477  abbreviations:  abbreviations:
# Line 1137  worth setting PCRE_DOTALL in order to ob Line 1543  worth setting PCRE_DOTALL in order to ob
1543  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1544  .P  .P
1545  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1546  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1547  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
1548  succeeds. Consider, for example:  succeeds. Consider, for example:
1549  .sp  .sp
# Line 1212  previous example can be rewritten as Line 1618  previous example can be rewritten as
1618  .sp  .sp
1619    \ed++foo    \ed++foo
1620  .sp  .sp
1621    Note that a possessive quantifier can be used with an entire group, for
1622    example:
1623    .sp
1624      (abc|xyz){2,3}+
1625    .sp
1626  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1627  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
1628  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 1285  no such problem when named parentheses a Line 1696  no such problem when named parentheses a
1696  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1697  .P  .P
1698  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
1699  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
1700  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
1701  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1702  .sp  .sp
1703    (ring), \e1    (ring), \e1
1704    (ring), \eg1    (ring), \eg1
1705    (ring), \eg{1}    (ring), \eg{1}
1706  .sp  .sp
1707  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1708  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
1709  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1710    example:
1711  .sp  .sp
1712    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1713  .sp  .sp
1714  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
1715  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.
1716  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
1717  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
1718  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1719  .P  .P
1720  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1721  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1325  back reference, the case of letters is r Line 1737  back reference, the case of letters is r
1737  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1738  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1739  .P  .P
1740  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1741  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1742    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1743    back reference syntax, in which \eg can be used for both numeric and named
1744    references, is also supported. We could rewrite the above example in any of
1745  the following ways:  the following ways:
1746  .sp  .sp
1747    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1748      (?'p1'(?i)rah)\es+\ek{p1}
1749    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1750      (?<p1>(?i)rah)\es+\eg{p1}
1751  .sp  .sp
1752  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1753  after the reference.  after the reference.
1754  .P  .P
1755  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
1756  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1757  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1758  .sp  .sp
1759    (a|(bc))\e2    (a|(bc))\e2
1760  .sp  .sp
1761  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
1762  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
1763  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1764  with a digit character, some delimiter must be used to terminate the back  .P
1765  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1766  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1767    If the pattern continues with a digit character, some delimiter must be used to
1768    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1769    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1770  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1771  .\" </a>  .\" </a>
1772  "Comments"  "Comments"
1773  .\"  .\"
1774  below) can be used.  below) can be used.
1775  .P  .
1776    .SS "Recursive back references"
1777    .rs
1778    .sp
1779  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
1780  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.
1781  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1366  to the previous iteration. In order for Line 1789  to the previous iteration. In order for
1789  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
1790  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
1791  minimum of zero.  minimum of zero.
1792    .P
1793    Back references of this type cause the group that they reference to be treated
1794    as an
1795    .\" HTML <a href="#atomicgroup">
1796    .\" </a>
1797    atomic group.
1798    .\"
1799    Once the whole group has been matched, a subsequent matching failure cannot
1800    cause backtracking into the middle of the group.
1801  .  .
1802  .  .
1803  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1419  lookbehind assertion is needed to achiev Line 1851  lookbehind assertion is needed to achiev
1851  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
1852  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
1853  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.
1854    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1855  .  .
1856  .  .
1857  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1443  is permitted, but Line 1876  is permitted, but
1876  .sp  .sp
1877  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1878  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
1879  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
1880  match the same length of string. An assertion such as  length of string. An assertion such as
1881  .sp  .sp
1882    (?<=ab(c|de))    (?<=ab(c|de))
1883  .sp  .sp
1884  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
1885  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
1886    branches:
1887  .sp  .sp
1888    (?<=abc|abde)    (?<=abc|abde)
1889  .sp  .sp
1890  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1891  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1892  .\" </a>  .\" </a>
1893  (see above)  (see above)
1894  .\"  .\"
1895  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
1896  fixed-length.  restriction.
1897  .P  .P
1898  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1899  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 1471  to appear in lookbehind assertions, beca Line 1905  to appear in lookbehind assertions, beca
1905  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
1906  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1907  .P  .P
1908    .\" HTML <a href="#subpatternsassubroutines">
1909    .\" </a>
1910    "Subroutine"
1911    .\"
1912    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1913    as the subpattern matches a fixed-length string.
1914    .\" HTML <a href="#recursion">
1915    .\" </a>
1916    Recursion,
1917    .\"
1918    however, is not supported.
1919    .P
1920  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1921  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
1922  pattern such as  strings. Consider a simple pattern such as
1923  .sp  .sp
1924    abcd$    abcd$
1925  .sp  .sp
# Line 1537  characters that are not "999". Line 1983  characters that are not "999".
1983  .sp  .sp
1984  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1985  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1986  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1987  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1988  .sp  .sp
1989    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1990    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1991  .sp  .sp
1992  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1993  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
1994  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1995    itself contain nested subpatterns of any form, including conditional
1996    subpatterns; the restriction to two alternatives applies only at the level of
1997    the condition. This pattern fragment is an example where the alternatives are
1998    complex:
1999    .sp
2000      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2001    .sp
2002  .P  .P
2003  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2004  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1554  recursion, a pseudo-condition called DEF Line 2007  recursion, a pseudo-condition called DEF
2007  .rs  .rs
2008  .sp  .sp
2009  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
2010  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2011  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
2012  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2013  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2014  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2015  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2016    section about duplicate subpattern numbers),
2017    .\"
2018    the condition is true if any of them have matched. An alternative notation is
2019    to precede the digits with a plus or minus sign. In this case, the subpattern
2020    number is relative rather than absolute. The most recently opened parentheses
2021    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2022    loops it can also make sense to refer to subsequent groups. The next
2023    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2024    zero in any of these forms is not used; it provokes a compile-time error.)
2025  .P  .P
2026  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2027  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 1570  three parts for ease of discussion: Line 2032  three parts for ease of discussion:
2032  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2033  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
2034  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
2035  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2036  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,
2037  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
2038  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2039  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2040  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2041  .P  .P
2042  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
2043  reference:  reference:
2044  .sp  .sp
2045    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1600  Rewriting the above example to use a nam Line 2062  Rewriting the above example to use a nam
2062  .sp  .sp
2063    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2064  .sp  .sp
2065    If the name used in a condition of this kind is a duplicate, the test is
2066    applied to all subpatterns of the same name, and is true if any one of them has
2067    matched.
2068  .  .
2069  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2070  .rs  .rs
# Line 1611  letter R, for example: Line 2076  letter R, for example:
2076  .sp  .sp
2077    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2078  .sp  .sp
2079  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
2080  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
2081  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2082    applied to all subpatterns of the same name, and is true if any one of them is
2083    the most recent recursion.
2084  .P  .P
2085  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2086  patterns are described below.  .\" HTML <a href="#recursion">
2087    .\" </a>
2088    The syntax for recursive patterns
2089    .\"
2090    is described below.
2091  .  .
2092    .\" HTML <a name="subdefine"></a>
2093  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2094  .rs  .rs
2095  .sp  .sp
# Line 1625  If the condition is the string (DEFINE), Line 2097  If the condition is the string (DEFINE),
2097  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
2098  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2099  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
2100  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
2101  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2102  written like this (ignore whitespace and line breaks):  .\" </a>
2103    "subroutines"
2104    .\"
2105    is described below.) For example, a pattern to match an IPv4 address such as
2106    "192.168.23.245" could be written like this (ignore whitespace and line
2107    breaks):
2108  .sp  .sp
2109    (?(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) )
2110    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1635  written like this (ignore whitespace and Line 2112  written like this (ignore whitespace and
2112  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
2113  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2114  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
2115  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2116  .P  pattern uses references to the named group to match the four dot-separated
2117  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.  
2118  .  .
2119  .SS "Assertion conditions"  .SS "Assertion conditions"
2120  .rs  .rs
# Line 1664  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2139  dd-aaa-dd or dd-dd-dd, where aaa are let
2139  .SH COMMENTS  .SH COMMENTS
2140  .rs  .rs
2141  .sp  .sp
2142  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
2143  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,
2144  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
2145    subpattern name or number. The characters that make up a comment play no part
2146    in the pattern matching.
2147  .P  .P
2148  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
2149  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2150  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2151    this case continues to immediately after the next newline character or
2152    character sequence in the pattern. Which characters are interpreted as newlines
2153    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2154    sequence at the start of the pattern, as described in the section entitled
2155    .\" HTML <a href="#newlines">
2156    .\" </a>
2157    "Newline conventions"
2158    .\"
2159    above. Note that the end of this type of comment is a literal newline sequence
2160    in the pattern; escape sequences that happen to represent a newline do not
2161    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2162    default newline convention is in force:
2163    .sp
2164      abc #comment \en still comment
2165    .sp
2166    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2167    a newline in the pattern. The sequence \en is still literal at this stage, so
2168    it does not terminate the comment. Only an actual character with the code value
2169    0x0a (the default newline) does so.
2170  .  .
2171  .  .
2172  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1696  recursively to the pattern in which it a Line 2192  recursively to the pattern in which it a
2192  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2193  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2194  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2195  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.
2196  .P  .P
2197  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
2198  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,
2199  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
2200    .\" HTML <a href="#subpatternsassubroutines">
2201    .\" </a>
2202    "subroutine"
2203    .\"
2204  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
2205  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2206  .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  
2207  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2208  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2209  .sp  .sp
2210    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2211  .sp  .sp
2212  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2213  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
2214  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2215  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2216    to avoid backtracking into sequences of non-parentheses.
2217  .P  .P
2218  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
2219  pattern, so instead you could use this:  pattern, so instead you could use this:
2220  .sp  .sp
2221    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2222  .sp  .sp
2223  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
2224  them instead of the whole pattern.  them instead of the whole pattern.
2225  .P  .P
2226  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
2227  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
2228  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
2229  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2230  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.  
2231  .P  .P
2232  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2233  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2234  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2235  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2236    .\" </a>
2237    "subroutine"
2238    .\"
2239    calls, as described in the next section.
2240  .P  .P
2241  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2242  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
2243  could rewrite the above example as follows:  could rewrite the above example as follows:
2244  .sp  .sp
2245    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2246  .sp  .sp
2247  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
2248  used.  used.
2249  .P  .P
2250  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2251  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
2252  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
2253  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2254  .sp  .sp
2255    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2256  .sp  .sp
2257  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,
2258  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
2259  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
2260  before failure can be reported.  before failure can be reported.
2261  .P  .P
2262  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
2263  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
2264  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  
2265  .\" HREF  .\" HREF
2266  \fBpcrecallout\fP  \fBpcrecallout\fP
2267  .\"  .\"
# Line 1771  documentation). If the pattern above is Line 2269  documentation). If the pattern above is
2269  .sp  .sp
2270    (ab(cd)ef)    (ab(cd)ef)
2271  .sp  .sp
2272  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
2273  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
2274  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2275    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2276       ^                        ^  .P
2277       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2278  .sp  obtain extra memory to store data during a recursion, which it does by using
2279  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
2280  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.  
2281  .P  .P
2282  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.
2283  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1796  different alternatives for the recursive Line 2291  different alternatives for the recursive
2291  is the actual recursive call.  is the actual recursive call.
2292  .  .
2293  .  .
2294    .\" HTML <a name="recursiondifference"></a>
2295    .SS "Recursion difference from Perl"
2296    .rs
2297    .sp
2298    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2299    treated as an atomic group. That is, once it has matched some of the subject
2300    string, it is never re-entered, even if it contains untried alternatives and
2301    there is a subsequent matching failure. This can be illustrated by the
2302    following pattern, which purports to match a palindromic string that contains
2303    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2304    .sp
2305      ^(.|(.)(?1)\e2)$
2306    .sp
2307    The idea is that it either matches a single character, or two identical
2308    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2309    it does not if the pattern is longer than three characters. Consider the
2310    subject string "abcba":
2311    .P
2312    At the top level, the first character is matched, but as it is not at the end
2313    of the string, the first alternative fails; the second alternative is taken
2314    and the recursion kicks in. The recursive call to subpattern 1 successfully
2315    matches the next character ("b"). (Note that the beginning and end of line
2316    tests are not part of the recursion).
2317    .P
2318    Back at the top level, the next character ("c") is compared with what
2319    subpattern 2 matched, which was "a". This fails. Because the recursion is
2320    treated as an atomic group, there are now no backtracking points, and so the
2321    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2322    try the second alternative.) However, if the pattern is written with the
2323    alternatives in the other order, things are different:
2324    .sp
2325      ^((.)(?1)\e2|.)$
2326    .sp
2327    This time, the recursing alternative is tried first, and continues to recurse
2328    until it runs out of characters, at which point the recursion fails. But this
2329    time we do have another alternative to try at the higher level. That is the big
2330    difference: in the previous case the remaining alternative is at a deeper
2331    recursion level, which PCRE cannot use.
2332    .P
2333    To change the pattern so that it matches all palindromic strings, not just
2334    those with an odd number of characters, it is tempting to change the pattern to
2335    this:
2336    .sp
2337      ^((.)(?1)\e2|.?)$
2338    .sp
2339    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2340    deeper recursion has matched a single character, it cannot be entered again in
2341    order to match an empty string. The solution is to separate the two cases, and
2342    write out the odd and even cases as alternatives at the higher level:
2343    .sp
2344      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2345    .sp
2346    If you want to match typical palindromic phrases, the pattern has to ignore all
2347    non-word characters, which can be done like this:
2348    .sp
2349      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2350    .sp
2351    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2352    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2353    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2354    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2355    more) to match typical phrases, and Perl takes so long that you think it has
2356    gone into a loop.
2357    .P
2358    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2359    string does not start with a palindrome that is shorter than the entire string.
2360    For example, although "abcba" is correctly matched, if the subject is "ababa",
2361    PCRE finds the palindrome "aba" at the start, then fails at top level because
2362    the end of the string does not follow. Once again, it cannot jump back into the
2363    recursion to try other alternatives, so the entire match fails.
2364    .
2365    .
2366  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2367  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2368  .rs  .rs
# Line 1808  relative, as in these examples: Line 2375  relative, as in these examples:
2375  .sp  .sp
2376    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2377    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2378    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2379  .sp  .sp
2380  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2381  .sp  .sp
# Line 1822  matches "sense and sensibility" and "res Line 2389  matches "sense and sensibility" and "res
2389  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
2390  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2391  .P  .P
2392  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2393  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
2394  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
2395  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2396    call revert to their previous values afterwards.
2397  .P  .P
2398  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2399  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 1837  It matches "abcabc". It does not match " Line 2405  It matches "abcabc". It does not match "
2405  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2406  .  .
2407  .  .
2408    .\" HTML <a name="onigurumasubroutines"></a>
2409    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2410    .rs
2411    .sp
2412    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2413    a number enclosed either in angle brackets or single quotes, is an alternative
2414    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2415    are two of the examples used above, rewritten using this syntax:
2416    .sp
2417      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2418      (sens|respons)e and \eg'1'ibility
2419    .sp
2420    PCRE supports an extension to Oniguruma: if a number is preceded by a
2421    plus or a minus sign it is taken as a relative reference. For example:
2422    .sp
2423      (abc)(?i:\eg<-1>)
2424    .sp
2425    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2426    synonymous. The former is a back reference; the latter is a subroutine call.
2427    .
2428    .
2429  .SH CALLOUTS  .SH CALLOUTS
2430  .rs  .rs
2431  .sp  .sp
# Line 1873  description of the interface to the call Line 2462  description of the interface to the call
2462  documentation.  documentation.
2463  .  .
2464  .  .
2465    .\" HTML <a name="backtrackcontrol"></a>
2466    .SH "BACKTRACKING CONTROL"
2467    .rs
2468    .sp
2469    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2470    are described in the Perl documentation as "experimental and subject to change
2471    or removal in a future version of Perl". It goes on to say: "Their usage in
2472    production code should be noted to avoid problems during upgrades." The same
2473    remarks apply to the PCRE features described in this section.
2474    .P
2475    Since these verbs are specifically related to backtracking, most of them can be
2476    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2477    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2478    failing negative assertion, they cause an error if encountered by
2479    \fBpcre_dfa_exec()\fP.
2480    .P
2481    If any of these verbs are used in an assertion or subroutine subpattern
2482    (including recursive subpatterns), their effect is confined to that subpattern;
2483    it does not extend to the surrounding pattern. Note that such subpatterns are
2484    processed as anchored at the point where they are tested.
2485    .P
2486    The new verbs make use of what was previously invalid syntax: an opening
2487    parenthesis followed by an asterisk. They are generally of the form
2488    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2489    depending on whether or not an argument is present. An name is a sequence of
2490    letters, digits, and underscores. If the name is empty, that is, if the closing
2491    parenthesis immediately follows the colon, the effect is as if the colon were
2492    not there. Any number of these verbs may occur in a pattern.
2493    .P
2494    PCRE contains some optimizations that are used to speed up matching by running
2495    some checks at the start of each match attempt. For example, it may know the
2496    minimum length of matching subject, or that a particular character must be
2497    present. When one of these optimizations suppresses the running of a match, any
2498    included backtracking verbs will not, of course, be processed. You can suppress
2499    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2500    when calling \fBpcre_exec()\fP.
2501    .
2502    .
2503    .SS "Verbs that act immediately"
2504    .rs
2505    .sp
2506    The following verbs act as soon as they are encountered. They may not be
2507    followed by a name.
2508    .sp
2509       (*ACCEPT)
2510    .sp
2511    This verb causes the match to end successfully, skipping the remainder of the
2512    pattern. When inside a recursion, only the innermost pattern is ended
2513    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2514    captured. (This feature was added to PCRE at release 8.00.) For example:
2515    .sp
2516      A((?:A|B(*ACCEPT)|C)D)
2517    .sp
2518    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2519    the outer parentheses.
2520    .sp
2521      (*FAIL) or (*F)
2522    .sp
2523    This verb causes the match to fail, forcing backtracking to occur. It is
2524    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2525    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2526    Perl features that are not present in PCRE. The nearest equivalent is the
2527    callout feature, as for example in this pattern:
2528    .sp
2529      a+(?C)(*FAIL)
2530    .sp
2531    A match with the string "aaaa" always fails, but the callout is taken before
2532    each backtrack happens (in this example, 10 times).
2533    .
2534    .
2535    .SS "Recording which path was taken"
2536    .rs
2537    .sp
2538    There is one verb whose main purpose is to track how a match was arrived at,
2539    though it also has a secondary use in conjunction with advancing the match
2540    starting point (see (*SKIP) below).
2541    .sp
2542      (*MARK:NAME) or (*:NAME)
2543    .sp
2544    A name is always required with this verb. There may be as many instances of
2545    (*MARK) as you like in a pattern, and their names do not have to be unique.
2546    .P
2547    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2548    to the caller via the \fIpcre_extra\fP data structure, as described in the
2549    .\" HTML <a href="pcreapi.html#extradata">
2550    .\" </a>
2551    section on \fIpcre_extra\fP
2552    .\"
2553    in the
2554    .\" HREF
2555    \fBpcreapi\fP
2556    .\"
2557    documentation. No data is returned for a partial match. Here is an example of
2558    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2559    outputting of (*MARK) data:
2560    .sp
2561      /X(*MARK:A)Y|X(*MARK:B)Z/K
2562      XY
2563       0: XY
2564      MK: A
2565      XZ
2566       0: XZ
2567      MK: B
2568    .sp
2569    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2570    indicates which of the two alternatives matched. This is a more efficient way
2571    of obtaining this information than putting each alternative in its own
2572    capturing parentheses.
2573    .P
2574    A name may also be returned after a failed match if the final path through the
2575    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2576    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2577    starting point for matching is advanced, the final check is often with an empty
2578    string, causing a failure before (*MARK) is reached. For example:
2579    .sp
2580      /X(*MARK:A)Y|X(*MARK:B)Z/K
2581      XP
2582      No match
2583    .sp
2584    There are three potential starting points for this match (starting with X,
2585    starting with P, and with an empty string). If the pattern is anchored, the
2586    result is different:
2587    .sp
2588      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2589      XP
2590      No match, mark = B
2591    .sp
2592    PCRE's start-of-match optimizations can also interfere with this. For example,
2593    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2594    subject length for a match, a shorter subject will not be scanned at all.
2595    .P
2596    Note that similar anomalies (though different in detail) exist in Perl, no
2597    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2598    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2599    .
2600    .
2601    .SS "Verbs that act after backtracking"
2602    .rs
2603    .sp
2604    The following verbs do nothing when they are encountered. Matching continues
2605    with what follows, but if there is no subsequent match, causing a backtrack to
2606    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2607    the verb. However, when one of these verbs appears inside an atomic group, its
2608    effect is confined to that group, because once the group has been matched,
2609    there is never any backtracking into it. In this situation, backtracking can
2610    "jump back" to the left of the entire atomic group. (Remember also, as stated
2611    above, that this localization also applies in subroutine calls and assertions.)
2612    .P
2613    These verbs differ in exactly what kind of failure occurs when backtracking
2614    reaches them.
2615    .sp
2616      (*COMMIT)
2617    .sp
2618    This verb, which may not be followed by a name, causes the whole match to fail
2619    outright if the rest of the pattern does not match. Even if the pattern is
2620    unanchored, no further attempts to find a match by advancing the starting point
2621    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2622    finding a match at the current starting point, or not at all. For example:
2623    .sp
2624      a+(*COMMIT)b
2625    .sp
2626    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2627    dynamic anchor, or "I've started, so I must finish." The name of the most
2628    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2629    match failure.
2630    .P
2631    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2632    unless PCRE's start-of-match optimizations are turned off, as shown in this
2633    \fBpcretest\fP example:
2634    .sp
2635      /(*COMMIT)abc/
2636      xyzabc
2637       0: abc
2638      xyzabc\eY
2639      No match
2640    .sp
2641    PCRE knows that any match must start with "a", so the optimization skips along
2642    the subject to "a" before running the first match attempt, which succeeds. When
2643    the optimization is disabled by the \eY escape in the second subject, the match
2644    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2645    starting points.
2646    .sp
2647      (*PRUNE) or (*PRUNE:NAME)
2648    .sp
2649    This verb causes the match to fail at the current starting position in the
2650    subject if the rest of the pattern does not match. If the pattern is
2651    unanchored, the normal "bumpalong" advance to the next starting character then
2652    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2653    reached, or when matching to the right of (*PRUNE), but if there is no match to
2654    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2655    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2656    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2657    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2658    match fails completely; the name is passed back if this is the final attempt.
2659    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2660    pattern (*PRUNE) has the same effect as (*COMMIT).
2661    .sp
2662      (*SKIP)
2663    .sp
2664    This verb, when given without a name, is like (*PRUNE), except that if the
2665    pattern is unanchored, the "bumpalong" advance is not to the next character,
2666    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2667    signifies that whatever text was matched leading up to it cannot be part of a
2668    successful match. Consider:
2669    .sp
2670      a+(*SKIP)b
2671    .sp
2672    If the subject is "aaaac...", after the first match attempt fails (starting at
2673    the first character in the string), the starting point skips on to start the
2674    next attempt at "c". Note that a possessive quantifer does not have the same
2675    effect as this example; although it would suppress backtracking during the
2676    first match attempt, the second attempt would start at the second character
2677    instead of skipping on to "c".
2678    .sp
2679      (*SKIP:NAME)
2680    .sp
2681    When (*SKIP) has an associated name, its behaviour is modified. If the
2682    following pattern fails to match, the previous path through the pattern is
2683    searched for the most recent (*MARK) that has the same name. If one is found,
2684    the "bumpalong" advance is to the subject position that corresponds to that
2685    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2686    matching name is found, normal "bumpalong" of one character happens (the
2687    (*SKIP) is ignored).
2688    .sp
2689      (*THEN) or (*THEN:NAME)
2690    .sp
2691    This verb causes a skip to the next alternation in the innermost enclosing
2692    group if the rest of the pattern does not match. That is, it cancels pending
2693    backtracking, but only within the current alternation. Its name comes from the
2694    observation that it can be used for a pattern-based if-then-else block:
2695    .sp
2696      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2697    .sp
2698    If the COND1 pattern matches, FOO is tried (and possibly further items after
2699    the end of the group if FOO succeeds); on failure the matcher skips to the
2700    second alternative and tries COND2, without backtracking into COND1. The
2701    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2702    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2703    like (*PRUNE).
2704    .
2705    .P
2706    The above verbs provide four different "strengths" of control when subsequent
2707    matching fails. (*THEN) is the weakest, carrying on the match at the next
2708    alternation. (*PRUNE) comes next, failing the match at the current starting
2709    position, but allowing an advance to the next character (for an unanchored
2710    pattern). (*SKIP) is similar, except that the advance may be more than one
2711    character. (*COMMIT) is the strongest, causing the entire match to fail.
2712    .P
2713    If more than one is present in a pattern, the "stongest" one wins. For example,
2714    consider this pattern, where A, B, etc. are complex pattern fragments:
2715    .sp
2716      (A(*COMMIT)B(*THEN)C|D)
2717    .sp
2718    Once A has matched, PCRE is committed to this match, at the current starting
2719    position. If subsequently B matches, but C does not, the normal (*THEN) action
2720    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2721    overrides.
2722    .
2723    .
2724  .SH "SEE ALSO"  .SH "SEE ALSO"
2725  .rs  .rs
2726  .sp  .sp
2727  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2728    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2729  .  .
2730  .  .
2731  .SH AUTHOR  .SH AUTHOR
# Line 1893  Cambridge CB2 3QH, England. Line 2742  Cambridge CB2 3QH, England.
2742  .rs  .rs
2743  .sp  .sp
2744  .nf  .nf
2745  Last updated: 29 May 2007  Last updated: 21 November 2010
2746  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2747  .fi  .fi

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