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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    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
61  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
63  \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
64  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
65  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
66    alternative function, and how it differs from the normal function, are
67    discussed in the
68  .\" HREF  .\" HREF
69  \fBpcrematching\fP  \fBpcrematching\fP
70  .\"  .\"
71  page.  page.
72  .  .
73  .  .
74    .\" HTML <a name="newlines"></a>
75    .SH "NEWLINE CONVENTIONS"
76    .rs
77    .sp
78    PCRE supports five different conventions for indicating line breaks in
79    strings: a single CR (carriage return) character, a single LF (linefeed)
80    character, the two-character sequence CRLF, any of the three preceding, or any
81    Unicode newline sequence. The
82    .\" HREF
83    \fBpcreapi\fP
84    .\"
85    page has
86    .\" HTML <a href="pcreapi.html#newlines">
87    .\" </a>
88    further discussion
89    .\"
90    about newlines, and shows how to set the newline convention in the
91    \fIoptions\fP arguments for the compiling and matching functions.
92    .P
93    It is also possible to specify a newline convention by starting a pattern
94    string with one of the following five sequences:
95    .sp
96      (*CR)        carriage return
97      (*LF)        linefeed
98      (*CRLF)      carriage return, followed by linefeed
99      (*ANYCRLF)   any of the three above
100      (*ANY)       all Unicode newline sequences
101    .sp
102    These override the default and the options given to \fBpcre_compile()\fP or
103    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
104    newline sequence, the pattern
105    .sp
106      (*CR)a.b
107    .sp
108    changes the convention to CR. That pattern matches "a\enb" because LF is no
109    longer a newline. Note that these special settings, which are not
110    Perl-compatible, are recognized only at the very start of a pattern, and that
111    they must be in upper case. If more than one of them is present, the last one
112    is used.
113    .P
114    The newline convention affects the interpretation of the dot metacharacter when
115    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
116    affect what the \eR escape sequence matches. By default, this is any Unicode
117    newline sequence, for Perl compatibility. However, this can be changed; see the
118    description of \eR in the section entitled
119    .\" HTML <a href="#newlineseq">
120    .\" </a>
121    "Newline sequences"
122    .\"
123    below. A change of \eR setting can be combined with a change of newline
124    convention.
125    .
126    .
127  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
128  .rs  .rs
129  .sp  .sp
# Line 101  The following sections describe the use Line 187  The following sections describe the use
187  .rs  .rs
188  .sp  .sp
189  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
190  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
191  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
192  outside character classes.  both inside and outside character classes.
193  .P  .P
194  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
195  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 111  otherwise be interpreted as a metacharac Line 197  otherwise be interpreted as a metacharac
197  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
198  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
199  .P  .P
200    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201    backslash. All other characters (in particular, those whose codepoints are
202    greater than 127) are treated as literals.
203    .P
204  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
205  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
206  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 130  Perl, $ and @ cause variable interpolati Line 220  Perl, $ and @ cause variable interpolati
220    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
221  .sp  .sp
222  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
223    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224    by \eE later in the pattern, the literal interpretation continues to the end of
225    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226    a character class, this causes an error, because the character class is not
227    terminated.
228  .  .
229  .  .
230  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 139  The \eQ...\eE sequence is recognized bot Line 234  The \eQ...\eE sequence is recognized bot
234  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
235  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
236  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
237  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
238  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:  
239  .sp  .sp
240    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
241    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
242    \ee        escape (hex 1B)    \ee        escape (hex 1B)
243    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
244    \en        newline (hex 0A)    \en        linefeed (hex 0A)
245    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
246    \et        tab (hex 09)    \et        tab (hex 09)
247    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
248    \exhh      character with hex code hh    \exhh      character with hex code hh
249    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
250  .sp  .sp
251  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
252  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
253  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
254  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255    than 127, a compile-time error occurs. This locks out non-ASCII characters in
256    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257    values are valid. A lower case letter is converted to upper case, and then the
258    0xc0 bits are flipped.)
259  .P  .P
260  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
261  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
262  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
263  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
264  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
265  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
266  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
267  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
268    there is no terminating }, this form of escape is not recognized. Instead, the
269    initial \ex will be interpreted as a basic hexadecimal escape, with no
270    following digits, giving a character whose value is zero.
271  .P  .P
272  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
273  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 225  zero, because no more than three octal d Line 326  zero, because no more than three octal d
326  .P  .P
327  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
328  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
329  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
330  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
331  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
332  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
333  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
334  .  .
335  .  .
336  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
337  .rs  .rs
338  .sp  .sp
339  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
340  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
341  discussed  reference can be coded as \eg{name}. Back references are discussed
342  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
343  .\" </a>  .\" </a>
344  later,  later,
# Line 252  parenthesized subpatterns. Line 350  parenthesized subpatterns.
350  .\"  .\"
351  .  .
352  .  .
353    .SS "Absolute and relative subroutine calls"
354    .rs
355    .sp
356    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
357    a number enclosed either in angle brackets or single quotes, is an alternative
358    syntax for referencing a subpattern as a "subroutine". Details are discussed
359    .\" HTML <a href="#onigurumasubroutines">
360    .\" </a>
361    later.
362    .\"
363    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
364    synonymous. The former is a back reference; the latter is a
365    .\" HTML <a href="#subpatternsassubroutines">
366    .\" </a>
367    subroutine
368    .\"
369    call.
370    .
371    .
372    .\" HTML <a name="genericchartypes"></a>
373  .SS "Generic character types"  .SS "Generic character types"
374  .rs  .rs
375  .sp  .sp
376  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
377  .sp  .sp
378    \ed     any decimal digit    \ed     any decimal digit
379    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
380      \eh     any horizontal whitespace character
381      \eH     any character that is not a horizontal whitespace character
382    \es     any whitespace character    \es     any whitespace character
383    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
384      \ev     any vertical whitespace character
385      \eV     any character that is not a vertical whitespace character
386    \ew     any "word" character    \ew     any "word" character
387    \eW     any "non-word" character    \eW     any "non-word" character
388  .sp  .sp
389  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
390  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
391    .\" HTML <a href="#fullstopdot">
392    .\" </a>
393    the "." metacharacter
394    .\"
395    when PCRE_DOTALL is not set.
396  .P  .P
397  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
398    of characters into two disjoint sets. Any given character matches one, and only
399    one, of each pair. The sequences can appear both inside and outside character
400  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
401  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
402  there is no character to match.  there is no character to match.
403  .P  .P
404  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
405  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
406  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
407  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
408  does.)  does.
409  .P  .P
410  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.
411  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
412  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
413  place (see  place (see
414  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 293  in the Line 421  in the
421  .\"  .\"
422  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,
423  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
424  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
425    Unicode is discouraged.
426  .P  .P
427  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
428  \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
429  character property support is available. The use of locales with Unicode is  their original meanings from before UTF-8 support was available, mainly for
430  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
431    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
432    properties are used to determine character types, as follows:
433    .sp
434      \ed  any character that \ep{Nd} matches (decimal digit)
435      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
436      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
437    .sp
438    The upper case escapes match the inverse sets of characters. Note that \ed
439    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
440    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
441    \eB because they are defined in terms of \ew and \eW. Matching these sequences
442    is noticeably slower when PCRE_UCP is set.
443    .P
444    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
445    release 5.10. In contrast to the other sequences, which match only ASCII
446    characters by default, these always match certain high-valued codepoints in
447    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448    are:
449    .sp
450      U+0009     Horizontal tab
451      U+0020     Space
452      U+00A0     Non-break space
453      U+1680     Ogham space mark
454      U+180E     Mongolian vowel separator
455      U+2000     En quad
456      U+2001     Em quad
457      U+2002     En space
458      U+2003     Em space
459      U+2004     Three-per-em space
460      U+2005     Four-per-em space
461      U+2006     Six-per-em space
462      U+2007     Figure space
463      U+2008     Punctuation space
464      U+2009     Thin space
465      U+200A     Hair space
466      U+202F     Narrow no-break space
467      U+205F     Medium mathematical space
468      U+3000     Ideographic space
469    .sp
470    The vertical space characters are:
471    .sp
472      U+000A     Linefeed
473      U+000B     Vertical tab
474      U+000C     Formfeed
475      U+000D     Carriage return
476      U+0085     Next line
477      U+2028     Line separator
478      U+2029     Paragraph separator
479  .  .
480  .  .
481    .\" HTML <a name="newlineseq"></a>
482  .SS "Newline sequences"  .SS "Newline sequences"
483  .rs  .rs
484  .sp  .sp
485  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
486  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:  
487  .sp  .sp
488    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489  .sp  .sp
# Line 326  are added: LS (line separator, U+2028) a Line 503  are added: LS (line separator, U+2028) a
503  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
504  recognized.  recognized.
505  .P  .P
506  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
507    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
508    either at compile time or when the pattern is matched. (BSR is an abbrevation
509    for "backslash R".) This can be made the default when PCRE is built; if this is
510    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
511    It is also possible to specify these settings by starting a pattern string with
512    one of the following sequences:
513    .sp
514      (*BSR_ANYCRLF)   CR, LF, or CRLF only
515      (*BSR_UNICODE)   any Unicode newline sequence
516    .sp
517    These override the default and the options given to \fBpcre_compile()\fP or
518    \fBpcre_compile2()\fP, but they can be overridden by options given to
519    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
520    which are not Perl-compatible, are recognized only at the very start of a
521    pattern, and that they must be in upper case. If more than one of them is
522    present, the last one is used. They can be combined with a change of newline
523    convention; for example, a pattern can start with:
524    .sp
525      (*ANY)(*BSR_ANYCRLF)
526    .sp
527    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
528    a character class, \eR is treated as an unrecognized escape sequence, and so
529    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
530  .  .
531  .  .
532  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 334  Inside a character class, \eR matches th Line 534  Inside a character class, \eR matches th
534  .rs  .rs
535  .sp  .sp
536  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
537  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
538  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
539    characters whose codepoints are less than 256, but they do work in this mode.
540    The extra escape sequences are:
541  .sp  .sp
542    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
543    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
544    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
545  .sp  .sp
546  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
547  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
548  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
549  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
550  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
551    .\" </a>
552    next section).
553    .\"
554    Other Perl properties such as "InMusicalSymbols" are not currently supported by
555    PCRE. Note that \eP{Any} does not match any characters, so always causes a
556    match failure.
557  .P  .P
558  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
559  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 359  Those that are not part of an identified Line 567  Those that are not part of an identified
567  .P  .P
568  Arabic,  Arabic,
569  Armenian,  Armenian,
570    Avestan,
571  Balinese,  Balinese,
572    Bamum,
573  Bengali,  Bengali,
574  Bopomofo,  Bopomofo,
575  Braille,  Braille,
576  Buginese,  Buginese,
577  Buhid,  Buhid,
578  Canadian_Aboriginal,  Canadian_Aboriginal,
579    Carian,
580    Cham,
581  Cherokee,  Cherokee,
582  Common,  Common,
583  Coptic,  Coptic,
# Line 374  Cypriot, Line 586  Cypriot,
586  Cyrillic,  Cyrillic,
587  Deseret,  Deseret,
588  Devanagari,  Devanagari,
589    Egyptian_Hieroglyphs,
590  Ethiopic,  Ethiopic,
591  Georgian,  Georgian,
592  Glagolitic,  Glagolitic,
# Line 386  Hangul, Line 599  Hangul,
599  Hanunoo,  Hanunoo,
600  Hebrew,  Hebrew,
601  Hiragana,  Hiragana,
602    Imperial_Aramaic,
603  Inherited,  Inherited,
604    Inscriptional_Pahlavi,
605    Inscriptional_Parthian,
606    Javanese,
607    Kaithi,
608  Kannada,  Kannada,
609  Katakana,  Katakana,
610    Kayah_Li,
611  Kharoshthi,  Kharoshthi,
612  Khmer,  Khmer,
613  Lao,  Lao,
614  Latin,  Latin,
615    Lepcha,
616  Limbu,  Limbu,
617  Linear_B,  Linear_B,
618    Lisu,
619    Lycian,
620    Lydian,
621  Malayalam,  Malayalam,
622    Meetei_Mayek,
623  Mongolian,  Mongolian,
624  Myanmar,  Myanmar,
625  New_Tai_Lue,  New_Tai_Lue,
# Line 403  Nko, Line 627  Nko,
627  Ogham,  Ogham,
628  Old_Italic,  Old_Italic,
629  Old_Persian,  Old_Persian,
630    Old_South_Arabian,
631    Old_Turkic,
632    Ol_Chiki,
633  Oriya,  Oriya,
634  Osmanya,  Osmanya,
635  Phags_Pa,  Phags_Pa,
636  Phoenician,  Phoenician,
637    Rejang,
638  Runic,  Runic,
639    Samaritan,
640    Saurashtra,
641  Shavian,  Shavian,
642  Sinhala,  Sinhala,
643    Sundanese,
644  Syloti_Nagri,  Syloti_Nagri,
645  Syriac,  Syriac,
646  Tagalog,  Tagalog,
647  Tagbanwa,  Tagbanwa,
648  Tai_Le,  Tai_Le,
649    Tai_Tham,
650    Tai_Viet,
651  Tamil,  Tamil,
652  Telugu,  Telugu,
653  Thaana,  Thaana,
# Line 422  Thai, Line 655  Thai,
655  Tibetan,  Tibetan,
656  Tifinagh,  Tifinagh,
657  Ugaritic,  Ugaritic,
658    Vai,
659  Yi.  Yi.
660  .P  .P
661  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
662  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
663  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
664  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
665  .P  .P
666  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
667  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 487  The special property L& is also supporte Line 721  The special property L& is also supporte
721  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
722  a modifier or "other".  a modifier or "other".
723  .P  .P
724  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
725    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
726    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
727    (see the discussion of PCRE_NO_UTF8_CHECK in the
728    .\" HREF
729    \fBpcreapi\fP
730    .\"
731    page). Perl does not support the Cs property.
732    .P
733    The long synonyms for property names that Perl supports (such as \ep{Letter})
734  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
735  properties with "Is".  properties with "Is".
736  .P  .P
# Line 511  atomic group Line 754  atomic group
754  (see below).  (see below).
755  .\"  .\"
756  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
757  preceding character.  preceding character. None of them have codepoints less than 256, so in
758    non-UTF-8 mode \eX matches any one character.
759    .P
760    Note that recent versions of Perl have changed \eX to match what Unicode calls
761    an "extended grapheme cluster", which has a more complicated definition.
762  .P  .P
763  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
764  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
765  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
766  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
767    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
768    (*UCP).
769    .
770    .
771    .\" HTML <a name="extraprops"></a>
772    .SS PCRE's additional properties
773    .rs
774    .sp
775    As well as the standard Unicode properties described in the previous
776    section, PCRE supports four more that make it possible to convert traditional
777    escape sequences such as \ew and \es and POSIX character classes to use Unicode
778    properties. PCRE uses these non-standard, non-Perl properties internally when
779    PCRE_UCP is set. They are:
780    .sp
781      Xan   Any alphanumeric character
782      Xps   Any POSIX space character
783      Xsp   Any Perl space character
784      Xwd   Any Perl "word" character
785    .sp
786    Xan matches characters that have either the L (letter) or the N (number)
787    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
788    carriage return, and any other character that has the Z (separator) property.
789    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
790    same characters as Xan, plus underscore.
791    .
792    .
793    .\" HTML <a name="resetmatchstart"></a>
794    .SS "Resetting the match start"
795    .rs
796    .sp
797    The escape sequence \eK causes any previously matched characters not to be
798    included in the final matched sequence. For example, the pattern:
799    .sp
800      foo\eKbar
801    .sp
802    matches "foobar", but reports that it has matched "bar". This feature is
803    similar to a lookbehind assertion
804    .\" HTML <a href="#lookbehind">
805    .\" </a>
806    (described below).
807    .\"
808    However, in this case, the part of the subject before the real match does not
809    have to be of fixed length, as lookbehind assertions do. The use of \eK does
810    not interfere with the setting of
811    .\" HTML <a href="#subpattern">
812    .\" </a>
813    captured substrings.
814    .\"
815    For example, when the pattern
816    .sp
817      (foo)\eKbar
818    .sp
819    matches "foobar", the first substring is still set to "foo".
820    .P
821    Perl documents that the use of \eK within assertions is "not well defined". In
822    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
823    ignored in negative assertions.
824  .  .
825  .  .
826  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 541  The backslashed assertions are: Line 845  The backslashed assertions are:
845    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
846    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
847  .sp  .sp
848  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
849  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
850    default it matches the corresponding literal character (for example, \eB
851    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
852    escape sequence" error is generated instead.
853  .P  .P
854  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
855  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
856  \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
857  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
858    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
859    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
860    of word" or "end of word" metasequence. However, whatever follows \eb normally
861    determines which it is. For example, the fragment \eba matches "a" at the start
862    of a word.
863  .P  .P
864  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
865  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 631  end of the subject in both modes, and if Line 943  end of the subject in both modes, and if
943  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
944  .  .
945  .  .
946  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
947    .SH "FULL STOP (PERIOD, DOT) AND \eN"
948  .rs  .rs
949  .sp  .sp
950  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 653  to match it. Line 966  to match it.
966  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
967  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
968  special meaning in a character class.  special meaning in a character class.
969    .P
970    The escape sequence \eN behaves like a dot, except that it is not affected by
971    the PCRE_DOTALL option. In other words, it matches any character except one
972    that signifies the end of a line.
973  .  .
974  .  .
975  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 661  special meaning in a character class. Line 978  special meaning in a character class.
978  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
979  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
980  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
981  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
982  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,
983  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
984  .P  .P
985  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 679  the lookbehind. Line 996  the lookbehind.
996  .rs  .rs
997  .sp  .sp
998  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
999  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.
1000  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
1001  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
1002  escaped with a backslash.  a member of the class, it should be the first data character in the class
1003    (after an initial circumflex, if present) or escaped with a backslash.
1004  .P  .P
1005  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
1006  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
1007  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
1008  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
1009  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
1010  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 696  For example, the character class [aeiou] Line 1014  For example, the character class [aeiou]
1014  [^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
1015  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1016  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
1017  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
1018  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
1019  string.  string.
1020  .P  .P
# Line 710  caseful version would. In UTF-8 mode, PC Line 1028  caseful version would. In UTF-8 mode, PC
1028  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
1029  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1030  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1031  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,
1032  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
1033  UTF-8 support.  with UTF-8 support.
1034  .P  .P
1035  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
1036  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 746  characters in both cases. In UTF-8 mode, Line 1064  characters in both cases. In UTF-8 mode,
1064  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
1065  property support.  property support.
1066  .P  .P
1067  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,
1068  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
1069  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1070  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
1071  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
1072  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1073    .\" HTML <a href="#genericchartypes">
1074    .\" </a>
1075    "Generic character types"
1076    .\"
1077    above. The escape sequence \eb has a different meaning inside a character
1078    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1079    are not special inside a character class. Like any other unrecognized escape
1080    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1081    default, but cause an error if the PCRE_EXTRA option is set.
1082    .P
1083    A circumflex can conveniently be used with the upper case character types to
1084    specify a more restricted set of characters than the matching lower case type.
1085    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1086    whereas [\ew] includes underscore. A positive character class should be read as
1087    "something OR something OR ..." and a negative class as "NOT something AND NOT
1088    something AND NOT ...".
1089  .P  .P
1090  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1091  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 771  this notation. For example, Line 1105  this notation. For example,
1105    [01[:alpha:]%]    [01[:alpha:]%]
1106  .sp  .sp
1107  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1108  are  are:
1109  .sp  .sp
1110    alnum    letters and digits    alnum    letters and digits
1111    alpha    letters    alpha    letters
# Line 782  are Line 1116  are
1116    graph    printing characters, excluding space    graph    printing characters, excluding space
1117    lower    lower case letters    lower    lower case letters
1118    print    printing characters, including space    print    printing characters, including space
1119    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1120    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1121    upper    upper case letters    upper    upper case letters
1122    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 803  matches "1", "2", or any non-digit. PCRE Line 1137  matches "1", "2", or any non-digit. PCRE
1137  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
1138  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1139  .P  .P
1140  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
1141  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1142    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1143    character properties are used. This is achieved by replacing the POSIX classes
1144    by other sequences, as follows:
1145    .sp
1146      [:alnum:]  becomes  \ep{Xan}
1147      [:alpha:]  becomes  \ep{L}
1148      [:blank:]  becomes  \eh
1149      [:digit:]  becomes  \ep{Nd}
1150      [:lower:]  becomes  \ep{Ll}
1151      [:space:]  becomes  \ep{Xps}
1152      [:upper:]  becomes  \ep{Lu}
1153      [:word:]   becomes  \ep{Xwd}
1154    .sp
1155    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1156    classes are unchanged, and match only characters with code points less than
1157    128.
1158  .  .
1159  .  .
1160  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 831  alternative in the subpattern. Line 1181  alternative in the subpattern.
1181  .rs  .rs
1182  .sp  .sp
1183  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1184  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
1185  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1186    The option letters are
1187  .sp  .sp
1188    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1189    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1197  PCRE_MULTILINE while unsetting PCRE_DOTA
1197  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
1198  unset.  unset.
1199  .P  .P
1200  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
1201  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
1202  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1203  the global options (and it will therefore show up in data extracted by the  .P
1204  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1205    subpattern parentheses), the change applies to the remainder of the pattern
1206    that follows. If the change is placed right at the start of a pattern, PCRE
1207    extracts it into the global options (and it will therefore show up in data
1208    extracted by the \fBpcre_fullinfo()\fP function).
1209  .P  .P
1210  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1211  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
1212  .sp  .sp
1213    (a(?i)b)c    (a(?i)b)c
1214  .sp  .sp
# Line 869  branch is abandoned before the option se Line 1224  branch is abandoned before the option se
1224  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1225  behaviour otherwise.  behaviour otherwise.
1226  .P  .P
1227  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
1228  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
1229  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1230    the application has set or what has been defaulted. Details are given in the
1231    section entitled
1232    .\" HTML <a href="#newlineseq">
1233    .\" </a>
1234    "Newline sequences"
1235    .\"
1236    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1237    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1238    PCRE_UTF8 and the PCRE_UCP options, respectively.
1239  .  .
1240  .  .
1241  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 885  Turning part of a pattern into a subpatt Line 1249  Turning part of a pattern into a subpatt
1249  .sp  .sp
1250    cat(aract|erpillar|)    cat(aract|erpillar|)
1251  .sp  .sp
1252  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1253  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1254  .sp  .sp
1255  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
1256  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
1257  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
1258  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1259  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1260  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1261  .sp  .sp
1262    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1263  .sp  .sp
# Line 926  is reached, an option setting in one bra Line 1289  is reached, an option setting in one bra
1289  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1290  .  .
1291  .  .
1292    .\" HTML <a name="dupsubpatternnumber"></a>
1293    .SH "DUPLICATE SUBPATTERN NUMBERS"
1294    .rs
1295    .sp
1296    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1297    the same numbers for its capturing parentheses. Such a subpattern starts with
1298    (?| and is itself a non-capturing subpattern. For example, consider this
1299    pattern:
1300    .sp
1301      (?|(Sat)ur|(Sun))day
1302    .sp
1303    Because the two alternatives are inside a (?| group, both sets of capturing
1304    parentheses are numbered one. Thus, when the pattern matches, you can look
1305    at captured substring number one, whichever alternative matched. This construct
1306    is useful when you want to capture part, but not all, of one of a number of
1307    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1308    number is reset at the start of each branch. The numbers of any capturing
1309    parentheses that follow the subpattern start after the highest number used in
1310    any branch. The following example is taken from the Perl documentation. The
1311    numbers underneath show in which buffer the captured content will be stored.
1312    .sp
1313      # before  ---------------branch-reset----------- after
1314      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1315      # 1            2         2  3        2     3     4
1316    .sp
1317    A back reference to a numbered subpattern uses the most recent value that is
1318    set for that number by any subpattern. The following pattern matches "abcabc"
1319    or "defdef":
1320    .sp
1321      /(?|(abc)|(def))\e1/
1322    .sp
1323    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1324    refers to the first one in the pattern with the given number. The following
1325    pattern matches "abcabc" or "defabc":
1326    .sp
1327      /(?|(abc)|(def))(?1)/
1328    .sp
1329    If a
1330    .\" HTML <a href="#conditions">
1331    .\" </a>
1332    condition test
1333    .\"
1334    for a subpattern's having matched refers to a non-unique number, the test is
1335    true if any of the subpatterns of that number have matched.
1336    .P
1337    An alternative approach to using this "branch reset" feature is to use
1338    duplicate named subpatterns, as described in the next section.
1339    .
1340    .
1341  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1342  .rs  .rs
1343  .sp  .sp
# Line 935  if an expression is modified, the number Line 1347  if an expression is modified, the number
1347  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1348  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
1349  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
1350  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1351    have different names, but PCRE does not.
1352  .P  .P
1353  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
1354  (?'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
1355  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1356  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1357  .\" </a>  .\" </a>
1358  backreferences,  back references,
1359  .\"  .\"
1360  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1361  .\" </a>  .\" </a>
# Line 962  extracting the name-to-number translatio Line 1375  extracting the name-to-number translatio
1375  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1376  .P  .P
1377  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
1378  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
1379  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
1380  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
1381  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
1382  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
1383    name, and in both cases you want to extract the abbreviation. This pattern
1384    (ignoring the line breaks) does the job:
1385  .sp  .sp
1386    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1387    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 975  abbreviation. This pattern (ignoring the Line 1390  abbreviation. This pattern (ignoring the
1390    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1391  .sp  .sp
1392  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.
1393    (An alternative way of solving this problem is to use a "branch reset"
1394    subpattern, as described in the previous section.)
1395    .P
1396  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1397  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
1398  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1399  make a reference to a non-unique named subpattern from elsewhere in the  .P
1400  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
1401  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1402    used. In the absence of duplicate numbers (see the previous section) this is
1403    the one with the lowest number. If you use a named reference in a condition
1404    test (see the
1405    .\"
1406    .\" HTML <a href="#conditions">
1407    .\" </a>
1408    section about conditions
1409    .\"
1410    below), either to check whether a subpattern has matched, or to check for
1411    recursion, all subpatterns with the same name are tested. If the condition is
1412    true for any one of them, the overall condition is true. This is the same
1413    behaviour as testing by number. For further details of the interfaces for
1414    handling named subpatterns, see the
1415  .\" HREF  .\" HREF
1416  \fBpcreapi\fP  \fBpcreapi\fP
1417  .\"  .\"
1418  documentation.  documentation.
1419    .P
1420    \fBWarning:\fP You cannot use different names to distinguish between two
1421    subpatterns with the same number because PCRE uses only the numbers when
1422    matching. For this reason, an error is given at compile time if different names
1423    are given to subpatterns with the same number. However, you can give the same
1424    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1425  .  .
1426  .  .
1427  .SH REPETITION  .SH REPETITION
# Line 998  items: Line 1435  items:
1435    the \eC escape sequence    the \eC escape sequence
1436    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1437    the \eR escape sequence    the \eR escape sequence
1438    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1439    a character class    a character class
1440    a back reference (see next section)    a back reference (see next section)
1441    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1442      a recursive or "subroutine" call to a subpattern
1443  .sp  .sp
1444  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1445  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1033  support is available, \eX{3} matches thr Line 1471  support is available, \eX{3} matches thr
1471  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).
1472  .P  .P
1473  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
1474  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1475    subpatterns that are referenced as
1476    .\" HTML <a href="#subpatternsassubroutines">
1477    .\" </a>
1478    subroutines
1479    .\"
1480    from elsewhere in the pattern (but see also the section entitled
1481    .\" HTML <a href="#subdefine">
1482    .\" </a>
1483    "Defining subpatterns for use by reference only"
1484    .\"
1485    below). Items other than subpatterns that have a {0} quantifier are omitted
1486    from the compiled pattern.
1487  .P  .P
1488  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1489  abbreviations:  abbreviations:
# Line 1105  worth setting PCRE_DOTALL in order to ob Line 1555  worth setting PCRE_DOTALL in order to ob
1555  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1556  .P  .P
1557  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1558  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1559  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
1560  succeeds. Consider, for example:  succeeds. Consider, for example:
1561  .sp  .sp
# Line 1180  previous example can be rewritten as Line 1630  previous example can be rewritten as
1630  .sp  .sp
1631    \ed++foo    \ed++foo
1632  .sp  .sp
1633    Note that a possessive quantifier can be used with an entire group, for
1634    example:
1635    .sp
1636      (abc|xyz){2,3}+
1637    .sp
1638  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1639  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
1640  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 1253  no such problem when named parentheses a Line 1708  no such problem when named parentheses a
1708  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1709  .P  .P
1710  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
1711  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
1712  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
1713  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1714  .sp  .sp
1715    (ring), \e1    (ring), \e1
1716    (ring), \eg1    (ring), \eg1
1717    (ring), \eg{1}    (ring), \eg{1}
1718  .sp  .sp
1719  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1720  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
1721  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1722    example:
1723  .sp  .sp
1724    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1725  .sp  .sp
1726  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
1727  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.
1728  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
1729  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
1730  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1731  .P  .P
1732  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1733  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1293  back reference, the case of letters is r Line 1749  back reference, the case of letters is r
1749  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
1750  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1751  .P  .P
1752  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
1753  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
1754    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1755    back reference syntax, in which \eg can be used for both numeric and named
1756    references, is also supported. We could rewrite the above example in any of
1757  the following ways:  the following ways:
1758  .sp  .sp
1759    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1760      (?'p1'(?i)rah)\es+\ek{p1}
1761    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1762      (?<p1>(?i)rah)\es+\eg{p1}
1763  .sp  .sp
1764  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
1765  after the reference.  after the reference.
1766  .P  .P
1767  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
1768  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1769  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1770  .sp  .sp
1771    (a|(bc))\e2    (a|(bc))\e2
1772  .sp  .sp
1773  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
1774  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
1775  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1776  with a digit character, some delimiter must be used to terminate the back  .P
1777  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1778  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1779    If the pattern continues with a digit character, some delimiter must be used to
1780    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1781    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1782  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1783  .\" </a>  .\" </a>
1784  "Comments"  "Comments"
1785  .\"  .\"
1786  below) can be used.  below) can be used.
1787  .P  .
1788    .SS "Recursive back references"
1789    .rs
1790    .sp
1791  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
1792  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.
1793  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1334  to the previous iteration. In order for Line 1801  to the previous iteration. In order for
1801  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
1802  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
1803  minimum of zero.  minimum of zero.
1804    .P
1805    Back references of this type cause the group that they reference to be treated
1806    as an
1807    .\" HTML <a href="#atomicgroup">
1808    .\" </a>
1809    atomic group.
1810    .\"
1811    Once the whole group has been matched, a subsequent matching failure cannot
1812    cause backtracking into the middle of the group.
1813  .  .
1814  .  .
1815  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1353  those that look ahead of the current pos Line 1829  those that look ahead of the current pos
1829  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1830  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1831  .P  .P
1832  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1833  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1834  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1835  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1836  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1837  because it does not make sense for negative assertions.  .P
1838    For compatibility with Perl, assertion subpatterns may be repeated; though
1839    it makes no sense to assert the same thing several times, the side effect of
1840    capturing parentheses may occasionally be useful. In practice, there only three
1841    cases:
1842    .sp
1843    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1844    However, it may contain internal capturing parenthesized groups that are called
1845    from elsewhere via the
1846    .\" HTML <a href="#subpatternsassubroutines">
1847    .\" </a>
1848    subroutine mechanism.
1849    .\"
1850    .sp
1851    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1852    were {0,1}. At run time, the rest of the pattern match is tried with and
1853    without the assertion, the order depending on the greediness of the quantifier.
1854    .sp
1855    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1856    The assertion is obeyed just once when encountered during matching.
1857  .  .
1858  .  .
1859  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1387  lookbehind assertion is needed to achiev Line 1882  lookbehind assertion is needed to achiev
1882  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
1883  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
1884  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.
1885    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1886  .  .
1887  .  .
1888  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1411  is permitted, but Line 1907  is permitted, but
1907  .sp  .sp
1908  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1909  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
1910  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
1911  match the same length of string. An assertion such as  length of string. An assertion such as
1912  .sp  .sp
1913    (?<=ab(c|de))    (?<=ab(c|de))
1914  .sp  .sp
1915  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
1916  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
1917    branches:
1918  .sp  .sp
1919    (?<=abc|abde)    (?<=abc|abde)
1920  .sp  .sp
1921    In some cases, the escape sequence \eK
1922    .\" HTML <a href="#resetmatchstart">
1923    .\" </a>
1924    (see above)
1925    .\"
1926    can be used instead of a lookbehind assertion to get round the fixed-length
1927    restriction.
1928    .P
1929  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1930  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
1931  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1431  to appear in lookbehind assertions, beca Line 1936  to appear in lookbehind assertions, beca
1936  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
1937  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1938  .P  .P
1939    .\" HTML <a href="#subpatternsassubroutines">
1940    .\" </a>
1941    "Subroutine"
1942    .\"
1943    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1944    as the subpattern matches a fixed-length string.
1945    .\" HTML <a href="#recursion">
1946    .\" </a>
1947    Recursion,
1948    .\"
1949    however, is not supported.
1950    .P
1951  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1952  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
1953  pattern such as  strings. Consider a simple pattern such as
1954  .sp  .sp
1955    abcd$    abcd$
1956  .sp  .sp
# Line 1497  characters that are not "999". Line 2014  characters that are not "999".
2014  .sp  .sp
2015  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2016  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2017  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2018  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2019  .sp  .sp
2020    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2021    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2022  .sp  .sp
2023  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2024  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
2025  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2026    itself contain nested subpatterns of any form, including conditional
2027    subpatterns; the restriction to two alternatives applies only at the level of
2028    the condition. This pattern fragment is an example where the alternatives are
2029    complex:
2030    .sp
2031      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2032    .sp
2033  .P  .P
2034  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2035  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1514  recursion, a pseudo-condition called DEF Line 2038  recursion, a pseudo-condition called DEF
2038  .rs  .rs
2039  .sp  .sp
2040  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
2041  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2042  matched.  matched. If there is more than one capturing subpattern with the same number
2043    (see the earlier
2044    .\"
2045    .\" HTML <a href="#recursion">
2046    .\" </a>
2047    section about duplicate subpattern numbers),
2048    .\"
2049    the condition is true if any of them have matched. An alternative notation is
2050    to precede the digits with a plus or minus sign. In this case, the subpattern
2051    number is relative rather than absolute. The most recently opened parentheses
2052    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2053    loops it can also make sense to refer to subsequent groups. The next
2054    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2055    zero in any of these forms is not used; it provokes a compile-time error.)
2056  .P  .P
2057  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2058  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 1526  three parts for ease of discussion: Line 2063  three parts for ease of discussion:
2063  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2064  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
2065  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
2066  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2067  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,
2068  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
2069  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2070  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2071  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2072    .P
2073    If you were embedding this pattern in a larger one, you could use a relative
2074    reference:
2075    .sp
2076      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2077    .sp
2078    This makes the fragment independent of the parentheses in the larger pattern.
2079  .  .
2080  .SS "Checking for a used subpattern by name"  .SS "Checking for a used subpattern by name"
2081  .rs  .rs
# Line 1549  Rewriting the above example to use a nam Line 2093  Rewriting the above example to use a nam
2093  .sp  .sp
2094    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2095  .sp  .sp
2096    If the name used in a condition of this kind is a duplicate, the test is
2097    applied to all subpatterns of the same name, and is true if any one of them has
2098    matched.
2099  .  .
2100  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2101  .rs  .rs
# Line 1560  letter R, for example: Line 2107  letter R, for example:
2107  .sp  .sp
2108    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2109  .sp  .sp
2110  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
2111  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
2112  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2113    applied to all subpatterns of the same name, and is true if any one of them is
2114    the most recent recursion.
2115  .P  .P
2116  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2117  patterns are described below.  .\" HTML <a href="#recursion">
2118    .\" </a>
2119    The syntax for recursive patterns
2120    .\"
2121    is described below.
2122  .  .
2123    .\" HTML <a name="subdefine"></a>
2124  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2125  .rs  .rs
2126  .sp  .sp
# Line 1574  If the condition is the string (DEFINE), Line 2128  If the condition is the string (DEFINE),
2128  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
2129  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2130  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
2131  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
2132  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2133  written like this (ignore whitespace and line breaks):  .\" </a>
2134    "subroutines"
2135    .\"
2136    is described below.) For example, a pattern to match an IPv4 address such as
2137    "192.168.23.245" could be written like this (ignore whitespace and line
2138    breaks):
2139  .sp  .sp
2140    (?(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) )
2141    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1584  written like this (ignore whitespace and Line 2143  written like this (ignore whitespace and
2143  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
2144  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2145  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
2146  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2147  .P  pattern uses references to the named group to match the four dot-separated
2148  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.  
2149  .  .
2150  .SS "Assertion conditions"  .SS "Assertion conditions"
2151  .rs  .rs
# Line 1613  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2170  dd-aaa-dd or dd-dd-dd, where aaa are let
2170  .SH COMMENTS  .SH COMMENTS
2171  .rs  .rs
2172  .sp  .sp
2173  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
2174  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,
2175  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
2176    subpattern name or number. The characters that make up a comment play no part
2177    in the pattern matching.
2178  .P  .P
2179  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
2180  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2181  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2182    this case continues to immediately after the next newline character or
2183    character sequence in the pattern. Which characters are interpreted as newlines
2184    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2185    sequence at the start of the pattern, as described in the section entitled
2186    .\" HTML <a href="#newlines">
2187    .\" </a>
2188    "Newline conventions"
2189    .\"
2190    above. Note that the end of this type of comment is a literal newline sequence
2191    in the pattern; escape sequences that happen to represent a newline do not
2192    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2193    default newline convention is in force:
2194    .sp
2195      abc #comment \en still comment
2196    .sp
2197    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2198    a newline in the pattern. The sequence \en is still literal at this stage, so
2199    it does not terminate the comment. Only an actual character with the code value
2200    0x0a (the default newline) does so.
2201  .  .
2202  .  .
2203  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1645  recursively to the pattern in which it a Line 2223  recursively to the pattern in which it a
2223  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2224  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2225  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2226  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.
2227  .P  .P
2228  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
2229  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,
2230  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
2231    .\" HTML <a href="#subpatternsassubroutines">
2232    .\" </a>
2233    "subroutine"
2234    .\"
2235  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
2236  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2237  .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  
2238  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2239  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2240  .sp  .sp
2241    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2242  .sp  .sp
2243  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2244  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
2245  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2246  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2247    to avoid backtracking into sequences of non-parentheses.
2248  .P  .P
2249  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
2250  pattern, so instead you could use this:  pattern, so instead you could use this:
2251  .sp  .sp
2252    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2253  .sp  .sp
2254  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
2255  them instead of the whole pattern.  them instead of the whole pattern.
2256  .P  .P
2257  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
2258  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
2259  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
2260  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2261  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.  
2262  .P  .P
2263  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2264  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2265  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2266  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2267    .\" </a>
2268    "subroutine"
2269    .\"
2270    calls, as described in the next section.
2271  .P  .P
2272  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2273  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
2274  could rewrite the above example as follows:  could rewrite the above example as follows:
2275  .sp  .sp
2276    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2277  .sp  .sp
2278  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
2279  used.  used.
2280  .P  .P
2281  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2282  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
2283  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
2284  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2285  .sp  .sp
2286    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2287  .sp  .sp
2288  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,
2289  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
2290  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
2291  before failure can be reported.  before failure can be reported.
2292  .P  .P
2293  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
2294  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
2295  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  
2296  .\" HREF  .\" HREF
2297  \fBpcrecallout\fP  \fBpcrecallout\fP
2298  .\"  .\"
# Line 1720  documentation). If the pattern above is Line 2300  documentation). If the pattern above is
2300  .sp  .sp
2301    (ab(cd)ef)    (ab(cd)ef)
2302  .sp  .sp
2303  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
2304  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
2305  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2306    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2307       ^                        ^  .P
2308       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2309  .sp  obtain extra memory to store data during a recursion, which it does by using
2310  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
2311  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.  
2312  .P  .P
2313  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.
2314  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1745  different alternatives for the recursive Line 2322  different alternatives for the recursive
2322  is the actual recursive call.  is the actual recursive call.
2323  .  .
2324  .  .
2325    .\" HTML <a name="recursiondifference"></a>
2326    .SS "Recursion difference from Perl"
2327    .rs
2328    .sp
2329    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2330    treated as an atomic group. That is, once it has matched some of the subject
2331    string, it is never re-entered, even if it contains untried alternatives and
2332    there is a subsequent matching failure. This can be illustrated by the
2333    following pattern, which purports to match a palindromic string that contains
2334    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2335    .sp
2336      ^(.|(.)(?1)\e2)$
2337    .sp
2338    The idea is that it either matches a single character, or two identical
2339    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2340    it does not if the pattern is longer than three characters. Consider the
2341    subject string "abcba":
2342    .P
2343    At the top level, the first character is matched, but as it is not at the end
2344    of the string, the first alternative fails; the second alternative is taken
2345    and the recursion kicks in. The recursive call to subpattern 1 successfully
2346    matches the next character ("b"). (Note that the beginning and end of line
2347    tests are not part of the recursion).
2348    .P
2349    Back at the top level, the next character ("c") is compared with what
2350    subpattern 2 matched, which was "a". This fails. Because the recursion is
2351    treated as an atomic group, there are now no backtracking points, and so the
2352    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2353    try the second alternative.) However, if the pattern is written with the
2354    alternatives in the other order, things are different:
2355    .sp
2356      ^((.)(?1)\e2|.)$
2357    .sp
2358    This time, the recursing alternative is tried first, and continues to recurse
2359    until it runs out of characters, at which point the recursion fails. But this
2360    time we do have another alternative to try at the higher level. That is the big
2361    difference: in the previous case the remaining alternative is at a deeper
2362    recursion level, which PCRE cannot use.
2363    .P
2364    To change the pattern so that it matches all palindromic strings, not just
2365    those with an odd number of characters, it is tempting to change the pattern to
2366    this:
2367    .sp
2368      ^((.)(?1)\e2|.?)$
2369    .sp
2370    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2371    deeper recursion has matched a single character, it cannot be entered again in
2372    order to match an empty string. The solution is to separate the two cases, and
2373    write out the odd and even cases as alternatives at the higher level:
2374    .sp
2375      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2376    .sp
2377    If you want to match typical palindromic phrases, the pattern has to ignore all
2378    non-word characters, which can be done like this:
2379    .sp
2380      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2381    .sp
2382    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2383    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2384    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2385    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2386    more) to match typical phrases, and Perl takes so long that you think it has
2387    gone into a loop.
2388    .P
2389    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2390    string does not start with a palindrome that is shorter than the entire string.
2391    For example, although "abcba" is correctly matched, if the subject is "ababa",
2392    PCRE finds the palindrome "aba" at the start, then fails at top level because
2393    the end of the string does not follow. Once again, it cannot jump back into the
2394    recursion to try other alternatives, so the entire match fails.
2395    .
2396    .
2397  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2398  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2399  .rs  .rs
# Line 1757  relative, as in these examples: Line 2406  relative, as in these examples:
2406  .sp  .sp
2407    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2408    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2409    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2410  .sp  .sp
2411  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2412  .sp  .sp
# Line 1771  matches "sense and sensibility" and "res Line 2420  matches "sense and sensibility" and "res
2420  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
2421  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2422  .P  .P
2423  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2424  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
2425  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
2426  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2427    call revert to their previous values afterwards.
2428  .P  .P
2429  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2430  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 1786  It matches "abcabc". It does not match " Line 2436  It matches "abcabc". It does not match "
2436  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2437  .  .
2438  .  .
2439    .\" HTML <a name="onigurumasubroutines"></a>
2440    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2441    .rs
2442    .sp
2443    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2444    a number enclosed either in angle brackets or single quotes, is an alternative
2445    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2446    are two of the examples used above, rewritten using this syntax:
2447    .sp
2448      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2449      (sens|respons)e and \eg'1'ibility
2450    .sp
2451    PCRE supports an extension to Oniguruma: if a number is preceded by a
2452    plus or a minus sign it is taken as a relative reference. For example:
2453    .sp
2454      (abc)(?i:\eg<-1>)
2455    .sp
2456    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2457    synonymous. The former is a back reference; the latter is a subroutine call.
2458    .
2459    .
2460  .SH CALLOUTS  .SH CALLOUTS
2461  .rs  .rs
2462  .sp  .sp
# Line 1822  description of the interface to the call Line 2493  description of the interface to the call
2493  documentation.  documentation.
2494  .  .
2495  .  .
2496    .\" HTML <a name="backtrackcontrol"></a>
2497    .SH "BACKTRACKING CONTROL"
2498    .rs
2499    .sp
2500    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2501    are described in the Perl documentation as "experimental and subject to change
2502    or removal in a future version of Perl". It goes on to say: "Their usage in
2503    production code should be noted to avoid problems during upgrades." The same
2504    remarks apply to the PCRE features described in this section.
2505    .P
2506    Since these verbs are specifically related to backtracking, most of them can be
2507    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2508    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2509    failing negative assertion, they cause an error if encountered by
2510    \fBpcre_dfa_exec()\fP.
2511    .P
2512    If any of these verbs are used in an assertion or subroutine subpattern
2513    (including recursive subpatterns), their effect is confined to that subpattern;
2514    it does not extend to the surrounding pattern, with one exception: a *MARK that
2515    is encountered in a positive assertion \fIis\fP passed back (compare capturing
2516    parentheses in assertions). Note that such subpatterns are processed as
2517    anchored at the point where they are tested.
2518    .P
2519    The new verbs make use of what was previously invalid syntax: an opening
2520    parenthesis followed by an asterisk. They are generally of the form
2521    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2522    depending on whether or not an argument is present. An name is a sequence of
2523    letters, digits, and underscores. If the name is empty, that is, if the closing
2524    parenthesis immediately follows the colon, the effect is as if the colon were
2525    not there. Any number of these verbs may occur in a pattern.
2526    .P
2527    PCRE contains some optimizations that are used to speed up matching by running
2528    some checks at the start of each match attempt. For example, it may know the
2529    minimum length of matching subject, or that a particular character must be
2530    present. When one of these optimizations suppresses the running of a match, any
2531    included backtracking verbs will not, of course, be processed. You can suppress
2532    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2533    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2534    pattern with (*NO_START_OPT).
2535    .
2536    .
2537    .SS "Verbs that act immediately"
2538    .rs
2539    .sp
2540    The following verbs act as soon as they are encountered. They may not be
2541    followed by a name.
2542    .sp
2543       (*ACCEPT)
2544    .sp
2545    This verb causes the match to end successfully, skipping the remainder of the
2546    pattern. When inside a recursion, only the innermost pattern is ended
2547    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2548    captured. (This feature was added to PCRE at release 8.00.) For example:
2549    .sp
2550      A((?:A|B(*ACCEPT)|C)D)
2551    .sp
2552    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2553    the outer parentheses.
2554    .sp
2555      (*FAIL) or (*F)
2556    .sp
2557    This verb causes the match to fail, forcing backtracking to occur. It is
2558    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2559    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2560    Perl features that are not present in PCRE. The nearest equivalent is the
2561    callout feature, as for example in this pattern:
2562    .sp
2563      a+(?C)(*FAIL)
2564    .sp
2565    A match with the string "aaaa" always fails, but the callout is taken before
2566    each backtrack happens (in this example, 10 times).
2567    .
2568    .
2569    .SS "Recording which path was taken"
2570    .rs
2571    .sp
2572    There is one verb whose main purpose is to track how a match was arrived at,
2573    though it also has a secondary use in conjunction with advancing the match
2574    starting point (see (*SKIP) below).
2575    .sp
2576      (*MARK:NAME) or (*:NAME)
2577    .sp
2578    A name is always required with this verb. There may be as many instances of
2579    (*MARK) as you like in a pattern, and their names do not have to be unique.
2580    .P
2581    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2582    to the caller via the \fIpcre_extra\fP data structure, as described in the
2583    .\" HTML <a href="pcreapi.html#extradata">
2584    .\" </a>
2585    section on \fIpcre_extra\fP
2586    .\"
2587    in the
2588    .\" HREF
2589    \fBpcreapi\fP
2590    .\"
2591    documentation. No data is returned for a partial match. Here is an example of
2592    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2593    outputting of (*MARK) data:
2594    .sp
2595      /X(*MARK:A)Y|X(*MARK:B)Z/K
2596      XY
2597       0: XY
2598      MK: A
2599      XZ
2600       0: XZ
2601      MK: B
2602    .sp
2603    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2604    indicates which of the two alternatives matched. This is a more efficient way
2605    of obtaining this information than putting each alternative in its own
2606    capturing parentheses.
2607    .P
2608    If (*MARK) is encountered in a positive assertion, its name is recorded and
2609    passed back if it is the last-encountered. This does not happen for negative
2610    assetions.
2611    .P
2612    A name may also be returned after a failed match if the final path through the
2613    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2614    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2615    starting point for matching is advanced, the final check is often with an empty
2616    string, causing a failure before (*MARK) is reached. For example:
2617    .sp
2618      /X(*MARK:A)Y|X(*MARK:B)Z/K
2619      XP
2620      No match
2621    .sp
2622    There are three potential starting points for this match (starting with X,
2623    starting with P, and with an empty string). If the pattern is anchored, the
2624    result is different:
2625    .sp
2626      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2627      XP
2628      No match, mark = B
2629    .sp
2630    PCRE's start-of-match optimizations can also interfere with this. For example,
2631    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2632    subject length for a match, a shorter subject will not be scanned at all.
2633    .P
2634    Note that similar anomalies (though different in detail) exist in Perl, no
2635    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2636    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2637    .
2638    .
2639    .SS "Verbs that act after backtracking"
2640    .rs
2641    .sp
2642    The following verbs do nothing when they are encountered. Matching continues
2643    with what follows, but if there is no subsequent match, causing a backtrack to
2644    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2645    the verb. However, when one of these verbs appears inside an atomic group, its
2646    effect is confined to that group, because once the group has been matched,
2647    there is never any backtracking into it. In this situation, backtracking can
2648    "jump back" to the left of the entire atomic group. (Remember also, as stated
2649    above, that this localization also applies in subroutine calls and assertions.)
2650    .P
2651    These verbs differ in exactly what kind of failure occurs when backtracking
2652    reaches them.
2653    .sp
2654      (*COMMIT)
2655    .sp
2656    This verb, which may not be followed by a name, causes the whole match to fail
2657    outright if the rest of the pattern does not match. Even if the pattern is
2658    unanchored, no further attempts to find a match by advancing the starting point
2659    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2660    finding a match at the current starting point, or not at all. For example:
2661    .sp
2662      a+(*COMMIT)b
2663    .sp
2664    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2665    dynamic anchor, or "I've started, so I must finish." The name of the most
2666    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2667    match failure.
2668    .P
2669    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2670    unless PCRE's start-of-match optimizations are turned off, as shown in this
2671    \fBpcretest\fP example:
2672    .sp
2673      /(*COMMIT)abc/
2674      xyzabc
2675       0: abc
2676      xyzabc\eY
2677      No match
2678    .sp
2679    PCRE knows that any match must start with "a", so the optimization skips along
2680    the subject to "a" before running the first match attempt, which succeeds. When
2681    the optimization is disabled by the \eY escape in the second subject, the match
2682    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2683    starting points.
2684    .sp
2685      (*PRUNE) or (*PRUNE:NAME)
2686    .sp
2687    This verb causes the match to fail at the current starting position in the
2688    subject if the rest of the pattern does not match. If the pattern is
2689    unanchored, the normal "bumpalong" advance to the next starting character then
2690    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2691    reached, or when matching to the right of (*PRUNE), but if there is no match to
2692    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2693    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2694    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2695    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2696    match fails completely; the name is passed back if this is the final attempt.
2697    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2698    pattern (*PRUNE) has the same effect as (*COMMIT).
2699    .sp
2700      (*SKIP)
2701    .sp
2702    This verb, when given without a name, is like (*PRUNE), except that if the
2703    pattern is unanchored, the "bumpalong" advance is not to the next character,
2704    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2705    signifies that whatever text was matched leading up to it cannot be part of a
2706    successful match. Consider:
2707    .sp
2708      a+(*SKIP)b
2709    .sp
2710    If the subject is "aaaac...", after the first match attempt fails (starting at
2711    the first character in the string), the starting point skips on to start the
2712    next attempt at "c". Note that a possessive quantifer does not have the same
2713    effect as this example; although it would suppress backtracking during the
2714    first match attempt, the second attempt would start at the second character
2715    instead of skipping on to "c".
2716    .sp
2717      (*SKIP:NAME)
2718    .sp
2719    When (*SKIP) has an associated name, its behaviour is modified. If the
2720    following pattern fails to match, the previous path through the pattern is
2721    searched for the most recent (*MARK) that has the same name. If one is found,
2722    the "bumpalong" advance is to the subject position that corresponds to that
2723    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2724    matching name is found, normal "bumpalong" of one character happens (the
2725    (*SKIP) is ignored).
2726    .sp
2727      (*THEN) or (*THEN:NAME)
2728    .sp
2729    This verb causes a skip to the next alternation in the innermost enclosing
2730    group if the rest of the pattern does not match. That is, it cancels pending
2731    backtracking, but only within the current alternation. Its name comes from the
2732    observation that it can be used for a pattern-based if-then-else block:
2733    .sp
2734      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2735    .sp
2736    If the COND1 pattern matches, FOO is tried (and possibly further items after
2737    the end of the group if FOO succeeds); on failure the matcher skips to the
2738    second alternative and tries COND2, without backtracking into COND1. The
2739    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2740    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2741    like (*PRUNE).
2742    .
2743    .P
2744    The above verbs provide four different "strengths" of control when subsequent
2745    matching fails. (*THEN) is the weakest, carrying on the match at the next
2746    alternation. (*PRUNE) comes next, failing the match at the current starting
2747    position, but allowing an advance to the next character (for an unanchored
2748    pattern). (*SKIP) is similar, except that the advance may be more than one
2749    character. (*COMMIT) is the strongest, causing the entire match to fail.
2750    .P
2751    If more than one is present in a pattern, the "stongest" one wins. For example,
2752    consider this pattern, where A, B, etc. are complex pattern fragments:
2753    .sp
2754      (A(*COMMIT)B(*THEN)C|D)
2755    .sp
2756    Once A has matched, PCRE is committed to this match, at the current starting
2757    position. If subsequently B matches, but C does not, the normal (*THEN) action
2758    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2759    overrides.
2760    .
2761    .
2762  .SH "SEE ALSO"  .SH "SEE ALSO"
2763  .rs  .rs
2764  .sp  .sp
2765  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2766    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2767  .  .
2768  .  .
2769  .SH AUTHOR  .SH AUTHOR
# Line 1842  Cambridge CB2 3QH, England. Line 2780  Cambridge CB2 3QH, England.
2780  .rs  .rs
2781  .sp  .sp
2782  .nf  .nf
2783  Last updated: 06 March 2007  Last updated: 24 July 2011
2784  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2785  .fi  .fi

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