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1  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"  .TH PCREPATTERN 3 "25 November 2013" "PCRE 8.34"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 20  have copious examples. Jeffrey Friedl's Line 20  have copious examples. Jeffrey Friedl's
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23    This document discusses the patterns that are supported by PCRE when one its
24    main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25    (16- or 32-bit), is used. PCRE also has alternative matching functions,
26    \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27    different algorithm that is not Perl-compatible. Some of the features discussed
28    below are not available when DFA matching is used. The advantages and
29    disadvantages of the alternative functions, and how they differ from the normal
30    functions, are discussed in the
31    .\" HREF
32    \fBpcrematching\fP
33    .\"
34    page.
35    .
36    .
37    .SH "SPECIAL START-OF-PATTERN ITEMS"
38    .rs
39    .sp
40    A number of options that can be passed to \fBpcre_compile()\fP can also be set
41    by special items at the start of a pattern. These are not Perl-compatible, but
42    are provided to make these options accessible to pattern writers who are not
43    able to change the program that processes the pattern. Any number of these
44    items may appear, but they must all be together right at the start of the
45    pattern string, and the letters must be in upper case.
46    .
47    .
48    .SS "UTF support"
49    .rs
50    .sp
51  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,
52  there is now also support for UTF-8 strings in the original library, and a  there is now also support for UTF-8 strings in the original library, an
53  second library that supports 16-bit and UTF-16 character strings. To use these  extra library that supports 16-bit and UTF-16 character strings, and a
54    third library that supports 32-bit and UTF-32 character strings. To use these
55  features, PCRE must be built to include appropriate support. When using UTF  features, PCRE must be built to include appropriate support. When using UTF
56  strings you must either call the compiling function with the PCRE_UTF8 or  strings you must either call the compiling function with the PCRE_UTF8,
57  PCRE_UTF16 option, or the pattern must start with one of these special  PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58  sequences:  these special sequences:
59  .sp  .sp
60    (*UTF8)    (*UTF8)
61    (*UTF16)    (*UTF16)
62      (*UTF32)
63      (*UTF)
64  .sp  .sp
65    (*UTF) is a generic sequence that can be used with any of the libraries.
66  Starting a pattern with such a sequence is equivalent to setting the relevant  Starting a pattern with such a sequence is equivalent to setting the relevant
67  option. This feature is not Perl-compatible. How setting a UTF mode affects  option. How setting a UTF mode affects pattern matching is mentioned in several
68  pattern matching is mentioned in several places below. There is also a summary  places below. There is also a summary of features in the
 of features in the  
69  .\" HREF  .\" HREF
70  \fBpcreunicode\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73  .P  .P
74  Another special sequence that may appear at the start of a pattern or in  Some applications that allow their users to supply patterns may wish to
75  combination with (*UTF8) or (*UTF16) is:  restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76  .sp  option is set at compile time, (*UTF) etc. are not allowed, and their
77    (*UCP)  appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82  .sp  .sp
83    Another special sequence that may appear at the start of a pattern is (*UCP).
84  This has the same effect as setting the PCRE_UCP option: it causes sequences  This has the same effect as setting the PCRE_UCP option: it causes sequences
85  such as \ed and \ew to use Unicode properties to determine character types,  such as \ed and \ew to use Unicode properties to determine character types,
86  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
87  table.  table.
88  .P  .
89    .
90    .SS "Disabling auto-possessification"
91    .rs
92    .sp
93    If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
94    the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
95    quantifiers possessive when what follows cannot match the repeated item. For
96    example, by default a+b is treated as a++b. For more details, see the
97    .\" HREF
98    \fBpcreapi\fP
99    .\"
100    documentation.
101    .
102    .
103    .SS "Disabling start-up optimizations"
104    .rs
105    .sp
106  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
107  PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are  PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
108  also some more of these special sequences that are concerned with the handling  several optimizations for quickly reaching "no match" results. For more
109  of newlines; they are described below.  details, see the
 .P  
 The remainder of this document discusses the patterns that are supported by  
 PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or  
 \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching  
 functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using  
 a different algorithm that is not Perl-compatible. Some of the features  
 discussed below are not available when DFA matching is used. The advantages and  
 disadvantages of the alternative functions, and how they differ from the normal  
 functions, are discussed in the  
110  .\" HREF  .\" HREF
111  \fBpcrematching\fP  \fBpcreapi\fP
112  .\"  .\"
113  page.  documentation.
114  .  .
115  .  .
116  .\" HTML <a name="newlines"></a>  .\" HTML <a name="newlines"></a>
117  .SH "NEWLINE CONVENTIONS"  .SS "Newline conventions"
118  .rs  .rs
119  .sp  .sp
120  PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
# Line 103  example, on a Unix system where LF is th Line 147  example, on a Unix system where LF is th
147    (*CR)a.b    (*CR)a.b
148  .sp  .sp
149  changes the convention to CR. That pattern matches "a\enb" because LF is no  changes the convention to CR. That pattern matches "a\enb" because LF is no
150  longer a newline. Note that these special settings, which are not  longer a newline. If more than one of these settings is present, the last one
 Perl-compatible, are recognized only at the very start of a pattern, and that  
 they must be in upper case. If more than one of them is present, the last one  
151  is used.  is used.
152  .P  .P
153  The newline convention affects the interpretation of the dot metacharacter when  The newline convention affects where the circumflex and dollar assertions are
154  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not  true. It also affects the interpretation of the dot metacharacter when
155  affect what the \eR escape sequence matches. By default, this is any Unicode  PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
156  newline sequence, for Perl compatibility. However, this can be changed; see the  what the \eR escape sequence matches. By default, this is any Unicode newline
157    sequence, for Perl compatibility. However, this can be changed; see the
158  description of \eR in the section entitled  description of \eR in the section entitled
159  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
160  .\" </a>  .\" </a>
# Line 121  below. A change of \eR setting can be co Line 164  below. A change of \eR setting can be co
164  convention.  convention.
165  .  .
166  .  .
167    .SS "Setting match and recursion limits"
168    .rs
169    .sp
170    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
171    internal \fBmatch()\fP function is called and on the maximum depth of
172    recursive calls. These facilities are provided to catch runaway matches that
173    are provoked by patterns with huge matching trees (a typical example is a
174    pattern with nested unlimited repeats) and to avoid running out of system stack
175    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
176    gives an error return. The limits can also be set by items at the start of the
177    pattern of the form
178    .sp
179      (*LIMIT_MATCH=d)
180      (*LIMIT_RECURSION=d)
181    .sp
182    where d is any number of decimal digits. However, the value of the setting must
183    be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
184    for it to have any effect. In other words, the pattern writer can lower the
185    limits set by the programmer, but not raise them. If there is more than one
186    setting of one of these limits, the lower value is used.
187    .
188    .
189    .SH "EBCDIC CHARACTER CODES"
190    .rs
191    .sp
192    PCRE can be compiled to run in an environment that uses EBCDIC as its character
193    code rather than ASCII or Unicode (typically a mainframe system). In the
194    sections below, character code values are ASCII or Unicode; in an EBCDIC
195    environment these characters may have different code values, and there are no
196    code points greater than 255.
197    .
198    .
199  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
200  .rs  .rs
201  .sp  .sp
# Line 198  In a UTF mode, only ASCII numbers and le Line 273  In a UTF mode, only ASCII numbers and le
273  backslash. All other characters (in particular, those whose codepoints are  backslash. All other characters (in particular, those whose codepoints are
274  greater than 127) are treated as literals.  greater than 127) are treated as literals.
275  .P  .P
276  If a pattern is compiled with the PCRE_EXTENDED option, white space in the  If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
277  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class), and characters between a # outside a
278  a character class and the next newline are ignored. An escaping backslash can  character class and the next newline, inclusive, are ignored. An escaping
279  be used to include a white space or # character as part of the pattern.  backslash can be used to include a white space or # character as part of the
280    pattern.
281  .P  .P
282  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
283  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 241  one of the following escape sequences th Line 317  one of the following escape sequences th
317    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
318    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
319    \et        tab (hex 09)    \et        tab (hex 09)
320      \e0dd      character with octal code 0dd
321    \eddd      character with octal code ddd, or back reference    \eddd      character with octal code ddd, or back reference
322      \eo{ddd..} character with octal code ddd..
323    \exhh      character with hex code hh    \exhh      character with hex code hh
324    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
325    \euhhhh    character with hex code hhhh (JavaScript mode only)    \euhhhh    character with hex code hhhh (JavaScript mode only)
326  .sp  .sp
327  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
328  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
329  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
330  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
331  than 127, a compile-time error occurs. This locks out non-ASCII characters in  data item (byte or 16-bit value) following \ec has a value greater than 127, a
332  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A  compile-time error occurs. This locks out non-ASCII characters in all modes.
333  lower case letter is converted to upper case, and then the 0xc0 bits are  .P
334  flipped.)  The \ec facility was designed for use with ASCII characters, but with the
335  .P  extension to Unicode it is even less useful than it once was. It is, however,
336  By default, after \ex, from zero to two hexadecimal digits are read (letters  recognized when PCRE is compiled in EBCDIC mode, where data items are always
337  can be in upper or lower case). Any number of hexadecimal digits may appear  bytes. In this mode, all values are valid after \ec. If the next character is a
338  between \ex{ and }, but the character code is constrained as follows:  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
339  .sp  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
340    8-bit non-UTF mode    less than 0x100  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341    8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint  characters also generate different values.
   16-bit non-UTF mode   less than 0x10000  
   16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint  
 .sp  
 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called  
 "surrogate" codepoints).  
 .P  
 If characters other than hexadecimal digits appear between \ex{ and }, or if  
 there is no terminating }, this form of escape is not recognized. Instead, the  
 initial \ex will be interpreted as a basic hexadecimal escape, with no  
 following digits, giving a character whose value is zero.  
 .P  
 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is  
 as just described only when it is followed by two hexadecimal digits.  
 Otherwise, it matches a literal "x" character. In JavaScript mode, support for  
 code points greater than 256 is provided by \eu, which must be followed by  
 four hexadecimal digits; otherwise it matches a literal "u" character.  
 .P  
 Characters whose value is less than 256 can be defined by either of the two  
 syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the  
 way they are handled. For example, \exdc is exactly the same as \ex{dc} (or  
 \eu00dc in JavaScript mode).  
342  .P  .P
343  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
344  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 289  specifies two binary zeros followed by a Line 346  specifies two binary zeros followed by a
346  sure you supply two digits after the initial zero if the pattern character that  sure you supply two digits after the initial zero if the pattern character that
347  follows is itself an octal digit.  follows is itself an octal digit.
348  .P  .P
349  The handling of a backslash followed by a digit other than 0 is complicated.  The escape \eo must be followed by a sequence of octal digits, enclosed in
350  Outside a character class, PCRE reads it and any following digits as a decimal  braces. An error occurs if this is not the case. This escape is a recent
351  number. If the number is less than 10, or if there have been at least that many  addition to Perl; it provides way of specifying character code points as octal
352    numbers greater than 0777, and it also allows octal numbers and back references
353    to be unambiguously specified.
354    .P
355    For greater clarity and unambiguity, it is best to avoid following \e by a
356    digit greater than zero. Instead, use \eo{} or \ex{} to specify character
357    numbers, and \eg{} to specify back references. The following paragraphs
358    describe the old, ambiguous syntax.
359    .P
360    The handling of a backslash followed by a digit other than 0 is complicated,
361    and Perl has changed in recent releases, causing PCRE also to change. Outside a
362    character class, PCRE reads the digit and any following digits as a decimal
363    number. If the number is less than 8, or if there have been at least that many
364  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
365  taken as a \fIback reference\fP. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
366  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
# Line 304  following the discussion of Line 373  following the discussion of
373  parenthesized subpatterns.  parenthesized subpatterns.
374  .\"  .\"
375  .P  .P
376  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number following \e is greater than
377  have not been that many capturing subpatterns, PCRE re-reads up to three octal  7 and there have not been that many capturing subpatterns, PCRE handles \e8 and
378  digits following the backslash, and uses them to generate a data character. Any  \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
379  subsequent digits stand for themselves. The value of the character is  octal digits following the backslash, using them to generate a data character.
380  constrained in the same way as characters specified in hexadecimal.  Any subsequent digits stand for themselves. For example:
 For example:  
381  .sp  .sp
382    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
383  .\" JOIN  .\" JOIN
384    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
385              previous capturing subpatterns              previous capturing subpatterns
# Line 328  For example: Line 396  For example:
396    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
397              the value 255 (decimal)              the value 255 (decimal)
398  .\" JOIN  .\" JOIN
399    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or the two
400              followed by the two characters "8" and "1"              characters "8" and "1"
401  .sp  .sp
402  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater that are specified using this syntax
403  zero, because no more than three octal digits are ever read.  must not be introduced by a leading zero, because no more than three octal
404    digits are ever read.
405    .P
406    By default, after \ex that is not followed by {, from zero to two hexadecimal
407    digits are read (letters can be in upper or lower case). Any number of
408    hexadecimal digits may appear between \ex{ and }. If a character other than
409    a hexadecimal digit appears between \ex{ and }, or if there is no terminating
410    }, an error occurs.
411  .P  .P
412    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
413    as just described only when it is followed by two hexadecimal digits.
414    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
415    code points greater than 256 is provided by \eu, which must be followed by
416    four hexadecimal digits; otherwise it matches a literal "u" character.
417    .P
418    Characters whose value is less than 256 can be defined by either of the two
419    syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
420    way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
421    \eu00dc in JavaScript mode).
422    .
423    .
424    .SS "Constraints on character values"
425    .rs
426    .sp
427    Characters that are specified using octal or hexadecimal numbers are
428    limited to certain values, as follows:
429    .sp
430      8-bit non-UTF mode    less than 0x100
431      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
432      16-bit non-UTF mode   less than 0x10000
433      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
434      32-bit non-UTF mode   less than 0x100000000
435      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
436    .sp
437    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
438    "surrogate" codepoints), and 0xffef.
439    .
440    .
441    .SS "Escape sequences in character classes"
442    .rs
443    .sp
444  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
445  and outside character classes. In addition, inside a character class, \eb is  and outside character classes. In addition, inside a character class, \eb is
446  interpreted as the backspace character (hex 08).  interpreted as the backspace character (hex 08).
# Line 424  classes. They each match one character o Line 531  classes. They each match one character o
531  matching point is at the end of the subject string, all of them fail, because  matching point is at the end of the subject string, all of them fail, because
532  there is no character to match.  there is no character to match.
533  .P  .P
534  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es did not used to match the VT character (code
535  This makes it different from the the POSIX "space" class. The \es characters  11), which made it different from the the POSIX "space" class. However, Perl
536  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is  added VT at release 5.18, and PCRE followed suit at release 8.34. The default
537  included in a Perl script, \es may match the VT character. In PCRE, it never  \es characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
538  does.  (32), which are defined as white space in the "C" locale. This list may vary if
539    locale-specific matching is taking place. For example, in some locales the
540    "non-breaking space" character (\exA0) is recognized as white space, and in
541    others the VT character is not.
542  .P  .P
543  A "word" character is an underscore or any character that is a letter or digit.  A "word" character is an underscore or any character that is a letter or digit.
544  By default, the definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
# Line 443  in the Line 553  in the
553  \fBpcreapi\fP  \fBpcreapi\fP
554  .\"  .\"
555  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,
556  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 127 are used for
557  accented letters, and these are then matched by \ew. The use of locales with  accented letters, and these are then matched by \ew. The use of locales with
558  Unicode is discouraged.  Unicode is discouraged.
559  .P  .P
560  By default, in a UTF mode, characters with values greater than 128 never match  By default, characters whose code points are greater than 127 never match \ed,
561  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \es, or \ew, and always match \eD, \eS, and \eW, although this may vary for
562  their original meanings from before UTF support was available, mainly for  characters in the range 128-255 when locale-specific matching is happening.
563  efficiency reasons. However, if PCRE is compiled with Unicode property support,  These escape sequences retain their original meanings from before Unicode
564  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  support was available, mainly for efficiency reasons. If PCRE is compiled with
565  properties are used to determine character types, as follows:  Unicode property support, and the PCRE_UCP option is set, the behaviour is
566  .sp  changed so that Unicode properties are used to determine character types, as
567    \ed  any character that \ep{Nd} matches (decimal digit)  follows:
568    \es  any character that \ep{Z} matches, plus HT, LF, FF, CR  .sp
569    \ew  any character that \ep{L} or \ep{N} matches, plus underscore    \ed  any character that matches \ep{Nd} (decimal digit)
570      \es  any character that matches \ep{Z} or \eh or \ev
571      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
572  .sp  .sp
573  The upper case escapes match the inverse sets of characters. Note that \ed  The upper case escapes match the inverse sets of characters. Note that \ed
574  matches only decimal digits, whereas \ew matches any Unicode digit, as well as  matches only decimal digits, whereas \ew matches any Unicode digit, as well as
# Line 466  is noticeably slower when PCRE_UCP is se Line 578  is noticeably slower when PCRE_UCP is se
578  .P  .P
579  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
580  release 5.10. In contrast to the other sequences, which match only ASCII  release 5.10. In contrast to the other sequences, which match only ASCII
581  characters by default, these always match certain high-valued codepoints,  characters by default, these always match certain high-valued code points,
582  whether or not PCRE_UCP is set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
583  .sp  .sp
584    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
585    U+0020     Space    U+0020     Space
586    U+00A0     Non-break space    U+00A0     Non-break space
587    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 491  whether or not PCRE_UCP is set. The hori Line 603  whether or not PCRE_UCP is set. The hori
603  .sp  .sp
604  The vertical space characters are:  The vertical space characters are:
605  .sp  .sp
606    U+000A     Linefeed    U+000A     Linefeed (LF)
607    U+000B     Vertical tab    U+000B     Vertical tab (VT)
608    U+000C     Form feed    U+000C     Form feed (FF)
609    U+000D     Carriage return    U+000D     Carriage return (CR)
610    U+0085     Next line    U+0085     Next line (NEL)
611    U+2028     Line separator    U+2028     Line separator
612    U+2029     Paragraph separator    U+2029     Paragraph separator
613  .sp  .sp
# Line 549  change of newline convention; for exampl Line 661  change of newline convention; for exampl
661  .sp  .sp
662    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
663  .sp  .sp
664  They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
665  sequences. Inside a character class, \eR is treated as an unrecognized escape  (*UCP) special sequences. Inside a character class, \eR is treated as an
666  sequence, and so matches the letter "R" by default, but causes an error if  unrecognized escape sequence, and so matches the letter "R" by default, but
667  PCRE_EXTRA is set.  causes an error if PCRE_EXTRA is set.
668  .  .
669  .  .
670  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 567  The extra escape sequences are: Line 679  The extra escape sequences are:
679  .sp  .sp
680    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
681    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
682    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
683  .sp  .sp
684  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
685  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 760  a modifier or "other". Line 872  a modifier or "other".
872  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
873  U+DFFF. Such characters are not valid in Unicode strings and so  U+DFFF. Such characters are not valid in Unicode strings and so
874  cannot be tested by PCRE, unless UTF validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
875  (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
876    PCRE_NO_UTF32_CHECK in the
877  .\" HREF  .\" HREF
878  \fBpcreapi\fP  \fBpcreapi\fP
879  .\"  .\"
# Line 775  Instead, this property is assumed for an Line 888  Instead, this property is assumed for an
888  Unicode table.  Unicode table.
889  .P  .P
890  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
891  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
892    the behaviour of current versions of Perl.
893  .P  .P
894  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
895  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
896  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
897    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
898    PCRE_UCP option or by starting the pattern with (*UCP).
899    .
900    .
901    .SS Extended grapheme clusters
902    .rs
903  .sp  .sp
904  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
905  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
906  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
907  .\" </a>  .\" </a>
908  (see below).  (see below).
909  .\"  .\"
910  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
911  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
912  8-bit non-UTF-8 mode \eX matches any one character.  .sp
913  .P    (?>\ePM\epM*)
914  Note that recent versions of Perl have changed \eX to match what Unicode calls  .sp
915  an "extended grapheme cluster", which has a more complicated definition.  That is, it matched a character without the "mark" property, followed by zero
916  .P  or more characters with the "mark" property. Characters with the "mark"
917  Matching characters by Unicode property is not fast, because PCRE has to search  property are typically non-spacing accents that affect the preceding character.
918  a structure that contains data for over fifteen thousand characters. That is  .P
919  why the traditional escape sequences such as \ed and \ew do not use Unicode  This simple definition was extended in Unicode to include more complicated
920  properties in PCRE by default, though you can make them do so by setting the  kinds of composite character by giving each character a grapheme breaking
921  PCRE_UCP option or by starting the pattern with (*UCP).  property, and creating rules that use these properties to define the boundaries
922    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
923    one of these clusters.
924    .P
925    \eX always matches at least one character. Then it decides whether to add
926    additional characters according to the following rules for ending a cluster:
927    .P
928    1. End at the end of the subject string.
929    .P
930    2. Do not end between CR and LF; otherwise end after any control character.
931    .P
932    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
933    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
934    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
935    character; an LVT or T character may be follwed only by a T character.
936    .P
937    4. Do not end before extending characters or spacing marks. Characters with
938    the "mark" property always have the "extend" grapheme breaking property.
939    .P
940    5. Do not end after prepend characters.
941    .P
942    6. Otherwise, end the cluster.
943  .  .
944  .  .
945  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
946  .SS PCRE's additional properties  .SS PCRE's additional properties
947  .rs  .rs
948  .sp  .sp
949  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
950  section, PCRE supports four more that make it possible to convert traditional  more that make it possible to convert traditional escape sequences such as \ew
951  escape sequences such as \ew and \es and POSIX character classes to use Unicode  and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
952  properties. PCRE uses these non-standard, non-Perl properties internally when  properties internally when PCRE_UCP is set. However, they may also be used
953  PCRE_UCP is set. They are:  explicitly. These properties are:
954  .sp  .sp
955    Xan   Any alphanumeric character    Xan   Any alphanumeric character
956    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 821  PCRE_UCP is set. They are: Line 960  PCRE_UCP is set. They are:
960  Xan matches characters that have either the L (letter) or the N (number)  Xan matches characters that have either the L (letter) or the N (number)
961  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
962  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
963  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps; it used to exclude vertical tab, for Perl
964  same characters as Xan, plus underscore.  compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
965    matches the same characters as Xan, plus underscore.
966    .P
967    There is another non-standard property, Xuc, which matches any character that
968    can be represented by a Universal Character Name in C++ and other programming
969    languages. These are the characters $, @, ` (grave accent), and all characters
970    with Unicode code points greater than or equal to U+00A0, except for the
971    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
972    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
973    where H is a hexadecimal digit. Note that the Xuc property does not match these
974    sequences but the characters that they represent.)
975  .  .
976  .  .
977  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 928  regular expression. Line 1077  regular expression.
1077  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1078  .rs  .rs
1079  .sp  .sp
1080    The circumflex and dollar metacharacters are zero-width assertions. That is,
1081    they test for a particular condition being true without consuming any
1082    characters from the subject string.
1083    .P
1084  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1085  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
1086  at the start of the subject string. If the \fIstartoffset\fP argument of  the start of the subject string. If the \fIstartoffset\fP argument of
1087  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1088  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1089  meaning  meaning
# Line 947  constrained to match only at the start o Line 1100  constrained to match only at the start o
1100  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1101  to be anchored.)  to be anchored.)
1102  .P  .P
1103  A dollar character is an assertion that is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1104  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1105  at the end of the string (by default). Dollar need not be the last character of  the end of the string (by default). Note, however, that it does not actually
1106  the pattern if a number of alternatives are involved, but it should be the last  match the newline. Dollar need not be the last character of the pattern if a
1107  item in any branch in which it appears. Dollar has no special meaning in a  number of alternatives are involved, but it should be the last item in any
1108  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1109  .P  .P
1110  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1111  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
# Line 1013  name; PCRE does not support this. Line 1166  name; PCRE does not support this.
1166  .sp  .sp
1167  Outside a character class, the escape sequence \eC matches any one data unit,  Outside a character class, the escape sequence \eC matches any one data unit,
1168  whether or not a UTF mode is set. In the 8-bit library, one data unit is one  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1169  byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always  byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1170    a 32-bit unit. Unlike a dot, \eC always
1171  matches line-ending characters. The feature is provided in Perl in order to  matches line-ending characters. The feature is provided in Perl in order to
1172  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1173  used. Because \eC breaks up characters into individual data units, matching one  used. Because \eC breaks up characters into individual data units, matching one
1174  unit with \eC in a UTF mode means that the rest of the string may start with a  unit with \eC in a UTF mode means that the rest of the string may start with a
1175  malformed UTF character. This has undefined results, because PCRE assumes that  malformed UTF character. This has undefined results, because PCRE assumes that
1176  it is dealing with valid UTF strings (and by default it checks this at the  it is dealing with valid UTF strings (and by default it checks this at the
1177  start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option  start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1178  is used).  PCRE_NO_UTF32_CHECK option is used).
1179  .P  .P
1180  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1181  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 1080  circumflex is not an assertion; it still Line 1234  circumflex is not an assertion; it still
1234  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
1235  string.  string.
1236  .P  .P
1237  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1238  included in a class as a literal string of data units, or by using the \ex{  can be included in a class as a literal string of data units, or by using the
1239  escaping mechanism.  \ex{ escaping mechanism.
1240  .P  .P
1241  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
1242  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
# Line 1104  The minus (hyphen) character can be used Line 1258  The minus (hyphen) character can be used
1258  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
1259  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
1260  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
1261  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class, or
1262    immediately after a range. For example, [b-d-z] matches letters in the range b
1263    to d, a hyphen character, or z.
1264  .P  .P
1265  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
1266  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
# Line 1114  the end of range, so [W-\e]46] is interp Line 1270  the end of range, so [W-\e]46] is interp
1270  followed by two other characters. The octal or hexadecimal representation of  followed by two other characters. The octal or hexadecimal representation of
1271  "]" can also be used to end a range.  "]" can also be used to end a range.
1272  .P  .P
1273    An error is generated if a POSIX character class (see below) or an escape
1274    sequence other than one that defines a single character appears at a point
1275    where a range ending character is expected. For example, [z-\exff] is valid,
1276    but [A-\ed] and [A-[:digit:]] are not.
1277    .P
1278  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
1279  used for characters specified numerically, for example [\e000-\e037]. Ranges  used for characters specified numerically, for example [\e000-\e037]. Ranges
1280  can include any characters that are valid for the current mode.  can include any characters that are valid for the current mode.
# Line 1179  are: Line 1340  are:
1340    lower    lower case letters    lower    lower case letters
1341    print    printing characters, including space    print    printing characters, including space
1342    punct    printing characters, excluding letters and digits and space    punct    printing characters, excluding letters and digits and space
1343    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1344    upper    upper case letters    upper    upper case letters
1345    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1346    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1347  .sp  .sp
1348  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1349  space (32). Notice that this list includes the VT character (code 11). This  and space (32). If locale-specific matching is taking place, the list of space
1350  makes "space" different to \es, which does not include VT (for Perl  characters may be different; there may be fewer or more of them. "Space" used
1351  compatibility).  to be different to \es, which did not include VT, for Perl compatibility.
1352    However, Perl changed at release 5.18, and PCRE followed at release 8.34.
1353    "Space" and \es now match the same set of characters.
1354  .P  .P
1355  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1356  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
# Line 1199  matches "1", "2", or any non-digit. PCRE Line 1362  matches "1", "2", or any non-digit. PCRE
1362  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
1363  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1364  .P  .P
1365  By default, in UTF modes, characters with values greater than 128 do not match  By default, characters with values greater than 128 do not match any of the
1366  any of the POSIX character classes. However, if the PCRE_UCP option is passed  POSIX character classes. However, if the PCRE_UCP option is passed to
1367  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  \fBpcre_compile()\fP, some of the classes are changed so that Unicode character
1368  character properties are used. This is achieved by replacing the POSIX classes  properties are used. This is achieved by replacing certain POSIX classes by
1369  by other sequences, as follows:  other sequences, as follows:
1370  .sp  .sp
1371    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1372    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
# Line 1214  by other sequences, as follows: Line 1377  by other sequences, as follows:
1377    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1378    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1379  .sp  .sp
1380  Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX  Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1381  classes are unchanged, and match only characters with code points less than  classes are handled specially in UCP mode:
1382  128.  .TP 10
1383    [:graph:]
1384    This matches characters that have glyphs that mark the page when printed. In
1385    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1386    properties, except for:
1387    .sp
1388      U+061C           Arabic Letter Mark
1389      U+180E           Mongolian Vowel Separator
1390      U+2066 - U+2069  Various "isolate"s
1391    .sp
1392    .TP 10
1393    [:print:]
1394    This matches the same characters as [:graph:] plus space characters that are
1395    not controls, that is, characters with the Zs property.
1396    .TP 10
1397    [:punct:]
1398    This matches all characters that have the Unicode P (punctuation) property,
1399    plus those characters whose code points are less than 128 that have the S
1400    (Symbol) property.
1401    .P
1402    The other POSIX classes are unchanged, and match only characters with code
1403    points less than 128.
1404  .  .
1405  .  .
1406  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1295  the section entitled Line 1479  the section entitled
1479  .\" </a>  .\" </a>
1480  "Newline sequences"  "Newline sequences"
1481  .\"  .\"
1482  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1483  can be used to set UTF and Unicode property modes; they are equivalent to  sequences that can be used to set UTF and Unicode property modes; they are
1484  setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1485    options, respectively. The (*UTF) sequence is a generic version that can be
1486    used with any of the libraries. However, the application can set the
1487    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1488  .  .
1489  .  .
1490  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1433  conditions, Line 1620  conditions,
1620  .\"  .\"
1621  can be made by name as well as by number.  can be made by name as well as by number.
1622  .P  .P
1623  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores, but must
1624  capturing parentheses are still allocated numbers as well as names, exactly as  start with a non-digit. Named capturing parentheses are still allocated numbers
1625  if the names were not present. The PCRE API provides function calls for  as well as names, exactly as if the names were not present. The PCRE API
1626  extracting the name-to-number translation table from a compiled pattern. There  provides function calls for extracting the name-to-number translation table
1627  is also a convenience function for extracting a captured substring by name.  from a compiled pattern. There is also a convenience function for extracting a
1628    captured substring by name.
1629  .P  .P
1630  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
1631  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
# Line 1463  for the first (and in this example, the Line 1651  for the first (and in this example, the
1651  matched. This saves searching to find which numbered subpattern it was.  matched. This saves searching to find which numbered subpattern it was.
1652  .P  .P
1653  If you make a back reference to a non-unique named subpattern from elsewhere in  If you make a back reference to a non-unique named subpattern from elsewhere in
1654  the pattern, the one that corresponds to the first occurrence of the name is  the pattern, the subpatterns to which the name refers are checked in the order
1655  used. In the absence of duplicate numbers (see the previous section) this is  in which they appear in the overall pattern. The first one that is set is used
1656  the one with the lowest number. If you use a named reference in a condition  for the reference. For example, this pattern matches both "foofoo" and
1657    "barbar" but not "foobar" or "barfoo":
1658    .sp
1659      (?:(?<n>foo)|(?<n>bar))\ek<n>
1660    .sp
1661    .P
1662    If you make a subroutine call to a non-unique named subpattern, the one that
1663    corresponds to the first occurrence of the name is used. In the absence of
1664    duplicate numbers (see the previous section) this is the one with the lowest
1665    number.
1666    .P
1667    If you use a named reference in a condition
1668  test (see the  test (see the
1669  .\"  .\"
1670  .\" HTML <a href="#conditions">  .\" HTML <a href="#conditions">
# Line 1485  documentation. Line 1684  documentation.
1684  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1685  subpatterns with the same number because PCRE uses only the numbers when  subpatterns with the same number because PCRE uses only the numbers when
1686  matching. For this reason, an error is given at compile time if different names  matching. For this reason, an error is given at compile time if different names
1687  are given to subpatterns with the same number. However, you can give the same  are given to subpatterns with the same number. However, you can always give the
1688  name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.  same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1689    set.
1690  .  .
1691  .  .
1692  .SH REPETITION  .SH REPETITION
# Line 1532  quantifier, but a literal string of four Line 1732  quantifier, but a literal string of four
1732  In UTF modes, quantifiers apply to characters rather than to individual data  In UTF modes, quantifiers apply to characters rather than to individual data
1733  units. Thus, for example, \ex{100}{2} matches two characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1734  which is represented by a two-byte sequence in a UTF-8 string. Similarly,  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1735  \eX{3} matches three Unicode extended sequences, each of which may be several  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1736  data units long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1737  .P  .P
1738  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
1739  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
# Line 1619  In cases where it is known that the subj Line 1819  In cases where it is known that the subj
1819  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1820  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1821  .P  .P
1822  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1823  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1824  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
1825  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1629  succeeds. Consider, for example: Line 1829  succeeds. Consider, for example:
1829  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1830  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1831  .P  .P
1832    Another case where implicit anchoring is not applied is when the leading .* is
1833    inside an atomic group. Once again, a match at the start may fail where a later
1834    one succeeds. Consider this pattern:
1835    .sp
1836      (?>.*?a)b
1837    .sp
1838    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1839    (*PRUNE) and (*SKIP) also disable this optimization.
1840    .P
1841  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1842  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1843  .sp  .sp
# Line 1897  except that it does not cause the curren Line 2106  except that it does not cause the curren
2106  Assertion subpatterns are not capturing subpatterns. If such an assertion  Assertion subpatterns are not capturing subpatterns. If such an assertion
2107  contains capturing subpatterns within it, these are counted for the purposes of  contains capturing subpatterns within it, these are counted for the purposes of
2108  numbering the capturing subpatterns in the whole pattern. However, substring  numbering the capturing subpatterns in the whole pattern. However, substring
2109  capturing is carried out only for positive assertions, because it does not make  capturing is carried out only for positive assertions. (Perl sometimes, but not
2110  sense for negative assertions.  always, does do capturing in negative assertions.)
2111  .P  .P
2112  For compatibility with Perl, assertion subpatterns may be repeated; though  For compatibility with Perl, assertion subpatterns may be repeated; though
2113  it makes no sense to assert the same thing several times, the side effect of  it makes no sense to assert the same thing several times, the side effect of
# Line 2148  This makes the fragment independent of t Line 2357  This makes the fragment independent of t
2357  .sp  .sp
2358  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2359  subpattern by name. For compatibility with earlier versions of PCRE, which had  subpattern by name. For compatibility with earlier versions of PCRE, which had
2360  this facility before Perl, the syntax (?(name)...) is also recognized. However,  this facility before Perl, the syntax (?(name)...) is also recognized.
 there is a possible ambiguity with this syntax, because subpattern names may  
 consist entirely of digits. PCRE looks first for a named subpattern; if it  
 cannot find one and the name consists entirely of digits, PCRE looks for a  
 subpattern of that number, which must be greater than zero. Using subpattern  
 names that consist entirely of digits is not recommended.  
2361  .P  .P
2362  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2363  .sp  .sp
# Line 2550  same pair of parentheses when there is a Line 2754  same pair of parentheses when there is a
2754  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2755  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
2756  function by putting its entry point in the global variable \fIpcre_callout\fP  function by putting its entry point in the global variable \fIpcre_callout\fP
2757  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this  (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2758  variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2759  .P  .P
2760  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2761  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
# Line 2562  For example, this pattern has two callou Line 2766  For example, this pattern has two callou
2766  .sp  .sp
2767  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2768  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2769  255.  255. If there is a conditional group in the pattern whose condition is an
2770    assertion, an additional callout is inserted just before the condition. An
2771    explicit callout may also be set at this position, as in this example:
2772    .sp
2773      (?(?C9)(?=a)abc|def)
2774    .sp
2775    Note that this applies only to assertion conditions, not to other types of
2776    condition.
2777  .P  .P
2778  During matching, when PCRE reaches a callout point, the external function is  During matching, when PCRE reaches a callout point, the external function is
2779  called. It is provided with the number of the callout, the position in the  called. It is provided with the number of the callout, the position in the
2780  pattern, and, optionally, one item of data originally supplied by the caller of  pattern, and, optionally, one item of data originally supplied by the caller of
2781  the matching function. The callout function may cause matching to proceed, to  the matching function. The callout function may cause matching to proceed, to
2782  backtrack, or to fail altogether. A complete description of the interface to  backtrack, or to fail altogether.
2783  the callout function is given in the  .P
2784    By default, PCRE implements a number of optimizations at compile time and
2785    matching time, and one side-effect is that sometimes callouts are skipped. If
2786    you need all possible callouts to happen, you need to set options that disable
2787    the relevant optimizations. More details, and a complete description of the
2788    interface to the callout function, are given in the
2789  .\" HREF  .\" HREF
2790  \fBpcrecallout\fP  \fBpcrecallout\fP
2791  .\"  .\"
# Line 2581  documentation. Line 2797  documentation.
2797  .rs  .rs
2798  .sp  .sp
2799  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2800  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2801  or removal in a future version of Perl". It goes on to say: "Their usage in  change or removal in a future version of Perl". It goes on to say: "Their usage
2802  production code should be noted to avoid problems during upgrades." The same  in production code should be noted to avoid problems during upgrades." The same
2803  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2804  .P  .P
2805    The new verbs make use of what was previously invalid syntax: an opening
2806    parenthesis followed by an asterisk. They are generally of the form
2807    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2808    differently depending on whether or not a name is present. A name is any
2809    sequence of characters that does not include a closing parenthesis. The maximum
2810    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2811    libraries. If the name is empty, that is, if the closing parenthesis
2812    immediately follows the colon, the effect is as if the colon were not there.
2813    Any number of these verbs may occur in a pattern.
2814    .P
2815  Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
2816  used only when the pattern is to be matched using one of the traditional  used only when the pattern is to be matched using one of the traditional
2817  matching functions, which use a backtracking algorithm. With the exception of  matching functions, because these use a backtracking algorithm. With the
2818  (*FAIL), which behaves like a failing negative assertion, they cause an error  exception of (*FAIL), which behaves like a failing negative assertion, the
2819  if encountered by a DFA matching function.  backtracking control verbs cause an error if encountered by a DFA matching
2820  .P  function.
 If any of these verbs are used in an assertion or in a subpattern that is  
 called as a subroutine (whether or not recursively), their effect is confined  
 to that subpattern; it does not extend to the surrounding pattern, with one  
 exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in  
 a successful positive assertion \fIis\fP passed back when a match succeeds  
 (compare capturing parentheses in assertions). Note that such subpatterns are  
 processed as anchored at the point where they are tested. Note also that Perl's  
 treatment of subroutines and assertions is different in some cases.  
2821  .P  .P
2822  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2823  parenthesis followed by an asterisk. They are generally of the form  .\" HTML <a href="#btrepeat">
2824  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  .\" </a>
2825  depending on whether or not an argument is present. A name is any sequence of  repeated groups,
2826  characters that does not include a closing parenthesis. The maximum length of  .\"
2827  name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name  .\" HTML <a href="#btassert">
2828  is empty, that is, if the closing parenthesis immediately follows the colon,  .\" </a>
2829  the effect is as if the colon were not there. Any number of these verbs may  assertions,
2830  occur in a pattern.  .\"
2831    and in
2832    .\" HTML <a href="#btsub">
2833    .\" </a>
2834    subpatterns called as subroutines
2835    .\"
2836    (whether or not recursively) is documented below.
2837  .  .
2838  .  .
2839  .\" HTML <a name="nooptimize"></a>  .\" HTML <a name="nooptimize"></a>
# Line 2619  occur in a pattern. Line 2843  occur in a pattern.
2843  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2844  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
2845  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2846  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations bypasses the running of a match, any
2847  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2848  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2849  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
# Line 2633  in the Line 2857  in the
2857  .\" HREF  .\" HREF
2858  \fBpcreapi\fP  \fBpcreapi\fP
2859  .\"  .\"
2860  documentation.  documentation.
2861  .P  .P
2862  Experiments with Perl suggest that it too has similar optimizations, sometimes  Experiments with Perl suggest that it too has similar optimizations, sometimes
2863  leading to anomalous results.  leading to anomalous results.
# Line 2650  followed by a name. Line 2874  followed by a name.
2874  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2875  pattern. However, when it is inside a subpattern that is called as a  pattern. However, when it is inside a subpattern that is called as a
2876  subroutine, only that subpattern is ended successfully. Matching then continues  subroutine, only that subpattern is ended successfully. Matching then continues
2877  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so  at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2878  far is captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2879    .P
2880    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2881    example:
2882  .sp  .sp
2883    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2884  .sp  .sp
# Line 2684  starting point (see (*SKIP) below). Line 2911  starting point (see (*SKIP) below).
2911  A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
2912  (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
2913  .P  .P
2914  When a match succeeds, the name of the last-encountered (*MARK) on the matching  When a match succeeds, the name of the last-encountered (*MARK:NAME),
2915  path is passed back to the caller as described in the section entitled  (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2916    caller as described in the section entitled
2917  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2918  .\" </a>  .\" </a>
2919  "Extra data for \fBpcre_exec()\fP"  "Extra data for \fBpcre_exec()\fP"
# Line 2710  indicates which of the two alternatives Line 2938  indicates which of the two alternatives
2938  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2939  capturing parentheses.  capturing parentheses.
2940  .P  .P
2941  If (*MARK) is encountered in a positive assertion, its name is recorded and  If a verb with a name is encountered in a positive assertion that is true, the
2942  passed back if it is the last-encountered. This does not happen for negative  name is recorded and passed back if it is the last-encountered. This does not
2943  assertions.  happen for negative assertions or failing positive assertions.
2944  .P  .P
2945  After a partial match or a failed match, the name of the last encountered  After a partial match or a failed match, the last encountered name in the
2946  (*MARK) in the entire match process is returned. For example:  entire match process is returned. For example:
2947  .sp  .sp
2948      re> /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2949    data> XP    data> XP
# Line 2727  attempts starting at "P" and then with a Line 2955  attempts starting at "P" and then with a
2955  (*MARK) item, but nevertheless do not reset it.  (*MARK) item, but nevertheless do not reset it.
2956  .P  .P
2957  If you are interested in (*MARK) values after failed matches, you should  If you are interested in (*MARK) values after failed matches, you should
2958  probably set the PCRE_NO_START_OPTIMIZE option  probably set the PCRE_NO_START_OPTIMIZE option
2959  .\" HTML <a href="#nooptimize">  .\" HTML <a href="#nooptimize">
2960  .\" </a>  .\" </a>
2961  (see above)  (see above)
2962  .\"  .\"
2963  to ensure that the match is always attempted.  to ensure that the match is always attempted.
2964  .  .
# Line 2741  to ensure that the match is always attem Line 2969  to ensure that the match is always attem
2969  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2970  with what follows, but if there is no subsequent match, causing a backtrack to  with what follows, but if there is no subsequent match, causing a backtrack to
2971  the verb, a failure is forced. That is, backtracking cannot pass to the left of  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2972  the verb. However, when one of these verbs appears inside an atomic group, its  the verb. However, when one of these verbs appears inside an atomic group or an
2973  effect is confined to that group, because once the group has been matched,  assertion that is true, its effect is confined to that group, because once the
2974  there is never any backtracking into it. In this situation, backtracking can  group has been matched, there is never any backtracking into it. In this
2975  "jump back" to the left of the entire atomic group. (Remember also, as stated  situation, backtracking can "jump back" to the left of the entire atomic group
2976  above, that this localization also applies in subroutine calls and assertions.)  or assertion. (Remember also, as stated above, that this localization also
2977    applies in subroutine calls.)
2978  .P  .P
2979  These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
2980  reaches them.  reaches them. The behaviour described below is what happens when the verb is
2981    not in a subroutine or an assertion. Subsequent sections cover these special
2982    cases.
2983  .sp  .sp
2984    (*COMMIT)    (*COMMIT)
2985  .sp  .sp
2986  This verb, which may not be followed by a name, causes the whole match to fail  This verb, which may not be followed by a name, causes the whole match to fail
2987  outright if the rest of the pattern does not match. Even if the pattern is  outright if there is a later matching failure that causes backtracking to reach
2988  unanchored, no further attempts to find a match by advancing the starting point  it. Even if the pattern is unanchored, no further attempts to find a match by
2989  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to  advancing the starting point take place. If (*COMMIT) is the only backtracking
2990  finding a match at the current starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2991    committed to finding a match at the current starting point, or not at all. For
2992    example:
2993  .sp  .sp
2994    a+(*COMMIT)b    a+(*COMMIT)b
2995  .sp  .sp
# Line 2765  dynamic anchor, or "I've started, so I m Line 2998  dynamic anchor, or "I've started, so I m
2998  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2999  match failure.  match failure.
3000  .P  .P
3001    If there is more than one backtracking verb in a pattern, a different one that
3002    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3003    match does not always guarantee that a match must be at this starting point.
3004    .P
3005  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
3006  unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
3007  \fBpcretest\fP example:  \fBpcretest\fP example:
# Line 2784  starting points. Line 3021  starting points.
3021    (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
3022  .sp  .sp
3023  This verb causes the match to fail at the current starting position in the  This verb causes the match to fail at the current starting position in the
3024  subject if the rest of the pattern does not match. If the pattern is  subject if there is a later matching failure that causes backtracking to reach
3025  unanchored, the normal "bumpalong" advance to the next starting character then  it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3026  happens. Backtracking can occur as usual to the left of (*PRUNE), before it is  starting character then happens. Backtracking can occur as usual to the left of
3027  reached, or when matching to the right of (*PRUNE), but if there is no match to  (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3028  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  if there is no match to the right, backtracking cannot cross (*PRUNE). In
3029  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3030  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3031  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an  expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3032  anchored pattern (*PRUNE) has the same effect as (*COMMIT).  as (*COMMIT).
3033    .P
3034    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3035    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3036    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3037  .sp  .sp
3038    (*SKIP)    (*SKIP)
3039  .sp  .sp
# Line 2813  instead of skipping on to "c". Line 3054  instead of skipping on to "c".
3054  .sp  .sp
3055    (*SKIP:NAME)    (*SKIP:NAME)
3056  .sp  .sp
3057  When (*SKIP) has an associated name, its behaviour is modified. If the  When (*SKIP) has an associated name, its behaviour is modified. When it is
3058  following pattern fails to match, the previous path through the pattern is  triggered, the previous path through the pattern is searched for the most
3059  searched for the most recent (*MARK) that has the same name. If one is found,  recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3060  the "bumpalong" advance is to the subject position that corresponds to that  is to the subject position that corresponds to that (*MARK) instead of to where
3061  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3062  matching name is found, the (*SKIP) is ignored.  (*SKIP) is ignored.
3063    .P
3064    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3065    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3066  .sp  .sp
3067    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
3068  .sp  .sp
3069  This verb causes a skip to the next innermost alternative if the rest of the  This verb causes a skip to the next innermost alternative when backtracking
3070  pattern does not match. That is, it cancels pending backtracking, but only  reaches it. That is, it cancels any further backtracking within the current
3071  within the current alternative. Its name comes from the observation that it can  alternative. Its name comes from the observation that it can be used for a
3072  be used for a pattern-based if-then-else block:  pattern-based if-then-else block:
3073  .sp  .sp
3074    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3075  .sp  .sp
3076  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
3077  the end of the group if FOO succeeds); on failure, the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
3078  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. If that
3079  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3080  If (*THEN) is not inside an alternation, it acts like (*PRUNE).  more alternatives, so there is a backtrack to whatever came before the entire
3081    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3082    .P
3083    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3084    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3085    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3086  .P  .P
3087  Note that a subpattern that does not contain a | character is just a part of  A subpattern that does not contain a | character is just a part of the
3088  the enclosing alternative; it is not a nested alternation with only one  enclosing alternative; it is not a nested alternation with only one
3089  alternative. The effect of (*THEN) extends beyond such a subpattern to the  alternative. The effect of (*THEN) extends beyond such a subpattern to the
3090  enclosing alternative. Consider this pattern, where A, B, etc. are complex  enclosing alternative. Consider this pattern, where A, B, etc. are complex
3091  pattern fragments that do not contain any | characters at this level:  pattern fragments that do not contain any | characters at this level:
# Line 2855  in C, matching moves to (*FAIL), which c Line 3104  in C, matching moves to (*FAIL), which c
3104  because there are no more alternatives to try. In this case, matching does now  because there are no more alternatives to try. In this case, matching does now
3105  backtrack into A.  backtrack into A.
3106  .P  .P
3107  Note also that a conditional subpattern is not considered as having two  Note that a conditional subpattern is not considered as having two
3108  alternatives, because only one is ever used. In other words, the | character in  alternatives, because only one is ever used. In other words, the | character in
3109  a conditional subpattern has a different meaning. Ignoring white space,  a conditional subpattern has a different meaning. Ignoring white space,
3110  consider:  consider:
# Line 2877  starting position, but allowing an advan Line 3126  starting position, but allowing an advan
3126  unanchored pattern). (*SKIP) is similar, except that the advance may be more  unanchored pattern). (*SKIP) is similar, except that the advance may be more
3127  than one character. (*COMMIT) is the strongest, causing the entire match to  than one character. (*COMMIT) is the strongest, causing the entire match to
3128  fail.  fail.
3129    .
3130    .
3131    .SS "More than one backtracking verb"
3132    .rs
3133    .sp
3134    If more than one backtracking verb is present in a pattern, the one that is
3135    backtracked onto first acts. For example, consider this pattern, where A, B,
3136    etc. are complex pattern fragments:
3137    .sp
3138      (A(*COMMIT)B(*THEN)C|ABD)
3139    .sp
3140    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3141    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3142    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3143    not always the same as Perl's. It means that if two or more backtracking verbs
3144    appear in succession, all the the last of them has no effect. Consider this
3145    example:
3146    .sp
3147      ...(*COMMIT)(*PRUNE)...
3148    .sp
3149    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3150    it to be triggered, and its action is taken. There can never be a backtrack
3151    onto (*COMMIT).
3152    .
3153    .
3154    .\" HTML <a name="btrepeat"></a>
3155    .SS "Backtracking verbs in repeated groups"
3156    .rs
3157    .sp
3158    PCRE differs from Perl in its handling of backtracking verbs in repeated
3159    groups. For example, consider:
3160    .sp
3161      /(a(*COMMIT)b)+ac/
3162    .sp
3163    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3164    the second repeat of the group acts.
3165    .
3166    .
3167    .\" HTML <a name="btassert"></a>
3168    .SS "Backtracking verbs in assertions"
3169    .rs
3170    .sp
3171    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3172    .P
3173    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3174    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3175    fail without any further processing.
3176    .P
3177    The other backtracking verbs are not treated specially if they appear in a
3178    positive assertion. In particular, (*THEN) skips to the next alternative in the
3179    innermost enclosing group that has alternations, whether or not this is within
3180    the assertion.
3181    .P
3182    Negative assertions are, however, different, in order to ensure that changing a
3183    positive assertion into a negative assertion changes its result. Backtracking
3184    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3185    without considering any further alternative branches in the assertion.
3186    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3187    within the assertion (the normal behaviour), but if the assertion does not have
3188    such an alternative, (*THEN) behaves like (*PRUNE).
3189    .
3190    .
3191    .\" HTML <a name="btsub"></a>
3192    .SS "Backtracking verbs in subroutines"
3193    .rs
3194    .sp
3195    These behaviours occur whether or not the subpattern is called recursively.
3196    Perl's treatment of subroutines is different in some cases.
3197    .P
3198    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3199    an immediate backtrack.
3200    .P
3201    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3202    succeed without any further processing. Matching then continues after the
3203    subroutine call.
3204    .P
3205    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3206    the subroutine match to fail.
3207  .P  .P
3208  If more than one such verb is present in a pattern, the "strongest" one wins.  (*THEN) skips to the next alternative in the innermost enclosing group within
3209  For example, consider this pattern, where A, B, etc. are complex pattern  the subpattern that has alternatives. If there is no such group within the
3210  fragments:  subpattern, (*THEN) causes the subroutine match to fail.
 .sp  
   (A(*COMMIT)B(*THEN)C|D)  
 .sp  
 Once A has matched, PCRE is committed to this match, at the current starting  
 position. If subsequently B matches, but C does not, the normal (*THEN) action  
 of trying the next alternative (that is, D) does not happen because (*COMMIT)  
 overrides.  
3211  .  .
3212  .  .
3213  .SH "SEE ALSO"  .SH "SEE ALSO"
3214  .rs  .rs
3215  .sp  .sp
3216  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3217  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3218  .  .
3219  .  .
3220  .SH AUTHOR  .SH AUTHOR
# Line 2911  Cambridge CB2 3QH, England. Line 3231  Cambridge CB2 3QH, England.
3231  .rs  .rs
3232  .sp  .sp
3233  .nf  .nf
3234  Last updated: 01 June 2012  Last updated: 25 November 2013
3235  Copyright (c) 1997-2012 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3236  .fi  .fi

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