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1  .TH PCREPATTERN 3 "17 March 2013" "PCRE 8.33"  .TH PCREPATTERN 3 "12 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, an  there is now also support for UTF-8 strings in the original library, an
53  extra library that supports 16-bit and UTF-16 character strings, and a  extra library that supports 16-bit and UTF-16 character strings, and a
# Line 36  these special sequences: Line 64  these special sequences:
64  .sp  .sp
65  (*UTF) is a generic sequence that can be used with any of the libraries.  (*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), (*UTF16), (*UTF32) or (*UTF) 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  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the  .
90  PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are  .SS "Disabling auto-possessification"
91  also some more of these special sequences that are concerned with the handling  .rs
92  of newlines; they are described below.  .sp
93  .P  If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
94  The remainder of this document discusses the patterns that are supported by  the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
95  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or  quantifiers possessive when what follows cannot match the repeated item. For
96  \fBpcre[16|32]_exec()\fP (16- or 32-bit), is used. PCRE also has alternative  example, by default a+b is treated as a++b. For more details, see the
 matching functions, \fBpcre_dfa_exec()\fP and \fBpcre[16|32_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  
97  .\" HREF  .\" HREF
98  \fBpcrematching\fP  \fBpcreapi\fP
99  .\"  .\"
100  page.  documentation.
101  .  .
102  .  .
103  .SH "EBCDIC CHARACTER CODES"  .SS "Disabling start-up optimizations"
104  .rs  .rs
105  .sp  .sp
106  PCRE can be compiled to run in an environment that uses EBCDIC as its character  If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
107  code rather than ASCII or Unicode (typically a mainframe system). In the  PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
108  sections below, character code values are ASCII or Unicode; in an EBCDIC  several optimizations for quickly reaching "no match" results. For more
109  environment these characters may have different code values, and there are no  details, see the
110  code points greater than 255.  .\" HREF
111    \fBpcreapi\fP
112    .\"
113    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 117  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 where the circumflex and dollar assertions are  The newline convention affects where the circumflex and dollar assertions are
# Line 136  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 213  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 256  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)
# Line 277  byte are inverted. Thus \ecA becomes hex Line 340  byte are inverted. Thus \ecA becomes hex
340  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341  characters also generate different values.  characters also generate different values.
342  .P  .P
 By default, after \ex, from zero to two hexadecimal digits are read (letters  
 can be in upper or lower case). Any number of hexadecimal digits may appear  
 between \ex{ and }, but the character code is constrained as follows:  
 .sp  
   8-bit non-UTF mode    less than 0x100  
   8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint  
   16-bit non-UTF mode   less than 0x10000  
   16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint  
   32-bit non-UTF mode   less than 0x80000000  
   32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint  
 .sp  
 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called  
 "surrogate" codepoints), and 0xffef.  
 .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.  
 Character codes specified by \eu in JavaScript mode are constrained in the same  
 was as those specified by \ex in non-JavaScript mode.  
 .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).  
 .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
345  specifies two binary zeros followed by a BEL character (code value 7). Make  specifies two binary zeros followed by a BEL character (code value 7). Make
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 330  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 an ASCII space    \e040   is another way of writing an ASCII space
383  .\" JOIN  .\" JOIN
# Line 354  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 450  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; in particular, in some locales the
540    "non-breaking space" character (\exA0) is recognized as white space.
541  .P  .P
542  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.
543  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 469  in the Line 552  in the
552  \fBpcreapi\fP  \fBpcreapi\fP
553  .\"  .\"
554  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,
555  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
556  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
557  Unicode is discouraged.  Unicode is discouraged.
558  .P  .P
559  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,
560  \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
561  their original meanings from before UTF support was available, mainly for  characters in the range 128-255 when locale-specific matching is happening.
562  efficiency reasons. However, if PCRE is compiled with Unicode property support,  These escape sequences retain their original meanings from before Unicode
563  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
564  properties are used to determine character types, as follows:  Unicode property support, and the PCRE_UCP option is set, the behaviour is
565  .sp  changed so that Unicode properties are used to determine character types, as
566    \ed  any character that \ep{Nd} matches (decimal digit)  follows:
567    \es  any character that \ep{Z} matches, plus HT, LF, FF, CR  .sp
568    \ew  any character that \ep{L} or \ep{N} matches, plus underscore    \ed  any character that matches \ep{Nd} (decimal digit)
569      \es  any character that matches \ep{Z} or \eh or \ev
570      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
571  .sp  .sp
572  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
573  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 492  is noticeably slower when PCRE_UCP is se Line 577  is noticeably slower when PCRE_UCP is se
577  .P  .P
578  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
579  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
580  characters by default, these always match certain high-valued codepoints,  characters by default, these always match certain high-valued code points,
581  whether or not PCRE_UCP is set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
582  .sp  .sp
583    U+0009     Horizontal tab (HT)    U+0009     Horizontal tab (HT)
# Line 802  Instead, this property is assumed for an Line 887  Instead, this property is assumed for an
887  Unicode table.  Unicode table.
888  .P  .P
889  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
890  example, \ep{Lu} always matches only upper case letters. This is different from  example, \ep{Lu} always matches only upper case letters. This is different from
891  the behaviour of current versions of Perl.  the behaviour of current versions of Perl.
892  .P  .P
893  Matching characters by Unicode property is not fast, because PCRE has to do a  Matching characters by Unicode property is not fast, because PCRE has to do a
# Line 862  the "mark" property always have the "ext Line 947  the "mark" property always have the "ext
947  .sp  .sp
948  As well as the standard Unicode properties described above, PCRE supports four  As well as the standard Unicode properties described above, PCRE supports four
949  more that make it possible to convert traditional escape sequences such as \ew  more that make it possible to convert traditional escape sequences such as \ew
950  and \es and POSIX character classes to use Unicode properties. PCRE uses these  and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
951  non-standard, non-Perl properties internally when PCRE_UCP is set. However,  properties internally when PCRE_UCP is set. However, they may also be used
952  they may also be used explicitly. These properties are:  explicitly. These properties are:
953  .sp  .sp
954    Xan   Any alphanumeric character    Xan   Any alphanumeric character
955    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 874  they may also be used explicitly. These Line 959  they may also be used explicitly. These
959  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)
960  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
961  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
962  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
963  same characters as Xan, plus underscore.  compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
964  .P  matches the same characters as Xan, plus underscore.
965  There is another non-standard property, Xuc, which matches any character that  .P
966  can be represented by a Universal Character Name in C++ and other programming  There is another non-standard property, Xuc, which matches any character that
967  languages. These are the characters $, @, ` (grave accent), and all characters  can be represented by a Universal Character Name in C++ and other programming
968  with Unicode code points greater than or equal to U+00A0, except for the  languages. These are the characters $, @, ` (grave accent), and all characters
969  surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are  with Unicode code points greater than or equal to U+00A0, except for the
970    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
971  excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH  excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
972  where H is a hexadecimal digit. Note that the Xuc property does not match these  where H is a hexadecimal digit. Note that the Xuc property does not match these
973  sequences but the characters that they represent.)  sequences but the characters that they represent.)
974  .  .
975  .  .
# Line 1171  The minus (hyphen) character can be used Line 1257  The minus (hyphen) character can be used
1257  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,
1258  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
1259  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
1260  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
1261    immediately after a range. For example, [b-d-z] matches letters in the range b
1262    to d, a hyphen character, or z.
1263  .P  .P
1264  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
1265  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 1181  the end of range, so [W-\e]46] is interp Line 1269  the end of range, so [W-\e]46] is interp
1269  followed by two other characters. The octal or hexadecimal representation of  followed by two other characters. The octal or hexadecimal representation of
1270  "]" can also be used to end a range.  "]" can also be used to end a range.
1271  .P  .P
1272    An error is generated if a POSIX character class (see below) or an escape
1273    sequence other than one that defines a single character appears at a point
1274    where a range ending character is expected. For example, [z-\exff] is valid,
1275    but [A-\ed] and [A-[:digit:]] are not.
1276    .P
1277  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
1278  used for characters specified numerically, for example [\e000-\e037]. Ranges  used for characters specified numerically, for example [\e000-\e037]. Ranges
1279  can include any characters that are valid for the current mode.  can include any characters that are valid for the current mode.
# Line 1246  are: Line 1339  are:
1339    lower    lower case letters    lower    lower case letters
1340    print    printing characters, including space    print    printing characters, including space
1341    punct    printing characters, excluding letters and digits and space    punct    printing characters, excluding letters and digits and space
1342    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1343    upper    upper case letters    upper    upper case letters
1344    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1345    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1346  .sp  .sp
1347  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),
1348  space (32). Notice that this list includes the VT character (code 11). This  and space (32). If locale-specific matching is taking place, there may be
1349  makes "space" different to \es, which does not include VT (for Perl  additional space characters. "Space" used to be different to \es, which did not
1350  compatibility).  include VT, for Perl compatibility. However, Perl changed at release 5.18, and
1351    PCRE followed at release 8.34. "Space" and \es now match the same set of
1352    characters.
1353  .P  .P
1354  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
1355  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 1266  matches "1", "2", or any non-digit. PCRE Line 1361  matches "1", "2", or any non-digit. PCRE
1361  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
1362  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1363  .P  .P
1364  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
1365  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
1366  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
1367  character properties are used. This is achieved by replacing the POSIX classes  properties are used. This is achieved by replacing certain POSIX classes by
1368  by other sequences, as follows:  other sequences, as follows:
1369  .sp  .sp
1370    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1371    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
# Line 1281  by other sequences, as follows: Line 1376  by other sequences, as follows:
1376    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1377    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1378  .sp  .sp
1379  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
1380  classes are unchanged, and match only characters with code points less than  classes are handled specially in UCP mode:
1381  128.  .TP 10
1382    [:graph:]
1383    This matches characters that have glyphs that mark the page when printed. In
1384    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1385    properties, except for:
1386    .sp
1387      U+061C           Arabic Letter Mark
1388      U+180E           Mongolian Vowel Separator
1389      U+2066 - U+2069  Various "isolate"s
1390    .sp
1391    .TP 10
1392    [:print:]
1393    This matches the same characters as [:graph:] plus space characters that are
1394    not controls, that is, characters with the Zs property.
1395    .TP 10
1396    [:punct:]
1397    This matches all characters that have the Unicode P (punctuation) property,
1398    plus those characters whose code points are less than 128 that have the S
1399    (Symbol) property.
1400    .P
1401    The other POSIX classes are unchanged, and match only characters with code
1402    points less than 128.
1403  .  .
1404  .  .
1405  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1366  above. There are also the (*UTF8), (*UTF Line 1482  above. There are also the (*UTF8), (*UTF
1482  sequences that can be used to set UTF and Unicode property modes; they are  sequences that can be used to set UTF and Unicode property modes; they are
1483  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1484  options, respectively. The (*UTF) sequence is a generic version that can be  options, respectively. The (*UTF) sequence is a generic version that can be
1485  used with any of the libraries.  used with any of the libraries. However, the application can set the
1486    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1487  .  .
1488  .  .
1489  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1502  conditions, Line 1619  conditions,
1619  .\"  .\"
1620  can be made by name as well as by number.  can be made by name as well as by number.
1621  .P  .P
1622  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores, but must
1623  capturing parentheses are still allocated numbers as well as names, exactly as  start with a non-digit. Named capturing parentheses are still allocated numbers
1624  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
1625  extracting the name-to-number translation table from a compiled pattern. There  provides function calls for extracting the name-to-number translation table
1626  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
1627    captured substring by name.
1628  .P  .P
1629  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
1630  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 1532  for the first (and in this example, the Line 1650  for the first (and in this example, the
1650  matched. This saves searching to find which numbered subpattern it was.  matched. This saves searching to find which numbered subpattern it was.
1651  .P  .P
1652  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
1653  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
1654  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
1655  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
1656    "barbar" but not "foobar" or "barfoo":
1657    .sp
1658      (?:(?<n>foo)|(?<n>bar))\k<n>
1659    .sp
1660    .P
1661    If you make a subroutine call to a non-unique named subpattern, the one that
1662    corresponds to the first occurrence of the name is used. In the absence of
1663    duplicate numbers (see the previous section) this is the one with the lowest
1664    number.
1665    .P
1666    If you use a named reference in a condition
1667  test (see the  test (see the
1668  .\"  .\"
1669  .\" HTML <a href="#conditions">  .\" HTML <a href="#conditions">
# Line 1554  documentation. Line 1683  documentation.
1683  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1684  subpatterns with the same number because PCRE uses only the numbers when  subpatterns with the same number because PCRE uses only the numbers when
1685  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
1686  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
1687  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
1688    set.
1689  .  .
1690  .  .
1691  .SH REPETITION  .SH REPETITION
# Line 1975  except that it does not cause the curren Line 2105  except that it does not cause the curren
2105  Assertion subpatterns are not capturing subpatterns. If such an assertion  Assertion subpatterns are not capturing subpatterns. If such an assertion
2106  contains capturing subpatterns within it, these are counted for the purposes of  contains capturing subpatterns within it, these are counted for the purposes of
2107  numbering the capturing subpatterns in the whole pattern. However, substring  numbering the capturing subpatterns in the whole pattern. However, substring
2108  capturing is carried out only for positive assertions. (Perl sometimes, but not  capturing is carried out only for positive assertions. (Perl sometimes, but not
2109  always, does do capturing in negative assertions.)  always, does do capturing in negative assertions.)
2110  .P  .P
2111  For compatibility with Perl, assertion subpatterns may be repeated; though  For compatibility with Perl, assertion subpatterns may be repeated; though
# Line 2226  This makes the fragment independent of t Line 2356  This makes the fragment independent of t
2356  .sp  .sp
2357  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2358  subpattern by name. For compatibility with earlier versions of PCRE, which had  subpattern by name. For compatibility with earlier versions of PCRE, which had
2359  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.  
2360  .P  .P
2361  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2362  .sp  .sp
# Line 2646  explicit callout may also be set at this Line 2771  explicit callout may also be set at this
2771  .sp  .sp
2772    (?(?C9)(?=a)abc|def)    (?(?C9)(?=a)abc|def)
2773  .sp  .sp
2774  Note that this applies only to assertion conditions, not to other types of  Note that this applies only to assertion conditions, not to other types of
2775  condition.  condition.
2776  .P  .P
2777  During matching, when PCRE reaches a callout point, the external function is  During matching, when PCRE reaches a callout point, the external function is
2778  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
2779  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
2780  the matching function. The callout function may cause matching to proceed, to  the matching function. The callout function may cause matching to proceed, to
2781  backtrack, or to fail altogether. A complete description of the interface to  backtrack, or to fail altogether.
2782  the callout function is given in the  .P
2783    By default, PCRE implements a number of optimizations at compile time and
2784    matching time, and one side-effect is that sometimes callouts are skipped. If
2785    you need all possible callouts to happen, you need to set options that disable
2786    the relevant optimizations. More details, and a complete description of the
2787    interface to the callout function, are given in the
2788  .\" HREF  .\" HREF
2789  \fBpcrecallout\fP  \fBpcrecallout\fP
2790  .\"  .\"
# Line 2666  documentation. Line 2796  documentation.
2796  .rs  .rs
2797  .sp  .sp
2798  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2799  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2800  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
2801  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
2802  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2803  .P  .P
2804    The new verbs make use of what was previously invalid syntax: an opening
2805    parenthesis followed by an asterisk. They are generally of the form
2806    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2807    differently depending on whether or not a name is present. A name is any
2808    sequence of characters that does not include a closing parenthesis. The maximum
2809    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2810    libraries. If the name is empty, that is, if the closing parenthesis
2811    immediately follows the colon, the effect is as if the colon were not there.
2812    Any number of these verbs may occur in a pattern.
2813    .P
2814  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
2815  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
2816  matching functions, which use a backtracking algorithm. With the exception of  matching functions, because these use a backtracking algorithm. With the
2817  (*FAIL), which behaves like a failing negative assertion, they cause an error  exception of (*FAIL), which behaves like a failing negative assertion, the
2818  if encountered by a DFA matching function.  backtracking control verbs cause an error if encountered by a DFA matching
2819  .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.  
2820  .P  .P
2821  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2822  parenthesis followed by an asterisk. They are generally of the form  .\" HTML <a href="#btrepeat">
2823  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  .\" </a>
2824  depending on whether or not an argument is present. A name is any sequence of  repeated groups,
2825  characters that does not include a closing parenthesis. The maximum length of  .\"
2826  name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit library.  .\" HTML <a href="#btassert">
2827  If the name is empty, that is, if the closing parenthesis immediately follows  .\" </a>
2828  the colon, the effect is as if the colon were not there. Any number of these  assertions,
2829  verbs may occur in a pattern.  .\"
2830    and in
2831    .\" HTML <a href="#btsub">
2832    .\" </a>
2833    subpatterns called as subroutines
2834    .\"
2835    (whether or not recursively) is documented below.
2836  .  .
2837  .  .
2838  .\" HTML <a name="nooptimize"></a>  .\" HTML <a name="nooptimize"></a>
# Line 2704  verbs may occur in a pattern. Line 2842  verbs may occur in a pattern.
2842  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
2843  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
2844  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2845  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
2846  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2847  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
2848  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 2735  followed by a name. Line 2873  followed by a name.
2873  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
2874  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
2875  subroutine, only that subpattern is ended successfully. Matching then continues  subroutine, only that subpattern is ended successfully. Matching then continues
2876  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
2877  far is captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2878    .P
2879    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2880    example:
2881  .sp  .sp
2882    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2883  .sp  .sp
# Line 2796  indicates which of the two alternatives Line 2937  indicates which of the two alternatives
2937  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2938  capturing parentheses.  capturing parentheses.
2939  .P  .P
2940  If a verb with a name is encountered in a positive assertion, its name is  If a verb with a name is encountered in a positive assertion that is true, the
2941  recorded and passed back if it is the last-encountered. This does not happen  name is recorded and passed back if it is the last-encountered. This does not
2942  for negative assertions.  happen for negative assertions or failing positive assertions.
2943  .P  .P
2944  After a partial match or a failed match, the last encountered name in the  After a partial match or a failed match, the last encountered name in the
2945  entire match process is returned. For example:  entire match process is returned. For example:
# Line 2827  to ensure that the match is always attem Line 2968  to ensure that the match is always attem
2968  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2969  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
2970  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
2971  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
2972  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
2973  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
2974  "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
2975  above, that this localization also applies in subroutine calls and assertions.)  or assertion. (Remember also, as stated above, that this localization also
2976    applies in subroutine calls.)
2977  .P  .P
2978  These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
2979  reaches them.  reaches them. The behaviour described below is what happens when the verb is
2980    not in a subroutine or an assertion. Subsequent sections cover these special
2981    cases.
2982  .sp  .sp
2983    (*COMMIT)    (*COMMIT)
2984  .sp  .sp
2985  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
2986  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
2987  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
2988  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
2989  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
2990    committed to finding a match at the current starting point, or not at all. For
2991    example:
2992  .sp  .sp
2993    a+(*COMMIT)b    a+(*COMMIT)b
2994  .sp  .sp
# Line 2851  dynamic anchor, or "I've started, so I m Line 2997  dynamic anchor, or "I've started, so I m
2997  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
2998  match failure.  match failure.
2999  .P  .P
3000    If there is more than one backtracking verb in a pattern, a different one that
3001    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3002    match does not always guarantee that a match must be at this starting point.
3003    .P
3004  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,
3005  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
3006  \fBpcretest\fP example:  \fBpcretest\fP example:
# Line 2870  starting points. Line 3020  starting points.
3020    (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
3021  .sp  .sp
3022  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
3023  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
3024  unanchored, the normal "bumpalong" advance to the next starting character then  it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3025  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
3026  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
3027  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
3028  (*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
3029  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
3030  In an anchored pattern (*PRUNE) has the same effect as (*COMMIT).  expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3031    as (*COMMIT).
3032  .P  .P
3033  The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).  The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3034  It is like (*MARK:NAME) in that the name is remembered for passing back to the  It is like (*MARK:NAME) in that the name is remembered for passing back to the
3035  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3036  .sp  .sp
# Line 2902  instead of skipping on to "c". Line 3053  instead of skipping on to "c".
3053  .sp  .sp
3054    (*SKIP:NAME)    (*SKIP:NAME)
3055  .sp  .sp
3056  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
3057  following pattern fails to match, the previous path through the pattern is  triggered, the previous path through the pattern is searched for the most
3058  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
3059  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
3060  (*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
3061  matching name is found, the (*SKIP) is ignored.  (*SKIP) is ignored.
3062  .P  .P
3063  Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores  Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3064  names that are set by (*PRUNE:NAME) or (*THEN:NAME).  names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3065  .sp  .sp
3066    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
3067  .sp  .sp
3068  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
3069  pattern does not match. That is, it cancels pending backtracking, but only  reaches it. That is, it cancels any further backtracking within the current
3070  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
3071  be used for a pattern-based if-then-else block:  pattern-based if-then-else block:
3072  .sp  .sp
3073    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3074  .sp  .sp
3075  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
3076  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
3077  second alternative and tries COND2, without backtracking into COND1.  second alternative and tries COND2, without backtracking into COND1. If that
3078  If (*THEN) is not inside an alternation, it acts like (*PRUNE).  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3079    more alternatives, so there is a backtrack to whatever came before the entire
3080    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3081  .P  .P
3082  The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).  The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3083  It is like (*MARK:NAME) in that the name is remembered for passing back to the  It is like (*MARK:NAME) in that the name is remembered for passing back to the
3084  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3085  .P  .P
3086  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
3087  the enclosing alternative; it is not a nested alternation with only one  enclosing alternative; it is not a nested alternation with only one
3088  alternative. The effect of (*THEN) extends beyond such a subpattern to the  alternative. The effect of (*THEN) extends beyond such a subpattern to the
3089  enclosing alternative. Consider this pattern, where A, B, etc. are complex  enclosing alternative. Consider this pattern, where A, B, etc. are complex
3090  pattern fragments that do not contain any | characters at this level:  pattern fragments that do not contain any | characters at this level:
# Line 2950  in C, matching moves to (*FAIL), which c Line 3103  in C, matching moves to (*FAIL), which c
3103  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
3104  backtrack into A.  backtrack into A.
3105  .P  .P
3106  Note also that a conditional subpattern is not considered as having two  Note that a conditional subpattern is not considered as having two
3107  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
3108  a conditional subpattern has a different meaning. Ignoring white space,  a conditional subpattern has a different meaning. Ignoring white space,
3109  consider:  consider:
# Line 2972  starting position, but allowing an advan Line 3125  starting position, but allowing an advan
3125  unanchored pattern). (*SKIP) is similar, except that the advance may be more  unanchored pattern). (*SKIP) is similar, except that the advance may be more
3126  than one character. (*COMMIT) is the strongest, causing the entire match to  than one character. (*COMMIT) is the strongest, causing the entire match to
3127  fail.  fail.
3128  .P  .
3129  If more than one such verb is present in a pattern, the one that is backtracked  .
3130  onto first acts. For example, consider this pattern, where A, B, etc. are  .SS "More than one backtracking verb"
3131  complex pattern fragments:  .rs
3132    .sp
3133    If more than one backtracking verb is present in a pattern, the one that is
3134    backtracked onto first acts. For example, consider this pattern, where A, B,
3135    etc. are complex pattern fragments:
3136  .sp  .sp
3137    (A(*COMMIT)B(*THEN)C|ABD)    (A(*COMMIT)B(*THEN)C|ABD)
3138  .sp  .sp
3139  If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to  If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3140  fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes  fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3141  the next alternative (ABD) to be tried. This behaviour is consistent, but is  the next alternative (ABD) to be tried. This behaviour is consistent, but is
3142  not always the same as Perl's.  not always the same as Perl's. It means that if two or more backtracking verbs
3143    appear in succession, all the the last of them has no effect. Consider this
3144    example:
3145    .sp
3146      ...(*COMMIT)(*PRUNE)...
3147    .sp
3148    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3149    it to be triggered, and its action is taken. There can never be a backtrack
3150    onto (*COMMIT).
3151    .
3152    .
3153    .\" HTML <a name="btrepeat"></a>
3154    .SS "Backtracking verbs in repeated groups"
3155    .rs
3156    .sp
3157    PCRE differs from Perl in its handling of backtracking verbs in repeated
3158    groups. For example, consider:
3159    .sp
3160      /(a(*COMMIT)b)+ac/
3161    .sp
3162    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3163    the second repeat of the group acts.
3164    .
3165    .
3166    .\" HTML <a name="btassert"></a>
3167    .SS "Backtracking verbs in assertions"
3168    .rs
3169    .sp
3170    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3171    .P
3172    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3173    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3174    fail without any further processing.
3175    .P
3176    The other backtracking verbs are not treated specially if they appear in a
3177    positive assertion. In particular, (*THEN) skips to the next alternative in the
3178    innermost enclosing group that has alternations, whether or not this is within
3179    the assertion.
3180    .P
3181    Negative assertions are, however, different, in order to ensure that changing a
3182    positive assertion into a negative assertion changes its result. Backtracking
3183    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3184    without considering any further alternative branches in the assertion.
3185    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3186    within the assertion (the normal behaviour), but if the assertion does not have
3187    such an alternative, (*THEN) behaves like (*PRUNE).
3188    .
3189    .
3190    .\" HTML <a name="btsub"></a>
3191    .SS "Backtracking verbs in subroutines"
3192    .rs
3193    .sp
3194    These behaviours occur whether or not the subpattern is called recursively.
3195    Perl's treatment of subroutines is different in some cases.
3196    .P
3197    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3198    an immediate backtrack.
3199    .P
3200    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3201    succeed without any further processing. Matching then continues after the
3202    subroutine call.
3203    .P
3204    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3205    the subroutine match to fail.
3206    .P
3207    (*THEN) skips to the next alternative in the innermost enclosing group within
3208    the subpattern that has alternatives. If there is no such group within the
3209    subpattern, (*THEN) causes the subroutine match to fail.
3210  .  .
3211  .  .
3212  .SH "SEE ALSO"  .SH "SEE ALSO"
# Line 3006  Cambridge CB2 3QH, England. Line 3230  Cambridge CB2 3QH, England.
3230  .rs  .rs
3231  .sp  .sp
3232  .nf  .nf
3233  Last updated: 17 March 2013  Last updated: 12 November 2013
3234  Copyright (c) 1997-2013 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3235  .fi  .fi

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