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1  .TH PCREPATTERN 3  .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 character strings. To use this,  there is now also support for UTF-8 strings in the original library, an
53  PCRE must be built to include UTF-8 support, and you must call  extra library that supports 16-bit and UTF-16 character strings, and a
54  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  third library that supports 32-bit and UTF-32 character strings. To use these
55  is also a special sequence that can be given at the start of a pattern:  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,
57    PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58    these special sequences:
59  .sp  .sp
60    (*UTF8)    (*UTF8)
61  .sp    (*UTF16)
62  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8    (*UTF32)
63  option. This feature is not Perl-compatible. How setting UTF-8 mode affects    (*UTF)
64  pattern matching is mentioned in several places below. There is also a summary  .sp
65  of UTF-8 features in the  (*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
67    option. How setting a UTF mode affects pattern matching is mentioned in several
68    places below. There is also a summary 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) 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 its main matching function, \fBpcre_exec()\fP, is used.  
 From release 6.0, PCRE offers a second matching function,  
 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  
 Perl-compatible. Some of the features discussed below are not available when  
 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  
 alternative function, and how it differs from the normal function, 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 94  string with one of the following five se Line 141  string with one of the following five se
141    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
142    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
143  .sp  .sp
144  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function. For
145  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  example, on a Unix system where LF is the default newline sequence, the pattern
 newline sequence, the pattern  
146  .sp  .sp
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 119  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 130  corresponding characters in the subject. Line 207  corresponding characters in the subject.
207  .sp  .sp
208  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
209  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
210  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
211  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
212  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
213  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
214  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
215  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
216  UTF-8 support.  UTF support.
217  .P  .P
218  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
219  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 192  otherwise be interpreted as a metacharac Line 269  otherwise be interpreted as a metacharac
269  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
270  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
271  .P  .P
272  In UTF-8 mode, only ASCII numbers and letters have any special meaning after a  In a UTF mode, only ASCII numbers and letters have any special meaning after a
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, whitespace 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 whitespace 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 235  one of the following escape sequences th Line 313  one of the following escape sequences th
313    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
314    \ecx       "control-x", where x is any ASCII character    \ecx       "control-x", where x is any ASCII character
315    \ee        escape (hex 1B)    \ee        escape (hex 1B)
316    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
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  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte  compile-time error occurs. This locks out non-ASCII characters in all modes.
333  values are valid. A lower case letter is converted to upper case, and then the  .P
334  0xc0 bits are 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 value of the character code must be less than 256  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
339  in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
340  value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341  Unicode code point, which is 10FFFF.  characters also generate different values.
 .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 282  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 297  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. In non-UTF-8 mode, the value of a  octal digits following the backslash, using them to generate a data character.
380  character specified in octal must be less than \e400. In UTF-8 mode, values up  Any subsequent digits stand for themselves. For example:
 to \e777 are permitted. 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 319  to \e777 are permitted. For example: Line 394  to \e777 are permitted. For example:
394              character with octal code 113              character with octal code 113
395  .\" JOIN  .\" JOIN
396    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
397              the byte consisting entirely of 1 bits              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 392  Another use of backslash is for specifyi Line 506  Another use of backslash is for specifyi
506  .sp  .sp
507    \ed     any decimal digit    \ed     any decimal digit
508    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
509    \eh     any horizontal whitespace character    \eh     any horizontal white space character
510    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
511    \es     any whitespace character    \es     any white space character
512    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
513    \ev     any vertical whitespace character    \ev     any vertical white space character
514    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
515    \ew     any "word" character    \ew     any "word" character
516    \eW     any "non-word" character    \eW     any "non-word" character
517  .sp  .sp
# Line 407  This is the same as Line 521  This is the same as
521  .\" </a>  .\" </a>
522  the "." metacharacter  the "." metacharacter
523  .\"  .\"
524  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
525  PCRE does not support this.  PCRE does not support this.
526  .P  .P
527  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
# Line 417  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 436  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 UTF-8 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-8 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 459  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 in  characters by default, these always match certain high-valued code points,
581  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters  whether or not PCRE_UCP is set. The horizontal space characters are:
 are:  
582  .sp  .sp
583    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
584    U+0020     Space    U+0020     Space
585    U+00A0     Non-break space    U+00A0     Non-break space
586    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 485  are: Line 602  are:
602  .sp  .sp
603  The vertical space characters are:  The vertical space characters are:
604  .sp  .sp
605    U+000A     Linefeed    U+000A     Linefeed (LF)
606    U+000B     Vertical tab    U+000B     Vertical tab (VT)
607    U+000C     Formfeed    U+000C     Form feed (FF)
608    U+000D     Carriage return    U+000D     Carriage return (CR)
609    U+0085     Next line    U+0085     Next line (NEL)
610    U+2028     Line separator    U+2028     Line separator
611    U+2029     Paragraph separator    U+2029     Paragraph separator
612    .sp
613    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
614    relevant.
615  .  .
616  .  .
617  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 499  The vertical space characters are: Line 619  The vertical space characters are:
619  .rs  .rs
620  .sp  .sp
621  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
622  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
623    following:
624  .sp  .sp
625    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
626  .sp  .sp
# Line 510  below. Line 631  below.
631  .\"  .\"
632  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
633  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
634  U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next  U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
635  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
636  cannot be split.  cannot be split.
637  .P  .P
638  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
639  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
640  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
641  recognized.  recognized.
# Line 530  one of the following sequences: Line 651  one of the following sequences:
651    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
652    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
653  .sp  .sp
654  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function, but
655  \fBpcre_compile2()\fP, but they can be overridden by options given to  they can themselves be overridden by options given to a matching function. Note
656  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  that these special settings, which are not Perl-compatible, are recognized only
657  which are not Perl-compatible, are recognized only at the very start of a  at the very start of a pattern, and that they must be in upper case. If more
658  pattern, and that they must be in upper case. If more than one of them is  than one of them is present, the last one is used. They can be combined with a
659  present, the last one is used. They can be combined with a change of newline  change of newline convention; for example, a pattern can start with:
 convention; for example, a pattern can start with:  
660  .sp  .sp
661    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
662  .sp  .sp
663  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
664  a character class, \eR is treated as an unrecognized escape sequence, and so  (*UCP) special sequences. Inside a character class, \eR is treated as an
665  matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.  unrecognized escape sequence, and so matches the letter "R" by default, but
666    causes an error if PCRE_EXTRA is set.
667  .  .
668  .  .
669  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 551  matches the letter "R" by default, but c Line 672  matches the letter "R" by default, but c
672  .sp  .sp
673  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
674  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
675  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
676  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
677  The extra escape sequences are:  The extra escape sequences are:
678  .sp  .sp
679    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
680    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
681    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
682  .sp  .sp
683  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
684  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 586  Armenian, Line 707  Armenian,
707  Avestan,  Avestan,
708  Balinese,  Balinese,
709  Bamum,  Bamum,
710    Batak,
711  Bengali,  Bengali,
712  Bopomofo,  Bopomofo,
713    Brahmi,
714  Braille,  Braille,
715  Buginese,  Buginese,
716  Buhid,  Buhid,
717  Canadian_Aboriginal,  Canadian_Aboriginal,
718  Carian,  Carian,
719    Chakma,
720  Cham,  Cham,
721  Cherokee,  Cherokee,
722  Common,  Common,
# Line 635  Lisu, Line 759  Lisu,
759  Lycian,  Lycian,
760  Lydian,  Lydian,
761  Malayalam,  Malayalam,
762    Mandaic,
763  Meetei_Mayek,  Meetei_Mayek,
764    Meroitic_Cursive,
765    Meroitic_Hieroglyphs,
766    Miao,
767  Mongolian,  Mongolian,
768  Myanmar,  Myanmar,
769  New_Tai_Lue,  New_Tai_Lue,
# Line 654  Rejang, Line 782  Rejang,
782  Runic,  Runic,
783  Samaritan,  Samaritan,
784  Saurashtra,  Saurashtra,
785    Sharada,
786  Shavian,  Shavian,
787  Sinhala,  Sinhala,
788    Sora_Sompeng,
789  Sundanese,  Sundanese,
790  Syloti_Nagri,  Syloti_Nagri,
791  Syriac,  Syriac,
# Line 664  Tagbanwa, Line 794  Tagbanwa,
794  Tai_Le,  Tai_Le,
795  Tai_Tham,  Tai_Tham,
796  Tai_Viet,  Tai_Viet,
797    Takri,
798  Tamil,  Tamil,
799  Telugu,  Telugu,
800  Thaana,  Thaana,
# Line 738  the Lu, Ll, or Lt property, in other wor Line 869  the Lu, Ll, or Lt property, in other wor
869  a modifier or "other".  a modifier or "other".
870  .P  .P
871  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
872  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
873  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
874  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
875    PCRE_NO_UTF32_CHECK in the
876  .\" HREF  .\" HREF
877  \fBpcreapi\fP  \fBpcreapi\fP
878  .\"  .\"
# Line 755  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.  example, \ep{Lu} always matches only upper case letters. This is different from
891    the behaviour of current versions of Perl.
892  .P  .P
893  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
894  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
895  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
896    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
897    PCRE_UCP option or by starting the pattern with (*UCP).
898    .
899    .
900    .SS Extended grapheme clusters
901    .rs
902  .sp  .sp
903  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
904  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  
905  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
906  .\" </a>  .\" </a>
907  (see below).  (see below).
908  .\"  .\"
909  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
910  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
911  non-UTF-8 mode \eX matches any one character.  .sp
912  .P    (?>\ePM\epM*)
913  Note that recent versions of Perl have changed \eX to match what Unicode calls  .sp
914  an "extended grapheme cluster", which has a more complicated definition.  That is, it matched a character without the "mark" property, followed by zero
915  .P  or more characters with the "mark" property. Characters with the "mark"
916  Matching characters by Unicode property is not fast, because PCRE has to search  property are typically non-spacing accents that affect the preceding character.
917  a structure that contains data for over fifteen thousand characters. That is  .P
918  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
919  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
920  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with  property, and creating rules that use these properties to define the boundaries
921  (*UCP).  of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
922    one of these clusters.
923    .P
924    \eX always matches at least one character. Then it decides whether to add
925    additional characters according to the following rules for ending a cluster:
926    .P
927    1. End at the end of the subject string.
928    .P
929    2. Do not end between CR and LF; otherwise end after any control character.
930    .P
931    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
932    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
933    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
934    character; an LVT or T character may be follwed only by a T character.
935    .P
936    4. Do not end before extending characters or spacing marks. Characters with
937    the "mark" property always have the "extend" grapheme breaking property.
938    .P
939    5. Do not end after prepend characters.
940    .P
941    6. Otherwise, end the cluster.
942  .  .
943  .  .
944  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
945  .SS PCRE's additional properties  .SS PCRE's additional properties
946  .rs  .rs
947  .sp  .sp
948  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
949  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
950  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
951  properties. PCRE uses these non-standard, non-Perl properties internally when  properties internally when PCRE_UCP is set. However, they may also be used
952  PCRE_UCP is set. They 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 800  PCRE_UCP is set. They are: Line 957  PCRE_UCP is set. They are:
957    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
958  .sp  .sp
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, formfeed, 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    matches the same characters as Xan, plus underscore.
965    .P
966    There is another non-standard property, Xuc, which matches any character that
967    can be represented by a Universal Character Name in C++ and other programming
968    languages. These are the characters $, @, ` (grave accent), and all characters
969    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
972    where H is a hexadecimal digit. Note that the Xuc property does not match these
973    sequences but the characters that they represent.)
974  .  .
975  .  .
976  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 870  escape sequence" error is generated inst Line 1037  escape sequence" error is generated inst
1037  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
1038  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
1039  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
1040  first or last character matches \ew, respectively. In UTF-8 mode, the meanings  first or last character matches \ew, respectively. In a UTF mode, the meanings
1041  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1042  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1043  of word" or "end of word" metasequence. However, whatever follows \eb normally  of word" or "end of word" metasequence. However, whatever follows \eb normally
# Line 909  regular expression. Line 1076  regular expression.
1076  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1077  .rs  .rs
1078  .sp  .sp
1079    The circumflex and dollar metacharacters are zero-width assertions. That is,
1080    they test for a particular condition being true without consuming any
1081    characters from the subject string.
1082    .P
1083  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1084  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
1085  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
1086  \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
1087  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1088  meaning  meaning
# Line 928  constrained to match only at the start o Line 1099  constrained to match only at the start o
1099  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1100  to be anchored.)  to be anchored.)
1101  .P  .P
1102  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
1103  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
1104  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
1105  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
1106  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
1107  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1108  .P  .P
1109  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
1110  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 965  end of the subject in both modes, and if Line 1136  end of the subject in both modes, and if
1136  .sp  .sp
1137  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
1138  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
1139  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1140  .P  .P
1141  When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
1142  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 989  that signifies the end of a line. Perl a Line 1160  that signifies the end of a line. Perl a
1160  name; PCRE does not support this.  name; PCRE does not support this.
1161  .  .
1162  .  .
1163  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE DATA UNIT"
1164  .rs  .rs
1165  .sp  .sp
1166  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one data unit,
1167  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1168  characters. The feature is provided in Perl in order to match individual bytes  byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1169  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC  a 32-bit unit. Unlike a dot, \eC always
1170  breaks up characters into individual bytes, matching one byte with \eC in UTF-8  matches line-ending characters. The feature is provided in Perl in order to
1171  mode means that the rest of the string may start with a malformed UTF-8  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1172  character. This has undefined results, because PCRE assumes that it is dealing  used. Because \eC breaks up characters into individual data units, matching one
1173  with valid UTF-8 strings (and by default it checks this at the start of  unit with \eC in a UTF mode means that the rest of the string may start with a
1174  processing unless the PCRE_NO_UTF8_CHECK option is used).  malformed UTF character. This has undefined results, because PCRE assumes that
1175    it is dealing with valid UTF strings (and by default it checks this at the
1176    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1177    PCRE_NO_UTF32_CHECK option is used).
1178  .P  .P
1179  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1180  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1181  .\" </a>  .\" </a>
1182  (described below)  (described below)
1183  .\"  .\"
1184  in UTF-8 mode, because this would make it impossible to calculate the length of  in a UTF mode, because this would make it impossible to calculate the length of
1185  the lookbehind.  the lookbehind.
1186  .P  .P
1187  In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one  In general, the \eC escape sequence is best avoided. However, one
1188  way of using it that avoids the problem of malformed UTF-8 characters is to  way of using it that avoids the problem of malformed UTF characters is to use a
1189  use a lookahead to check the length of the next character, as in this pattern  lookahead to check the length of the next character, as in this pattern, which
1190  (ignore white space and line breaks):  could be used with a UTF-8 string (ignore white space and line breaks):
1191  .sp  .sp
1192    (?| (?=[\ex00-\ex7f])(\eC) |    (?| (?=[\ex00-\ex7f])(\eC) |
1193        (?=[\ex80-\ex{7ff}])(\eC)(\eC) |        (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
# Line 1043  bracket causes a compile-time error. If Line 1217  bracket causes a compile-time error. If
1217  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1218  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1219  .P  .P
1220  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In a UTF mode, the
1221  character may be more than one byte long. A matched character must be in the  character may be more than one data unit long. A matched character must be in
1222  set of characters defined by the class, unless the first character in the class  the set of characters defined by the class, unless the first character in the
1223  definition is a circumflex, in which case the subject character must not be in  class definition is a circumflex, in which case the subject character must not
1224  the set defined by the class. If a circumflex is actually required as a member  be in the set defined by the class. If a circumflex is actually required as a
1225  of the class, ensure it is not the first character, or escape it with a  member of the class, ensure it is not the first character, or escape it with a
1226  backslash.  backslash.
1227  .P  .P
1228  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
# Line 1059  circumflex is not an assertion; it still Line 1233  circumflex is not an assertion; it still
1233  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
1234  string.  string.
1235  .P  .P
1236  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1237  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  can be included in a class as a literal string of data units, or by using the
1238    \ex{ escaping mechanism.
1239  .P  .P
1240  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
1241  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
1242  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1243  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1244  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1245  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1246  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1247  If you want to use caseless matching in UTF8-mode for characters 128 and above,  If you want to use caseless matching in a UTF mode for characters 128 and
1248  you must ensure that PCRE is compiled with Unicode property support as well as  above, you must ensure that PCRE is compiled with Unicode property support as
1249  with UTF-8 support.  well as with UTF support.
1250  .P  .P
1251  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1252  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 1082  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 1092  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]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1279  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1280  .P  .P
1281  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1282  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1283  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1284  tables for a French locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1285  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1286  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1287  property support.  property support.
1288  .P  .P
1289  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1290  \eV, \ew, and \eW may appear in a character class, and add the characters that  \eV, \ew, and \eW may appear in a character class, and add the characters that
1291  they match to the class. For example, [\edABCDEF] matches any hexadecimal  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1292  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1293  and their upper case partners, just as it does when they appear outside a  and their upper case partners, just as it does when they appear outside a
1294  character class, as described in the section entitled  character class, as described in the section entitled
1295  .\" HTML <a href="#genericchartypes">  .\" HTML <a href="#genericchartypes">
# Line 1158  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 1178  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-8 mode, 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 1193  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 1266  option settings happen at compile time. Line 1470  option settings happen at compile time.
1470  behaviour otherwise.  behaviour otherwise.
1471  .P  .P
1472  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1473  application when the compile or match functions are called. In some cases the  application when the compiling or matching functions are called. In some cases
1474  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1475  the application has set or what has been defaulted. Details are given in the  what the application has set or what has been defaulted. Details are given in
1476  section entitled  the section entitled
1477  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1478  .\" </a>  .\" </a>
1479  "Newline sequences"  "Newline sequences"
1480  .\"  .\"
1481  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1482  to set UTF-8 and Unicode property modes; they are equivalent to setting the  sequences that can be used to set UTF and Unicode property modes; they are
1483  PCRE_UTF8 and the PCRE_UCP options, respectively.  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
1485    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 1295  match "cataract", "erpillar" or an empty Line 1502  match "cataract", "erpillar" or an empty
1502  .sp  .sp
1503  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1504  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1505  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of the
1506  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1507  from 1) to obtain numbers for the capturing subpatterns. For example, if the  the DFA matching functions do not support capturing.)
1508  string "the red king" is matched against the pattern  .P
1509    Opening parentheses are counted from left to right (starting from 1) to obtain
1510    numbers for the capturing subpatterns. For example, if the string "the red
1511    king" is matched against the pattern
1512  .sp  .sp
1513    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1514  .sp  .sp
# Line 1409  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 1439  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 1461  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 1474  items: Line 1697  items:
1697    a literal data character    a literal data character
1698    the dot metacharacter    the dot metacharacter
1699    the \eC escape sequence    the \eC escape sequence
1700    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1701    the \eR escape sequence    the \eR escape sequence
1702    an escape such as \ed or \epL that matches a single character    an escape such as \ed or \epL that matches a single character
1703    a character class    a character class
# Line 1505  where a quantifier is not allowed, or on Line 1728  where a quantifier is not allowed, or on
1728  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1729  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1730  .P  .P
1731  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1732  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1733  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1734  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1735  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1736  .P  .P
1737  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
1738  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 1595  In cases where it is known that the subj Line 1818  In cases where it is known that the subj
1818  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1819  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1820  .P  .P
1821  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1822  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1823  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
1824  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1605  succeeds. Consider, for example: Line 1828  succeeds. Consider, for example:
1828  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
1829  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1830  .P  .P
1831    Another case where implicit anchoring is not applied is when the leading .* is
1832    inside an atomic group. Once again, a match at the start may fail where a later
1833    one succeeds. Consider this pattern:
1834    .sp
1835      (?>.*?a)b
1836    .sp
1837    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1838    (*PRUNE) and (*SKIP) also disable this optimization.
1839    .P
1840  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1841  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1842  .sp  .sp
# Line 1819  Because there may be many capturing pare Line 2051  Because there may be many capturing pare
2051  following a backslash are taken as part of a potential back reference number.  following a backslash are taken as part of a potential back reference number.
2052  If the pattern continues with a digit character, some delimiter must be used to  If the pattern continues with a digit character, some delimiter must be used to
2053  terminate the back reference. If the PCRE_EXTENDED option is set, this can be  terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2054  whitespace. Otherwise, the \eg{ syntax or an empty comment (see  white space. Otherwise, the \eg{ syntax or an empty comment (see
2055  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
2056  .\" </a>  .\" </a>
2057  "Comments"  "Comments"
# Line 1873  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, because it does not make  capturing is carried out only for positive assertions. (Perl sometimes, but not
2109  sense for 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
2112  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 1972  temporarily move the current position ba Line 2204  temporarily move the current position ba
2204  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2205  assertion fails.  assertion fails.
2206  .P  .P
2207  In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2208  even in UTF-8 mode) to appear in lookbehind assertions, because it makes it  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2209  impossible to calculate the length of the lookbehind. The \eX and \eR escapes,  it impossible to calculate the length of the lookbehind. The \eX and \eR
2210  which can match different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2211    permitted.
2212  .P  .P
2213  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2214  .\" </a>  .\" </a>
# Line 2123  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 2175  subroutines that can be referenced from Line 2403  subroutines that can be referenced from
2403  subroutines  subroutines
2404  .\"  .\"
2405  is described below.) For example, a pattern to match an IPv4 address such as  is described below.) For example, a pattern to match an IPv4 address such as
2406  "192.168.23.245" could be written like this (ignore whitespace and line  "192.168.23.245" could be written like this (ignore white space and line
2407  breaks):  breaks):
2408  .sp  .sp
2409    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
# Line 2222  closing parenthesis. Nested parentheses Line 2450  closing parenthesis. Nested parentheses
2450  option is set, an unescaped # character also introduces a comment, which in  option is set, an unescaped # character also introduces a comment, which in
2451  this case continues to immediately after the next newline character or  this case continues to immediately after the next newline character or
2452  character sequence in the pattern. Which characters are interpreted as newlines  character sequence in the pattern. Which characters are interpreted as newlines
2453  is controlled by the options passed to \fBpcre_compile()\fP or by a special  is controlled by the options passed to a compiling function or by a special
2454  sequence at the start of the pattern, as described in the section entitled  sequence at the start of the pattern, as described in the section entitled
2455  .\" HTML <a href="#newlines">  .\" HTML <a href="#newlines">
2456  .\" </a>  .\" </a>
# Line 2524  same pair of parentheses when there is a Line 2752  same pair of parentheses when there is a
2752  .P  .P
2753  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
2754  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
2755  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
2756    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2757  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2758  .P  .P
2759  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
# Line 2534  For example, this pattern has two callou Line 2763  For example, this pattern has two callou
2763  .sp  .sp
2764    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2765  .sp  .sp
2766  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2767  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2768  255.  255. If there is a conditional group in the pattern whose condition is an
2769  .P  assertion, an additional callout is inserted just before the condition. An
2770  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  explicit callout may also be set at this position, as in this example:
2771  set), the external function is called. It is provided with the number of the  .sp
2772  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2773  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2774  may cause matching to proceed, to backtrack, or to fail altogether. A complete  Note that this applies only to assertion conditions, not to other types of
2775  description of the interface to the callout function is given in the  condition.
2776    .P
2777    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
2779    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
2781    backtrack, or to fail altogether.
2782    .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 2555  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
 Since these verbs are specifically related to backtracking, most of them can be  
 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  
 a backtracking algorithm. With the exception of (*FAIL), which behaves like a  
 failing negative assertion, they cause an error if encountered by  
 \fBpcre_dfa_exec()\fP.  
 .P  
 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 is different in some cases.  
 .P  
2804  The new verbs make use of what was previously invalid syntax: an opening  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  parenthesis followed by an asterisk. They are generally of the form
2806  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2807  depending on whether or not an argument is present. A name is any sequence of  differently depending on whether or not a name is present. A name is any
2808  characters that does not include a closing parenthesis. If the name is empty,  sequence of characters that does not include a closing parenthesis. The maximum
2809  that is, if the closing parenthesis immediately follows the colon, the effect  length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2810  is as if the colon were not there. Any number of these verbs may occur in a  libraries. If the name is empty, that is, if the closing parenthesis
2811  pattern.  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
2815    used only when the pattern is to be matched using one of the traditional
2816    matching functions, because these use a backtracking algorithm. With the
2817    exception of (*FAIL), which behaves like a failing negative assertion, the
2818    backtracking control verbs cause an error if encountered by a DFA matching
2819    function.
2820  .P  .P
2821    The behaviour of these verbs in
2822    .\" HTML <a href="#btrepeat">
2823    .\" </a>
2824    repeated groups,
2825    .\"
2826    .\" HTML <a href="#btassert">
2827    .\" </a>
2828    assertions,
2829    .\"
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>
2839    .SS "Optimizations that affect backtracking verbs"
2840    .rs
2841    .sp
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
2849  pattern with (*NO_START_OPT).  pattern with (*NO_START_OPT). There is more discussion of this option in the
2850    section entitled
2851    .\" HTML <a href="pcreapi.html#execoptions">
2852    .\" </a>
2853    "Option bits for \fBpcre_exec()\fP"
2854    .\"
2855    in the
2856    .\" HREF
2857    \fBpcreapi\fP
2858    .\"
2859    documentation.
2860  .P  .P
2861  Experiments with Perl suggest that it too has similar optimizations, sometimes  Experiments with Perl suggest that it too has similar optimizations, sometimes
2862  leading to anomalous results.  leading to anomalous results.
2863  .  .
2864  .  .
# Line 2608  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 2642  starting point (see (*SKIP) below). Line 2910  starting point (see (*SKIP) below).
2910  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
2911  (*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.
2912  .P  .P
2913  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),
2914  path is passed back to the caller via the \fIpcre_extra\fP data structure, as  (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2915  described in the  caller as described in the section entitled
2916  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2917  .\" </a>  .\" </a>
2918  section on \fIpcre_extra\fP  "Extra data for \fBpcre_exec()\fP"
2919  .\"  .\"
2920  in the  in the
2921  .\" HREF  .\" HREF
# Line 2669  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 (*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
2941  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
2942  assertions.  happen for negative assertions or failing positive assertions.
2943  .P  .P
2944  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
2945  (*MARK) in the entire match process is returned. For example:  entire match process is returned. For example:
2946  .sp  .sp
2947      re> /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2948    data> XP    data> XP
2949    No match, mark = B    No match, mark = B
2950  .sp  .sp
2951  Note that in this unanchored example the mark is retained from the match  Note that in this unanchored example the mark is retained from the match
2952  attempt that started at the letter "X". Subsequent match attempts starting at  attempt that started at the letter "X" in the subject. Subsequent match
2953  "P" and then with an empty string do not get as far as the (*MARK) item, but  attempts starting at "P" and then with an empty string do not get as far as the
2954  nevertheless do not reset it.  (*MARK) item, but nevertheless do not reset it.
2955    .P
2956    If you are interested in (*MARK) values after failed matches, you should
2957    probably set the PCRE_NO_START_OPTIMIZE option
2958    .\" HTML <a href="#nooptimize">
2959    .\" </a>
2960    (see above)
2961    .\"
2962    to ensure that the match is always attempted.
2963  .  .
2964  .  .
2965  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2692  nevertheless do not reset it. Line 2968  nevertheless do not reset it.
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 2716  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 2735  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  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
3031  anchored pattern (*PRUNE) has the same effect as (*COMMIT).  as (*COMMIT).
3032    .P
3033    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
3035    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3036  .sp  .sp
3037    (*SKIP)    (*SKIP)
3038  .sp  .sp
# Line 2764  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
3063    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).
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. The  second alternative and tries COND2, without backtracking into COND1. If that
3078  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
3079  If (*THEN) is not inside an alternation, it acts like (*PRUNE).  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
3082    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
3084    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 2806  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 2828  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    .
3129    .
3130    .SS "More than one backtracking verb"
3131    .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
3137      (A(*COMMIT)B(*THEN)C|ABD)
3138    .sp
3139    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
3141    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3142    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  .P
3207  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
3208  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
3209  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.  
3210  .  .
3211  .  .
3212  .SH "SEE ALSO"  .SH "SEE ALSO"
3213  .rs  .rs
3214  .sp  .sp
3215  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3216  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3217  .  .
3218  .  .
3219  .SH AUTHOR  .SH AUTHOR
# Line 2862  Cambridge CB2 3QH, England. Line 3230  Cambridge CB2 3QH, England.
3230  .rs  .rs
3231  .sp  .sp
3232  .nf  .nf
3233  Last updated: 29 November 2011  Last updated: 12 November 2013
3234  Copyright (c) 1997-2011 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3235  .fi  .fi

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