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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "06 September 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  .\" HTML <a href="pcre.html#utf8support">  Starting a pattern with such a sequence is equivalent to setting the relevant
67  .\" </a>  option. How setting a UTF mode affects pattern matching is mentioned in several
68  section on UTF-8 support  places below. There is also a summary of features in the
 .\"  
 in the main  
69  .\" HREF  .\" HREF
70  \fBpcre\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    option is set at compile time, (*UTF) etc. are not allowed, and their
77    appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82    .sp
83    Another special sequence that may appear at the start of a pattern is
84  .sp  .sp
85    (*UCP)    (*UCP)
86  .sp  .sp
# Line 51  This has the same effect as setting the Line 88  This has the same effect as setting the
88  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,
89  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
90  table.  table.
91  .P  .
92    .
93    .SS "Disabling start-up optimizations"
94    .rs
95    .sp
96  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
97  PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are  PCRE_NO_START_OPTIMIZE option either at compile or matching time.
 also some more of these special sequences that are concerned with the handling  
 of newlines; they are described below.  
 .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  
 .\" HREF  
 \fBpcrematching\fP  
 .\"  
 page.  
98  .  .
99  .  .
100  .\" HTML <a name="newlines"></a>  .\" HTML <a name="newlines"></a>
101  .SH "NEWLINE CONVENTIONS"  .SS "Newline conventions"
102  .rs  .rs
103  .sp  .sp
104  PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
# Line 99  string with one of the following five se Line 125  string with one of the following five se
125    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
126    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
127  .sp  .sp
128  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
129  \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  
130  .sp  .sp
131    (*CR)a.b    (*CR)a.b
132  .sp  .sp
133  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
134  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  
135  is used.  is used.
136  .P  .P
137  The newline convention affects the interpretation of the dot metacharacter when  The newline convention affects where the circumflex and dollar assertions are
138  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
139  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
140  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
141    sequence, for Perl compatibility. However, this can be changed; see the
142  description of \eR in the section entitled  description of \eR in the section entitled
143  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
144  .\" </a>  .\" </a>
# Line 124  below. A change of \eR setting can be co Line 148  below. A change of \eR setting can be co
148  convention.  convention.
149  .  .
150  .  .
151    .SS "Setting match and recursion limits"
152    .rs
153    .sp
154    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
155    internal \fBmatch()\fP function is called and on the maximum depth of
156    recursive calls. These facilities are provided to catch runaway matches that
157    are provoked by patterns with huge matching trees (a typical example is a
158    pattern with nested unlimited repeats) and to avoid running out of system stack
159    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
160    gives an error return. The limits can also be set by items at the start of the
161    pattern of the form
162    .sp
163      (*LIMIT_MATCH=d)
164      (*LIMIT_RECURSION=d)
165    .sp
166    where d is any number of decimal digits. However, the value of the setting must
167    be less than the value set by the caller of \fBpcre_exec()\fP for it to have
168    any effect. In other words, the pattern writer can lower the limit set by the
169    programmer, but not raise it. If there is more than one setting of one of these
170    limits, the lower value is used.
171    .
172    .
173    .SH "EBCDIC CHARACTER CODES"
174    .rs
175    .sp
176    PCRE can be compiled to run in an environment that uses EBCDIC as its character
177    code rather than ASCII or Unicode (typically a mainframe system). In the
178    sections below, character code values are ASCII or Unicode; in an EBCDIC
179    environment these characters may have different code values, and there are no
180    code points greater than 255.
181    .
182    .
183  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
184  .rs  .rs
185  .sp  .sp
# Line 135  corresponding characters in the subject. Line 191  corresponding characters in the subject.
191  .sp  .sp
192  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
193  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
194  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
195  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
196  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
197  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
198  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
199  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
200  UTF-8 support.  UTF support.
201  .P  .P
202  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
203  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 197  otherwise be interpreted as a metacharac Line 253  otherwise be interpreted as a metacharac
253  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
254  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
255  .P  .P
256  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
257  backslash. All other characters (in particular, those whose codepoints are  backslash. All other characters (in particular, those whose codepoints are
258  greater than 127) are treated as literals.  greater than 127) are treated as literals.
259  .P  .P
260  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, white space in the
261  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
262  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
263  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
264  .P  .P
265  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
266  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 220  Perl, $ and @ cause variable interpolati Line 276  Perl, $ and @ cause variable interpolati
276    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
277  .sp  .sp
278  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
279  An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed  An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
280  by \eE later in the pattern, the literal interpretation continues to the end of  by \eE later in the pattern, the literal interpretation continues to the end of
281  the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside  the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
282  a character class, this causes an error, because the character class is not  a character class, this causes an error, because the character class is not
283  terminated.  terminated.
# Line 240  one of the following escape sequences th Line 296  one of the following escape sequences th
296    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
297    \ecx       "control-x", where x is any ASCII character    \ecx       "control-x", where x is any ASCII character
298    \ee        escape (hex 1B)    \ee        escape (hex 1B)
299    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
300    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
301    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
302    \et        tab (hex 09)    \et        tab (hex 09)
303    \eddd      character with octal code ddd, or back reference    \eddd      character with octal code ddd, or back reference
304    \exhh      character with hex code hh    \exhh      character with hex code hh
305    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
306      \euhhhh    character with hex code hhhh (JavaScript mode only)
307    .sp
308    The precise effect of \ecx on ASCII characters is as follows: if x is a lower
309    case letter, it is converted to upper case. Then bit 6 of the character (hex
310    40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
311    but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
312    data item (byte or 16-bit value) following \ec has a value greater than 127, a
313    compile-time error occurs. This locks out non-ASCII characters in all modes.
314    .P
315    The \ec facility was designed for use with ASCII characters, but with the
316    extension to Unicode it is even less useful than it once was. It is, however,
317    recognized when PCRE is compiled in EBCDIC mode, where data items are always
318    bytes. In this mode, all values are valid after \ec. If the next character is a
319    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
320    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
321    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
322    characters also generate different values.
323    .P
324    By default, after \ex, from zero to two hexadecimal digits are read (letters
325    can be in upper or lower case). Any number of hexadecimal digits may appear
326    between \ex{ and }, but the character code is constrained as follows:
327    .sp
328      8-bit non-UTF mode    less than 0x100
329      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
330      16-bit non-UTF mode   less than 0x10000
331      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
332      32-bit non-UTF mode   less than 0x80000000
333      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
334  .sp  .sp
335  The precise effect of \ecx is as follows: if x is a lower case letter, it  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
336  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  "surrogate" codepoints), and 0xffef.
 Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while  
 \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater  
 than 127, a compile-time error occurs. This locks out non-ASCII characters in  
 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte  
 values are valid. A lower case letter is converted to upper case, and then the  
 0xc0 bits are flipped.)  
 .P  
 After \ex, from zero to two hexadecimal digits are read (letters can be in  
 upper or lower case). Any number of hexadecimal digits may appear between \ex{  
 and }, but the value of the character code must be less than 256 in non-UTF-8  
 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  
 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  
 point, which is 10FFFF.  
337  .P  .P
338  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
339  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
340  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
341  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
342  .P  .P
343    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
344    as just described only when it is followed by two hexadecimal digits.
345    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
346    code points greater than 256 is provided by \eu, which must be followed by
347    four hexadecimal digits; otherwise it matches a literal "u" character.
348    Character codes specified by \eu in JavaScript mode are constrained in the same
349    was as those specified by \ex in non-JavaScript mode.
350    .P
351  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
352  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
353  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
354    \eu00dc in JavaScript mode).
355  .P  .P
356  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
357  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 297  parenthesized subpatterns. Line 377  parenthesized subpatterns.
377  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
378  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
379  digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
380  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. The value of the character is
381  character specified in octal must be less than \e400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
382  to \e777 are permitted. For example:  For example:
383  .sp  .sp
384    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
385  .\" JOIN  .\" JOIN
386    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
387              previous capturing subpatterns              previous capturing subpatterns
# Line 316  to \e777 are permitted. For example: Line 396  to \e777 are permitted. For example:
396              character with octal code 113              character with octal code 113
397  .\" JOIN  .\" JOIN
398    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
399              the byte consisting entirely of 1 bits              the value 255 (decimal)
400  .\" JOIN  .\" JOIN
401    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
402              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 325  Note that octal values of 100 or greater Line 405  Note that octal values of 100 or greater
405  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
406  .P  .P
407  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
408  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
409  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
410  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
411  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
412  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
413  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
414    error if the PCRE_EXTRA option is set. Outside a character class, these
415    sequences have different meanings.
416    .
417    .
418    .SS "Unsupported escape sequences"
419    .rs
420    .sp
421    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
422    handler and used to modify the case of following characters. By default, PCRE
423    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
424    option is set, \eU matches a "U" character, and \eu can be used to define a
425    character by code point, as described in the previous section.
426  .  .
427  .  .
428  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 377  Another use of backslash is for specifyi Line 469  Another use of backslash is for specifyi
469  .sp  .sp
470    \ed     any decimal digit    \ed     any decimal digit
471    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
472    \eh     any horizontal whitespace character    \eh     any horizontal white space character
473    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
474    \es     any whitespace character    \es     any white space character
475    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
476    \ev     any vertical whitespace character    \ev     any vertical white space character
477    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
478    \ew     any "word" character    \ew     any "word" character
479    \eW     any "non-word" character    \eW     any "non-word" character
480  .sp  .sp
# Line 392  This is the same as Line 484  This is the same as
484  .\" </a>  .\" </a>
485  the "." metacharacter  the "." metacharacter
486  .\"  .\"
487  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
488    PCRE does not support this.
489  .P  .P
490  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
491  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 424  or "french" in Windows, some character c Line 517  or "french" in Windows, some character c
517  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
518  Unicode is discouraged.  Unicode is discouraged.
519  .P  .P
520  By default, in UTF-8 mode, characters with values greater than 128 never match  By default, in a UTF mode, characters with values greater than 128 never match
521  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
522  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF support was available, mainly for
523  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
524  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  and the PCRE_UCP option is set, the behaviour is changed so that Unicode
525  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
# Line 443  is noticeably slower when PCRE_UCP is se Line 536  is noticeably slower when PCRE_UCP is se
536  .P  .P
537  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
538  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
539  characters by default, these always match certain high-valued codepoints in  characters by default, these always match certain high-valued codepoints,
540  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:  
541  .sp  .sp
542    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
543    U+0020     Space    U+0020     Space
544    U+00A0     Non-break space    U+00A0     Non-break space
545    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 469  are: Line 561  are:
561  .sp  .sp
562  The vertical space characters are:  The vertical space characters are:
563  .sp  .sp
564    U+000A     Linefeed    U+000A     Linefeed (LF)
565    U+000B     Vertical tab    U+000B     Vertical tab (VT)
566    U+000C     Formfeed    U+000C     Form feed (FF)
567    U+000D     Carriage return    U+000D     Carriage return (CR)
568    U+0085     Next line    U+0085     Next line (NEL)
569    U+2028     Line separator    U+2028     Line separator
570    U+2029     Paragraph separator    U+2029     Paragraph separator
571    .sp
572    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
573    relevant.
574  .  .
575  .  .
576  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 483  The vertical space characters are: Line 578  The vertical space characters are:
578  .rs  .rs
579  .sp  .sp
580  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
581  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
582    following:
583  .sp  .sp
584    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
585  .sp  .sp
# Line 494  below. Line 590  below.
590  .\"  .\"
591  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
592  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,
593  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
594  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
595  cannot be split.  cannot be split.
596  .P  .P
597  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
598  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).
599  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
600  recognized.  recognized.
# Line 514  one of the following sequences: Line 610  one of the following sequences:
610    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
611    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
612  .sp  .sp
613  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
614  \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
615  \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
616  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
617  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
618  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:  
619  .sp  .sp
620    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
621  .sp  .sp
622  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
623  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
624  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
625    causes an error if PCRE_EXTRA is set.
626  .  .
627  .  .
628  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 535  matches the letter "R" by default, but c Line 631  matches the letter "R" by default, but c
631  .sp  .sp
632  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
633  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
634  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
635  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.
636  The extra escape sequences are:  The extra escape sequences are:
637  .sp  .sp
638    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
639    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
640    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
641  .sp  .sp
642  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
643  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 570  Armenian, Line 666  Armenian,
666  Avestan,  Avestan,
667  Balinese,  Balinese,
668  Bamum,  Bamum,
669    Batak,
670  Bengali,  Bengali,
671  Bopomofo,  Bopomofo,
672    Brahmi,
673  Braille,  Braille,
674  Buginese,  Buginese,
675  Buhid,  Buhid,
676  Canadian_Aboriginal,  Canadian_Aboriginal,
677  Carian,  Carian,
678    Chakma,
679  Cham,  Cham,
680  Cherokee,  Cherokee,
681  Common,  Common,
# Line 619  Lisu, Line 718  Lisu,
718  Lycian,  Lycian,
719  Lydian,  Lydian,
720  Malayalam,  Malayalam,
721    Mandaic,
722  Meetei_Mayek,  Meetei_Mayek,
723    Meroitic_Cursive,
724    Meroitic_Hieroglyphs,
725    Miao,
726  Mongolian,  Mongolian,
727  Myanmar,  Myanmar,
728  New_Tai_Lue,  New_Tai_Lue,
# Line 638  Rejang, Line 741  Rejang,
741  Runic,  Runic,
742  Samaritan,  Samaritan,
743  Saurashtra,  Saurashtra,
744    Sharada,
745  Shavian,  Shavian,
746  Sinhala,  Sinhala,
747    Sora_Sompeng,
748  Sundanese,  Sundanese,
749  Syloti_Nagri,  Syloti_Nagri,
750  Syriac,  Syriac,
# Line 648  Tagbanwa, Line 753  Tagbanwa,
753  Tai_Le,  Tai_Le,
754  Tai_Tham,  Tai_Tham,
755  Tai_Viet,  Tai_Viet,
756    Takri,
757  Tamil,  Tamil,
758  Telugu,  Telugu,
759  Thaana,  Thaana,
# Line 722  the Lu, Ll, or Lt property, in other wor Line 828  the Lu, Ll, or Lt property, in other wor
828  a modifier or "other".  a modifier or "other".
829  .P  .P
830  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
831  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
832  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
833  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
834    PCRE_NO_UTF32_CHECK in the
835  .\" HREF  .\" HREF
836  \fBpcreapi\fP  \fBpcreapi\fP
837  .\"  .\"
# Line 739  Instead, this property is assumed for an Line 846  Instead, this property is assumed for an
846  Unicode table.  Unicode table.
847  .P  .P
848  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
849  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
850    the behaviour of current versions of Perl.
851  .P  .P
852  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
853  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
854  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
855    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
856    PCRE_UCP option or by starting the pattern with (*UCP).
857    .
858    .
859    .SS Extended grapheme clusters
860    .rs
861  .sp  .sp
862  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
863  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  
864  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
865  .\" </a>  .\" </a>
866  (see below).  (see below).
867  .\"  .\"
868  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
869  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
870  non-UTF-8 mode \eX matches any one character.  .sp
871  .P    (?>\ePM\epM*)
872  Note that recent versions of Perl have changed \eX to match what Unicode calls  .sp
873  an "extended grapheme cluster", which has a more complicated definition.  That is, it matched a character without the "mark" property, followed by zero
874  .P  or more characters with the "mark" property. Characters with the "mark"
875  Matching characters by Unicode property is not fast, because PCRE has to search  property are typically non-spacing accents that affect the preceding character.
876  a structure that contains data for over fifteen thousand characters. That is  .P
877  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
878  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
879  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
880  (*UCP).  of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
881    one of these clusters.
882    .P
883    \eX always matches at least one character. Then it decides whether to add
884    additional characters according to the following rules for ending a cluster:
885    .P
886    1. End at the end of the subject string.
887    .P
888    2. Do not end between CR and LF; otherwise end after any control character.
889    .P
890    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
891    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
892    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
893    character; an LVT or T character may be follwed only by a T character.
894    .P
895    4. Do not end before extending characters or spacing marks. Characters with
896    the "mark" property always have the "extend" grapheme breaking property.
897    .P
898    5. Do not end after prepend characters.
899    .P
900    6. Otherwise, end the cluster.
901  .  .
902  .  .
903  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
904  .SS PCRE's additional properties  .SS PCRE's additional properties
905  .rs  .rs
906  .sp  .sp
907  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
908  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
909  escape sequences such as \ew and \es and POSIX character classes to use Unicode  and \es and POSIX character classes to use Unicode properties. PCRE uses these
910  properties. PCRE uses these non-standard, non-Perl properties internally when  non-standard, non-Perl properties internally when PCRE_UCP is set. However,
911  PCRE_UCP is set. They are:  they may also be used explicitly. These properties are:
912  .sp  .sp
913    Xan   Any alphanumeric character    Xan   Any alphanumeric character
914    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 784  PCRE_UCP is set. They are: Line 916  PCRE_UCP is set. They are:
916    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
917  .sp  .sp
918  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)
919  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
920  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
921  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
922  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
923    .P
924    There is another non-standard property, Xuc, which matches any character that
925    can be represented by a Universal Character Name in C++ and other programming
926    languages. These are the characters $, @, ` (grave accent), and all characters
927    with Unicode code points greater than or equal to U+00A0, except for the
928    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
929    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
930    where H is a hexadecimal digit. Note that the Xuc property does not match these
931    sequences but the characters that they represent.)
932  .  .
933  .  .
934  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 854  escape sequence" error is generated inst Line 995  escape sequence" error is generated inst
995  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
996  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
997  \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
998  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
999  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
1000  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
1001  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 893  regular expression. Line 1034  regular expression.
1034  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1035  .rs  .rs
1036  .sp  .sp
1037    The circumflex and dollar metacharacters are zero-width assertions. That is,
1038    they test for a particular condition being true without consuming any
1039    characters from the subject string.
1040    .P
1041  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1042  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
1043  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
1044  \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
1045  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1046  meaning  meaning
# Line 912  constrained to match only at the start o Line 1057  constrained to match only at the start o
1057  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1058  to be anchored.)  to be anchored.)
1059  .P  .P
1060  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
1061  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
1062  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
1063  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
1064  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
1065  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1066  .P  .P
1067  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
1068  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 949  end of the subject in both modes, and if Line 1094  end of the subject in both modes, and if
1094  .sp  .sp
1095  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
1096  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
1097  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1098  .P  .P
1099  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
1100  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 969  special meaning in a character class. Line 1114  special meaning in a character class.
1114  .P  .P
1115  The escape sequence \eN behaves like a dot, except that it is not affected by  The escape sequence \eN behaves like a dot, except that it is not affected by
1116  the PCRE_DOTALL option. In other words, it matches any character except one  the PCRE_DOTALL option. In other words, it matches any character except one
1117  that signifies the end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
1118    name; PCRE does not support this.
1119  .  .
1120  .  .
1121  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE DATA UNIT"
1122  .rs  .rs
1123  .sp  .sp
1124  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,
1125  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1126  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
1127  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the  a 32-bit unit. Unlike a dot, \eC always
1128  rest of the string may start with a malformed UTF-8 character. For this reason,  matches line-ending characters. The feature is provided in Perl in order to
1129  the \eC escape sequence is best avoided.  match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1130    used. Because \eC breaks up characters into individual data units, matching one
1131    unit with \eC in a UTF mode means that the rest of the string may start with a
1132    malformed UTF character. This has undefined results, because PCRE assumes that
1133    it is dealing with valid UTF strings (and by default it checks this at the
1134    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1135    PCRE_NO_UTF32_CHECK option is used).
1136  .P  .P
1137  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1138  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1139  .\" </a>  .\" </a>
1140  (described below),  (described below)
1141  .\"  .\"
1142  because in UTF-8 mode 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
1143  the lookbehind.  the lookbehind.
1144    .P
1145    In general, the \eC escape sequence is best avoided. However, one
1146    way of using it that avoids the problem of malformed UTF characters is to use a
1147    lookahead to check the length of the next character, as in this pattern, which
1148    could be used with a UTF-8 string (ignore white space and line breaks):
1149    .sp
1150      (?| (?=[\ex00-\ex7f])(\eC) |
1151          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1152          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1153          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1154    .sp
1155    A group that starts with (?| resets the capturing parentheses numbers in each
1156    alternative (see
1157    .\" HTML <a href="#dupsubpatternnumber">
1158    .\" </a>
1159    "Duplicate Subpattern Numbers"
1160    .\"
1161    below). The assertions at the start of each branch check the next UTF-8
1162    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1163    character's individual bytes are then captured by the appropriate number of
1164    groups.
1165  .  .
1166  .  .
1167  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 1002  bracket causes a compile-time error. If Line 1175  bracket causes a compile-time error. If
1175  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
1176  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1177  .P  .P
1178  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
1179  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
1180  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
1181  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
1182  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
1183  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
1184  backslash.  backslash.
1185  .P  .P
1186  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 1018  circumflex is not an assertion; it still Line 1191  circumflex is not an assertion; it still
1191  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
1192  string.  string.
1193  .P  .P
1194  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)
1195  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
1196    \ex{ escaping mechanism.
1197  .P  .P
1198  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
1199  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
1200  "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
1201  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
1202  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
1203  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1204  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1205  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
1206  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
1207  with UTF-8 support.  well as with UTF support.
1208  .P  .P
1209  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
1210  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 1052  followed by two other characters. The oc Line 1226  followed by two other characters. The oc
1226  "]" can also be used to end a range.  "]" can also be used to end a range.
1227  .P  .P
1228  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
1229  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1230  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}].  
1231  .P  .P
1232  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
1233  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
1234  [][\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
1235  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
1236  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
1237  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
1238  property support.  property support.
1239  .P  .P
1240  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,
1241  \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
1242  they match to the class. For example, [\edABCDEF] matches any hexadecimal  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1243  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
1244  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
1245  character class, as described in the section entitled  character class, as described in the section entitled
1246  .\" HTML <a href="#genericchartypes">  .\" HTML <a href="#genericchartypes">
# Line 1137  matches "1", "2", or any non-digit. PCRE Line 1310  matches "1", "2", or any non-digit. PCRE
1310  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
1311  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1312  .P  .P
1313  By default, in UTF-8 mode, characters with values greater than 128 do not match  By default, in UTF modes, characters with values greater than 128 do not match
1314  any of the POSIX character classes. However, if the PCRE_UCP option is passed  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1315  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1316  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
# Line 1225  option settings happen at compile time. Line 1398  option settings happen at compile time.
1398  behaviour otherwise.  behaviour otherwise.
1399  .P  .P
1400  \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
1401  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
1402  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1403  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
1404  section entitled  the section entitled
1405  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1406  .\" </a>  .\" </a>
1407  "Newline sequences"  "Newline sequences"
1408  .\"  .\"
1409  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
1410  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
1411  PCRE_UTF8 and the PCRE_UCP options, respectively.  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1412    options, respectively. The (*UTF) sequence is a generic version that can be
1413    used with any of the libraries. However, the application can set the
1414    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1415  .  .
1416  .  .
1417  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1254  match "cataract", "erpillar" or an empty Line 1430  match "cataract", "erpillar" or an empty
1430  .sp  .sp
1431  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
1432  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
1433  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
1434  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1435  from 1) to obtain numbers for the capturing subpatterns. For example, if the  the DFA matching functions do not support capturing.)
1436  string "the red king" is matched against the pattern  .P
1437    Opening parentheses are counted from left to right (starting from 1) to obtain
1438    numbers for the capturing subpatterns. For example, if the string "the red
1439    king" is matched against the pattern
1440  .sp  .sp
1441    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1442  .sp  .sp
# Line 1320  or "defdef": Line 1499  or "defdef":
1499  .sp  .sp
1500    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1501  .sp  .sp
1502  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1503  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1504  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1505  .sp  .sp
1506    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1507  .sp  .sp
# Line 1398  for the first (and in this example, the Line 1577  for the first (and in this example, the
1577  matched. This saves searching to find which numbered subpattern it was.  matched. This saves searching to find which numbered subpattern it was.
1578  .P  .P
1579  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
1580  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
1581  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
1582  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
1583    "barbar" but not "foobar" or "barfoo":
1584    .sp
1585      (?:(?<n>foo)|(?<n>bar))\k<n>
1586    .sp
1587    .P
1588    If you make a subroutine call to a non-unique named subpattern, the one that
1589    corresponds to the first occurrence of the name is used. In the absence of
1590    duplicate numbers (see the previous section) this is the one with the lowest
1591    number.
1592    .P
1593    If you use a named reference in a condition
1594  test (see the  test (see the
1595  .\"  .\"
1596  .\" HTML <a href="#conditions">  .\" HTML <a href="#conditions">
# Line 1420  documentation. Line 1610  documentation.
1610  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1611  subpatterns with the same number because PCRE uses only the numbers when  subpatterns with the same number because PCRE uses only the numbers when
1612  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
1613  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
1614  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
1615    set.
1616  .  .
1617  .  .
1618  .SH REPETITION  .SH REPETITION
# Line 1433  items: Line 1624  items:
1624    a literal data character    a literal data character
1625    the dot metacharacter    the dot metacharacter
1626    the \eC escape sequence    the \eC escape sequence
1627    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1628    the \eR escape sequence    the \eR escape sequence
1629    an escape such as \ed or \epL that matches a single character    an escape such as \ed or \epL that matches a single character
1630    a character class    a character class
1631    a back reference (see next section)    a back reference (see next section)
1632    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1633    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1634  .sp  .sp
1635  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1636  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1464  where a quantifier is not allowed, or on Line 1655  where a quantifier is not allowed, or on
1655  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
1656  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1657  .P  .P
1658  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
1659  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
1660  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,
1661  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
1662  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1663  .P  .P
1664  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
1665  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 1554  In cases where it is known that the subj Line 1745  In cases where it is known that the subj
1745  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1746  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1747  .P  .P
1748  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1749  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1750  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
1751  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1564  succeeds. Consider, for example: Line 1755  succeeds. Consider, for example:
1755  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
1756  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1757  .P  .P
1758    Another case where implicit anchoring is not applied is when the leading .* is
1759    inside an atomic group. Once again, a match at the start may fail where a later
1760    one succeeds. Consider this pattern:
1761    .sp
1762      (?>.*?a)b
1763    .sp
1764    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1765    (*PRUNE) and (*SKIP) also disable this optimization.
1766    .P
1767  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1768  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1769  .sp  .sp
# Line 1778  Because there may be many capturing pare Line 1978  Because there may be many capturing pare
1978  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.
1979  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
1980  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
1981  whitespace. Otherwise, the \eg{ syntax or an empty comment (see  white space. Otherwise, the \eg{ syntax or an empty comment (see
1982  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1983  .\" </a>  .\" </a>
1984  "Comments"  "Comments"
# Line 1829  those that look ahead of the current pos Line 2029  those that look ahead of the current pos
2029  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
2030  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
2031  .P  .P
2032  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2033  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
2034  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2035  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2036  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2037  because it does not make sense for negative assertions.  .P
2038    For compatibility with Perl, assertion subpatterns may be repeated; though
2039    it makes no sense to assert the same thing several times, the side effect of
2040    capturing parentheses may occasionally be useful. In practice, there only three
2041    cases:
2042    .sp
2043    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2044    However, it may contain internal capturing parenthesized groups that are called
2045    from elsewhere via the
2046    .\" HTML <a href="#subpatternsassubroutines">
2047    .\" </a>
2048    subroutine mechanism.
2049    .\"
2050    .sp
2051    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2052    were {0,1}. At run time, the rest of the pattern match is tried with and
2053    without the assertion, the order depending on the greediness of the quantifier.
2054    .sp
2055    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2056    The assertion is obeyed just once when encountered during matching.
2057  .  .
2058  .  .
2059  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1912  temporarily move the current position ba Line 2131  temporarily move the current position ba
2131  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2132  assertion fails.  assertion fails.
2133  .P  .P
2134  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2135  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2136  the length of the lookbehind. The \eX and \eR escapes, which can match  it impossible to calculate the length of the lookbehind. The \eX and \eR
2137  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2138    permitted.
2139  .P  .P
2140  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2141  .\" </a>  .\" </a>
# Line 2109  If the condition is the string (DEFINE), Line 2329  If the condition is the string (DEFINE),
2329  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2330  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2331  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2332  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2333  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2334  .\" </a>  .\" </a>
2335  "subroutines"  subroutines
2336  .\"  .\"
2337  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
2338  "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
2339  breaks):  breaks):
2340  .sp  .sp
2341    (?(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 2162  closing parenthesis. Nested parentheses Line 2382  closing parenthesis. Nested parentheses
2382  option is set, an unescaped # character also introduces a comment, which in  option is set, an unescaped # character also introduces a comment, which in
2383  this case continues to immediately after the next newline character or  this case continues to immediately after the next newline character or
2384  character sequence in the pattern. Which characters are interpreted as newlines  character sequence in the pattern. Which characters are interpreted as newlines
2385  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
2386  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
2387  .\" HTML <a href="#newlines">  .\" HTML <a href="#newlines">
2388  .\" </a>  .\" </a>
# Line 2207  individual subpattern recursion. After i Line 2427  individual subpattern recursion. After i
2427  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2428  .P  .P
2429  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2430  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2431  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2432  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2433  .\" </a>  .\" </a>
2434  "subroutine"  non-recursive subroutine
2435  .\"  .\"
2436  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2437  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2246  references such as (?+2). However, these Line 2466  references such as (?+2). However, these
2466  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2467  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2468  .\" </a>  .\" </a>
2469  "subroutine"  non-recursive subroutine
2470  .\"  .\"
2471  calls, as described in the next section.  calls, as described in the next section.
2472  .P  .P
# Line 2283  documentation). If the pattern above is Line 2503  documentation). If the pattern above is
2503  .sp  .sp
2504  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2505  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2506  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2507  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2508  .P  .P
2509  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2510  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2304  is the actual recursive call. Line 2524  is the actual recursive call.
2524  .  .
2525  .  .
2526  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2527  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2528  .rs  .rs
2529  .sp  .sp
2530  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2531  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2532  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2533  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2534  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2535  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2536    characters (for example, "a", "aba", "abcba", "abcdcba"):
2537  .sp  .sp
2538    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2539  .sp  .sp
# Line 2373  For example, although "abcba" is correct Line 2594  For example, although "abcba" is correct
2594  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2595  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2596  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2597    .P
2598    The second way in which PCRE and Perl differ in their recursion processing is
2599    in the handling of captured values. In Perl, when a subpattern is called
2600    recursively or as a subpattern (see the next section), it has no access to any
2601    values that were captured outside the recursion, whereas in PCRE these values
2602    can be referenced. Consider this pattern:
2603    .sp
2604      ^(.)(\e1|a(?2))
2605    .sp
2606    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2607    then in the second group, when the back reference \e1 fails to match "b", the
2608    second alternative matches "a" and then recurses. In the recursion, \e1 does
2609    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2610    match because inside the recursive call \e1 cannot access the externally set
2611    value.
2612  .  .
2613  .  .
2614  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2615  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2616  .rs  .rs
2617  .sp  .sp
2618  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2619  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2620  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2621  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2622  relative, as in these examples:  relative, as in these examples:
2623  .sp  .sp
# Line 2401  matches "sense and sensibility" and "res Line 2637  matches "sense and sensibility" and "res
2637  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2638  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2639  .P  .P
2640  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2641  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2642  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2643  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2644  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2645  .P  .P
2646  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2647  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2648  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2649  .sp  .sp
2650    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2651  .sp  .sp
# Line 2448  same pair of parentheses when there is a Line 2684  same pair of parentheses when there is a
2684  .P  .P
2685  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
2686  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
2687  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
2688    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2689  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2690  .P  .P
2691  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 2458  For example, this pattern has two callou Line 2695  For example, this pattern has two callou
2695  .sp  .sp
2696    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2697  .sp  .sp
2698  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
2699  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2700  255.  255. If there is a conditional group in the pattern whose condition is an
2701  .P  assertion, an additional callout is inserted just before the condition. An
2702  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:
2703  set), the external function is called. It is provided with the number of the  .sp
2704  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2705  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2706  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
2707  description of the interface to the callout function is given in the  condition.
2708    .P
2709    During matching, when PCRE reaches a callout point, the external function is
2710    called. It is provided with the number of the callout, the position in the
2711    pattern, and, optionally, one item of data originally supplied by the caller of
2712    the matching function. The callout function may cause matching to proceed, to
2713    backtrack, or to fail altogether. A complete description of the interface to
2714    the callout function is given in the
2715  .\" HREF  .\" HREF
2716  \fBpcrecallout\fP  \fBpcrecallout\fP
2717  .\"  .\"
# Line 2479  documentation. Line 2723  documentation.
2723  .rs  .rs
2724  .sp  .sp
2725  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2726  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2727  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
2728  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
2729  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2730  .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 subroutine subpattern  
 (including recursive subpatterns), their effect is confined to that subpattern;  
 it does not extend to the surrounding pattern, with one exception: a *MARK that  
 is encountered in a positive assertion \fIis\fP passed back (compare capturing  
 parentheses in assertions). Note that such subpatterns are processed as  
 anchored at the point where they are tested.  
 .P  
2731  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
2732  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2733  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2734  depending on whether or not an argument is present. An name is a sequence of  differently depending on whether or not a name is present. A name is any
2735  letters, digits, and underscores. If the name is empty, that is, if the closing  sequence of characters that does not include a closing parenthesis. The maximum
2736  parenthesis immediately follows the colon, the effect is as if the colon were  length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2737  not there. Any number of these verbs may occur in a pattern.  libraries. If the name is empty, that is, if the closing parenthesis
2738    immediately follows the colon, the effect is as if the colon were not there.
2739    Any number of these verbs may occur in a pattern.
2740  .P  .P
2741    Since these verbs are specifically related to backtracking, most of them can be
2742    used only when the pattern is to be matched using one of the traditional
2743    matching functions, because these use a backtracking algorithm. With the
2744    exception of (*FAIL), which behaves like a failing negative assertion, the
2745    backtracking control verbs cause an error if encountered by a DFA matching
2746    function.
2747    .P
2748    The behaviour of these verbs in
2749    .\" HTML <a href="#btrepeat">
2750    .\" </a>
2751    repeated groups,
2752    .\"
2753    .\" HTML <a href="#btassert">
2754    .\" </a>
2755    assertions,
2756    .\"
2757    and in
2758    .\" HTML <a href="#btsub">
2759    .\" </a>
2760    subpatterns called as subroutines
2761    .\"
2762    (whether or not recursively) is documented below.
2763    .
2764    .
2765    .\" HTML <a name="nooptimize"></a>
2766    .SS "Optimizations that affect backtracking verbs"
2767    .rs
2768    .sp
2769  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
2770  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
2771  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2772  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
2773  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2774  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
2775  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
2776  pattern with (*NO_START_OPT).  pattern with (*NO_START_OPT). There is more discussion of this option in the
2777    section entitled
2778    .\" HTML <a href="pcreapi.html#execoptions">
2779    .\" </a>
2780    "Option bits for \fBpcre_exec()\fP"
2781    .\"
2782    in the
2783    .\" HREF
2784    \fBpcreapi\fP
2785    .\"
2786    documentation.
2787    .P
2788    Experiments with Perl suggest that it too has similar optimizations, sometimes
2789    leading to anomalous results.
2790  .  .
2791  .  .
2792  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2524  followed by a name. Line 2798  followed by a name.
2798     (*ACCEPT)     (*ACCEPT)
2799  .sp  .sp
2800  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
2801  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2802  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2803  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2804    assertion succeeds; in a negative assertion, the assertion fails.
2805    .P
2806    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2807    example:
2808  .sp  .sp
2809    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2810  .sp  .sp
# Line 2535  the outer parentheses. Line 2813  the outer parentheses.
2813  .sp  .sp
2814    (*FAIL) or (*F)    (*FAIL) or (*F)
2815  .sp  .sp
2816  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2817  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2818  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2819  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2559  starting point (see (*SKIP) below). Line 2837  starting point (see (*SKIP) below).
2837  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
2838  (*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.
2839  .P  .P
2840  When a match succeeds, the name of the last-encountered (*MARK) is passed back  When a match succeeds, the name of the last-encountered (*MARK:NAME),
2841  to the caller via the \fIpcre_extra\fP data structure, as described in the  (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2842    caller as described in the section entitled
2843  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2844  .\" </a>  .\" </a>
2845  section on \fIpcre_extra\fP  "Extra data for \fBpcre_exec()\fP"
2846  .\"  .\"
2847  in the  in the
2848  .\" HREF  .\" HREF
2849  \fBpcreapi\fP  \fBpcreapi\fP
2850  .\"  .\"
2851  documentation. No data is returned for a partial match. Here is an example of  documentation. Here is an example of \fBpcretest\fP output, where the /K
2852  \fBpcretest\fP output, where the /K modifier requests the retrieval and  modifier requests the retrieval and outputting of (*MARK) data:
 outputting of (*MARK) data:  
2853  .sp  .sp
2854    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2855    XY    data> XY
2856     0: XY     0: XY
2857    MK: A    MK: A
2858    XZ    XZ
# Line 2586  indicates which of the two alternatives Line 2864  indicates which of the two alternatives
2864  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2865  capturing parentheses.  capturing parentheses.
2866  .P  .P
2867  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
2868  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
2869  assetions.  happen for negative assertions or failing positive assertions.
2870  .P  .P
2871  A name may also be returned after a failed match if the final path through the  After a partial match or a failed match, the last encountered name in the
2872  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  entire match process is returned. For example:
 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  
 starting point for matching is advanced, the final check is often with an empty  
 string, causing a failure before (*MARK) is reached. For example:  
 .sp  
   /X(*MARK:A)Y|X(*MARK:B)Z/K  
   XP  
   No match  
 .sp  
 There are three potential starting points for this match (starting with X,  
 starting with P, and with an empty string). If the pattern is anchored, the  
 result is different:  
2873  .sp  .sp
2874    /^X(*MARK:A)Y|^X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2875    XP    data> XP
2876    No match, mark = B    No match, mark = B
2877  .sp  .sp
2878  PCRE's start-of-match optimizations can also interfere with this. For example,  Note that in this unanchored example the mark is retained from the match
2879  if, as a result of a call to \fBpcre_study()\fP, it knows the minimum  attempt that started at the letter "X" in the subject. Subsequent match
2880  subject length for a match, a shorter subject will not be scanned at all.  attempts starting at "P" and then with an empty string do not get as far as the
2881  .P  (*MARK) item, but nevertheless do not reset it.
2882  Note that similar anomalies (though different in detail) exist in Perl, no  .P
2883  doubt for the same reasons. The use of (*MARK) data after a failed match of an  If you are interested in (*MARK) values after failed matches, you should
2884  unanchored pattern is not recommended, unless (*COMMIT) is involved.  probably set the PCRE_NO_START_OPTIMIZE option
2885    .\" HTML <a href="#nooptimize">
2886    .\" </a>
2887    (see above)
2888    .\"
2889    to ensure that the match is always attempted.
2890  .  .
2891  .  .
2892  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2623  unanchored pattern is not recommended, u Line 2895  unanchored pattern is not recommended, u
2895  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2896  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
2897  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
2898  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
2899  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
2900  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
2901  "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
2902  above, that this localization also applies in subroutine calls and assertions.)  or assertion. (Remember also, as stated above, that this localization also
2903    applies in subroutine calls.)
2904  .P  .P
2905  These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
2906  reaches them.  reaches them. The behaviour described below is what happens when the verb is
2907    not in a subroutine or an assertion. Subsequent sections cover these special
2908    cases.
2909  .sp  .sp
2910    (*COMMIT)    (*COMMIT)
2911  .sp  .sp
2912  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
2913  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
2914  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
2915  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
2916  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
2917    committed to finding a match at the current starting point, or not at all. For
2918    example:
2919  .sp  .sp
2920    a+(*COMMIT)b    a+(*COMMIT)b
2921  .sp  .sp
# Line 2647  dynamic anchor, or "I've started, so I m Line 2924  dynamic anchor, or "I've started, so I m
2924  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
2925  match failure.  match failure.
2926  .P  .P
2927    If there is more than one backtracking verb in a pattern, a different one that
2928    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2929    match does not always guarantee that a match must be at this starting point.
2930    .P
2931  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,
2932  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
2933  \fBpcretest\fP example:  \fBpcretest\fP example:
2934  .sp  .sp
2935    /(*COMMIT)abc/      re> /(*COMMIT)abc/
2936    xyzabc    data> xyzabc
2937     0: abc     0: abc
2938    xyzabc\eY    xyzabc\eY
2939    No match    No match
# Line 2666  starting points. Line 2947  starting points.
2947    (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
2948  .sp  .sp
2949  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
2950  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
2951  unanchored, the normal "bumpalong" advance to the next starting character then  it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2952  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
2953  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
2954  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
2955  (*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
2956  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
2957  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the  expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2958  match fails completely; the name is passed back if this is the final attempt.  as (*COMMIT).
2959  (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored  .P
2960  pattern (*PRUNE) has the same effect as (*COMMIT).  The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2961    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2962    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2963  .sp  .sp
2964    (*SKIP)    (*SKIP)
2965  .sp  .sp
# Line 2697  instead of skipping on to "c". Line 2980  instead of skipping on to "c".
2980  .sp  .sp
2981    (*SKIP:NAME)    (*SKIP:NAME)
2982  .sp  .sp
2983  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
2984  following pattern fails to match, the previous path through the pattern is  triggered, the previous path through the pattern is searched for the most
2985  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
2986  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
2987  (*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
2988  matching name is found, normal "bumpalong" of one character happens (the  (*SKIP) is ignored.
2989  (*SKIP) is ignored).  .P
2990    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2991    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2992  .sp  .sp
2993    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2994  .sp  .sp
2995  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next innermost alternative when backtracking
2996  group if the rest of the pattern does not match. That is, it cancels pending  reaches it. That is, it cancels any further backtracking within the current
2997  backtracking, but only within the current alternation. Its name comes from the  alternative. Its name comes from the observation that it can be used for a
2998  observation that it can be used for a pattern-based if-then-else block:  pattern-based if-then-else block:
2999  .sp  .sp
3000    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3001  .sp  .sp
3002  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
3003  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
3004  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. If that
3005  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3006  overall match fails. If (*THEN) is not directly inside an alternation, it acts  more alternatives, so there is a backtrack to whatever came before the entire
3007  like (*PRUNE).  group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3008  .  .P
3009  .P  The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3010  The above verbs provide four different "strengths" of control when subsequent  It is like (*MARK:NAME) in that the name is remembered for passing back to the
3011  matching fails. (*THEN) is the weakest, carrying on the match at the next  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3012  alternation. (*PRUNE) comes next, failing the match at the current starting  .P
3013  position, but allowing an advance to the next character (for an unanchored  A subpattern that does not contain a | character is just a part of the
3014  pattern). (*SKIP) is similar, except that the advance may be more than one  enclosing alternative; it is not a nested alternation with only one
3015  character. (*COMMIT) is the strongest, causing the entire match to fail.  alternative. The effect of (*THEN) extends beyond such a subpattern to the
3016  .P  enclosing alternative. Consider this pattern, where A, B, etc. are complex
3017  If more than one is present in a pattern, the "stongest" one wins. For example,  pattern fragments that do not contain any | characters at this level:
3018  consider this pattern, where A, B, etc. are complex pattern fragments:  .sp
3019  .sp    A (B(*THEN)C) | D
3020    (A(*COMMIT)B(*THEN)C|D)  .sp
3021  .sp  If A and B are matched, but there is a failure in C, matching does not
3022  Once A has matched, PCRE is committed to this match, at the current starting  backtrack into A; instead it moves to the next alternative, that is, D.
3023  position. If subsequently B matches, but C does not, the normal (*THEN) action  However, if the subpattern containing (*THEN) is given an alternative, it
3024  of trying the next alternation (that is, D) does not happen because (*COMMIT)  behaves differently:
3025  overrides.  .sp
3026      A (B(*THEN)C | (*FAIL)) | D
3027    .sp
3028    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3029    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3030    because there are no more alternatives to try. In this case, matching does now
3031    backtrack into A.
3032    .P
3033    Note that a conditional subpattern is not considered as having two
3034    alternatives, because only one is ever used. In other words, the | character in
3035    a conditional subpattern has a different meaning. Ignoring white space,
3036    consider:
3037    .sp
3038      ^.*? (?(?=a) a | b(*THEN)c )
3039    .sp
3040    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3041    it initially matches zero characters. The condition (?=a) then fails, the
3042    character "b" is matched, but "c" is not. At this point, matching does not
3043    backtrack to .*? as might perhaps be expected from the presence of the |
3044    character. The conditional subpattern is part of the single alternative that
3045    comprises the whole pattern, and so the match fails. (If there was a backtrack
3046    into .*?, allowing it to match "b", the match would succeed.)
3047    .P
3048    The verbs just described provide four different "strengths" of control when
3049    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3050    next alternative. (*PRUNE) comes next, failing the match at the current
3051    starting position, but allowing an advance to the next character (for an
3052    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3053    than one character. (*COMMIT) is the strongest, causing the entire match to
3054    fail.
3055    .
3056    .
3057    .SS "More than one backtracking verb"
3058    .rs
3059    .sp
3060    If more than one backtracking verb is present in a pattern, the one that is
3061    backtracked onto first acts. For example, consider this pattern, where A, B,
3062    etc. are complex pattern fragments:
3063    .sp
3064      (A(*COMMIT)B(*THEN)C|ABD)
3065    .sp
3066    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3067    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3068    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3069    not always the same as Perl's. It means that if two or more backtracking verbs
3070    appear in succession, all the the last of them has no effect. Consider this
3071    example:
3072    .sp
3073      ...(*COMMIT)(*PRUNE)...
3074    .sp
3075    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3076    it to be triggered, and its action is taken. There can never be a backtrack
3077    onto (*COMMIT).
3078    .
3079    .
3080    .\" HTML <a name="btrepeat"></a>
3081    .SS "Backtracking verbs in repeated groups"
3082    .rs
3083    .sp
3084    PCRE differs from Perl in its handling of backtracking verbs in repeated
3085    groups. For example, consider:
3086    .sp
3087      /(a(*COMMIT)b)+ac/
3088    .sp
3089    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3090    the second repeat of the group acts.
3091    .
3092    .
3093    .\" HTML <a name="btassert"></a>
3094    .SS "Backtracking verbs in assertions"
3095    .rs
3096    .sp
3097    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3098    .P
3099    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3100    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3101    fail without any further processing.
3102    .P
3103    The other backtracking verbs are not treated specially if they appear in a
3104    positive assertion. In particular, (*THEN) skips to the next alternative in the
3105    innermost enclosing group that has alternations, whether or not this is within
3106    the assertion.
3107    .P
3108    Negative assertions are, however, different, in order to ensure that changing a
3109    positive assertion into a negative assertion changes its result. Backtracking
3110    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3111    without considering any further alternative branches in the assertion.
3112    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3113    within the assertion (the normal behaviour), but if the assertion does not have
3114    such an alternative, (*THEN) behaves like (*PRUNE).
3115    .
3116    .
3117    .\" HTML <a name="btsub"></a>
3118    .SS "Backtracking verbs in subroutines"
3119    .rs
3120    .sp
3121    These behaviours occur whether or not the subpattern is called recursively.
3122    Perl's treatment of subroutines is different in some cases.
3123    .P
3124    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3125    an immediate backtrack.
3126    .P
3127    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3128    succeed without any further processing. Matching then continues after the
3129    subroutine call.
3130    .P
3131    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3132    the subroutine match to fail.
3133    .P
3134    (*THEN) skips to the next alternative in the innermost enclosing group within
3135    the subpattern that has alternatives. If there is no such group within the
3136    subpattern, (*THEN) causes the subroutine match to fail.
3137  .  .
3138  .  .
3139  .SH "SEE ALSO"  .SH "SEE ALSO"
3140  .rs  .rs
3141  .sp  .sp
3142  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3143  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3144  .  .
3145  .  .
3146  .SH AUTHOR  .SH AUTHOR
# Line 2761  Cambridge CB2 3QH, England. Line 3157  Cambridge CB2 3QH, England.
3157  .rs  .rs
3158  .sp  .sp
3159  .nf  .nf
3160  Last updated: 22 July 2011  Last updated: 06 September 2013
3161  Copyright (c) 1997-2011 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3162  .fi  .fi

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