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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "12 November 2013" "PCRE 8.34"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 9  are described in detail below. There is Line 9  are described in detail below. There is
9  .\" HREF  .\" HREF
10  \fBpcresyntax\fP  \fBpcresyntax\fP
11  .\"  .\"
12  page. Perl's regular expressions are described in its own documentation, and  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18  regular expressions in general are covered in a number of books, some of which  regular expressions in general are covered in a number of books, some of which
19  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
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  The original operation of PCRE was on strings of one-byte characters. However,  This document discusses the patterns that are supported by PCRE when one its
24  there is now also support for UTF-8 character strings. To use this, you must  main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  (16- or 32-bit), is used. PCRE also has alternative matching functions,
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27  places below. There is also a summary of UTF-8 features in the  different algorithm that is not Perl-compatible. Some of the features discussed
28  .\" HTML <a href="pcre.html#utf8support">  below are not available when DFA matching is used. The advantages and
29  .\" </a>  disadvantages of the alternative functions, and how they differ from the normal
30  section on UTF-8 support  functions, are discussed in the
31    .\" HREF
32    \fBpcrematching\fP
33  .\"  .\"
34  in the main  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,
52    there is now also support for UTF-8 strings in the original library, an
53    extra library that supports 16-bit and UTF-16 character strings, and a
54    third library that supports 32-bit and UTF-32 character strings. To use these
55    features, PCRE must be built to include appropriate support. When using UTF
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
60      (*UTF8)
61      (*UTF16)
62      (*UTF32)
63      (*UTF)
64    .sp
65    (*UTF) is a generic sequence that can be used with any of the libraries.
66    Starting a pattern with such a sequence is equivalent to setting the relevant
67    option. How setting a UTF mode affects pattern matching is mentioned in several
68    places below. There is also a summary of features in the
69  .\" HREF  .\" HREF
70  \fBpcre\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73  .P  .P
74  The remainder of this document discusses the patterns that are supported by  Some applications that allow their users to supply patterns may wish to
75  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76  From release 6.0, PCRE offers a second matching function,  option is set at compile time, (*UTF) etc. are not allowed, and their
77  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  appearance causes an error.
78  Perl-compatible. Some of the features discussed below are not available when  .
79  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  .
80  alternative function, and how it differs from the normal function, are  .SS "Unicode property support"
81  discussed in the  .rs
82    .sp
83    Another special sequence that may appear at the start of a pattern is (*UCP).
84    This has the same effect as setting the PCRE_UCP option: it causes sequences
85    such as \ed and \ew to use Unicode properties to determine character types,
86    instead of recognizing only characters with codes less than 128 via a lookup
87    table.
88    .
89    .
90    .SS "Disabling auto-possessification"
91    .rs
92    .sp
93    If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
94    the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
95    quantifiers possessive when what follows cannot match the repeated item. For
96    example, by default a+b is treated as a++b. For more details, see the
97  .\" HREF  .\" HREF
98  \fBpcrematching\fP  \fBpcreapi\fP
99  .\"  .\"
100  page.  documentation.
101  .  .
102  .  .
103  .SH "NEWLINE CONVENTIONS"  .SS "Disabling start-up optimizations"
104    .rs
105    .sp
106    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
107    PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
108    several optimizations for quickly reaching "no match" results. For more
109    details, see the
110    .\" HREF
111    \fBpcreapi\fP
112    .\"
113    documentation.
114    .
115    .
116    .\" HTML <a name="newlines"></a>
117    .SS "Newline conventions"
118  .rs  .rs
119  .sp  .sp
120  PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
# Line 71  string with one of the following five se Line 141  string with one of the following five se
141    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
142    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
143  .sp  .sp
144  These override the default and the options given to \fBpcre_compile()\fP. For  These override the default and the options given to the compiling function. For
145  example, on a Unix system where LF is the default newline sequence, the pattern  example, on a Unix system where LF is the default newline sequence, the pattern
146  .sp  .sp
147    (*CR)a.b    (*CR)a.b
148  .sp  .sp
149  changes the convention to CR. That pattern matches "a\enb" because LF is no  changes the convention to CR. That pattern matches "a\enb" because LF is no
150  longer a newline. Note that these special settings, which are not  longer a newline. If more than one of these settings is present, the last one
 Perl-compatible, are recognized only at the very start of a pattern, and that  
 they must be in upper case. If more than one of them is present, the last one  
151  is used.  is used.
152  .P  .P
153  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects where the circumflex and dollar assertions are
154  default, this is any Unicode newline sequence, for Perl compatibility. However,  true. It also affects the interpretation of the dot metacharacter when
155  this can be changed; see the description of \eR in the section entitled  PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
156    what the \eR escape sequence matches. By default, this is any Unicode newline
157    sequence, for Perl compatibility. However, this can be changed; see the
158    description of \eR in the section entitled
159  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
160  .\" </a>  .\" </a>
161  "Newline sequences"  "Newline sequences"
162  .\"  .\"
163  below.  below. A change of \eR setting can be combined with a change of newline
164    convention.
165    .
166    .
167    .SS "Setting match and recursion limits"
168    .rs
169    .sp
170    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
171    internal \fBmatch()\fP function is called and on the maximum depth of
172    recursive calls. These facilities are provided to catch runaway matches that
173    are provoked by patterns with huge matching trees (a typical example is a
174    pattern with nested unlimited repeats) and to avoid running out of system stack
175    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
176    gives an error return. The limits can also be set by items at the start of the
177    pattern of the form
178    .sp
179      (*LIMIT_MATCH=d)
180      (*LIMIT_RECURSION=d)
181    .sp
182    where d is any number of decimal digits. However, the value of the setting must
183    be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
184    for it to have any effect. In other words, the pattern writer can lower the
185    limits set by the programmer, but not raise them. If there is more than one
186    setting of one of these limits, the lower value is used.
187    .
188    .
189    .SH "EBCDIC CHARACTER CODES"
190    .rs
191    .sp
192    PCRE can be compiled to run in an environment that uses EBCDIC as its character
193    code rather than ASCII or Unicode (typically a mainframe system). In the
194    sections below, character code values are ASCII or Unicode; in an EBCDIC
195    environment these characters may have different code values, and there are no
196    code points greater than 255.
197  .  .
198  .  .
199  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
# Line 103  corresponding characters in the subject. Line 207  corresponding characters in the subject.
207  .sp  .sp
208  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
209  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
210  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
211  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
212  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
213  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
214  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
215  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
216  UTF-8 support.  UTF support.
217  .P  .P
218  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
219  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 155  The following sections describe the use Line 259  The following sections describe the use
259  .rs  .rs
260  .sp  .sp
261  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
262  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
263  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
264  outside character classes.  both inside and outside character classes.
265  .P  .P
266  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
267  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 165  otherwise be interpreted as a metacharac Line 269  otherwise be interpreted as a metacharac
269  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
270  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
271  .P  .P
272  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  In a UTF mode, only ASCII numbers and letters have any special meaning after a
273  pattern (other than in a character class) and characters between a # outside  backslash. All other characters (in particular, those whose codepoints are
274  a character class and the next newline are ignored. An escaping backslash can  greater than 127) are treated as literals.
275  be used to include a whitespace or # character as part of the pattern.  .P
276    If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
277    pattern (other than in a character class), and characters between a # outside a
278    character class and the next newline, inclusive, are ignored. An escaping
279    backslash can be used to include a white space or # character as part of the
280    pattern.
281  .P  .P
282  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
283  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 184  Perl, $ and @ cause variable interpolati Line 293  Perl, $ and @ cause variable interpolati
293    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
294  .sp  .sp
295  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
296    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
297    by \eE later in the pattern, the literal interpretation continues to the end of
298    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
299    a character class, this causes an error, because the character class is not
300    terminated.
301  .  .
302  .  .
303  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 193  The \eQ...\eE sequence is recognized bot Line 307  The \eQ...\eE sequence is recognized bot
307  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
308  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
309  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
310  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
311  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
312  .sp  .sp
313    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
314    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
315    \ee        escape (hex 1B)    \ee        escape (hex 1B)
316    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
317    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
318    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
319    \et        tab (hex 09)    \et        tab (hex 09)
320    \eddd      character with octal code ddd, or backreference    \e0dd      character with octal code 0dd
321      \eddd      character with octal code ddd, or back reference
322      \eo{ddd..} character with octal code ddd..
323    \exhh      character with hex code hh    \exhh      character with hex code hh
324    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
325      \euhhhh    character with hex code hhhh (JavaScript mode only)
326  .sp  .sp
327  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
328  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
329  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
330  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
331  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
332  After \ex, from zero to two hexadecimal digits are read (letters can be in  compile-time error occurs. This locks out non-ASCII characters in all modes.
333  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
334  and }, but the value of the character code must be less than 256 in non-UTF-8  The \ec facility was designed for use with ASCII characters, but with the
335  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  extension to Unicode it is even less useful than it once was. It is, however,
336  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  recognized when PCRE is compiled in EBCDIC mode, where data items are always
337  point, which is 10FFFF.  bytes. In this mode, all values are valid after \ec. If the next character is a
338  .P  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
339  If characters other than hexadecimal digits appear between \ex{ and }, or if  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
340  there is no terminating }, this form of escape is not recognized. Instead, the  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341  initial \ex will be interpreted as a basic hexadecimal escape, with no  characters also generate different values.
 following digits, giving a character whose value is zero.  
 .P  
 Characters whose value is less than 256 can be defined by either of the two  
 syntaxes for \ex. There is no difference in the way they are handled. For  
 example, \exdc is exactly the same as \ex{dc}.  
342  .P  .P
343  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
344  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 235  specifies two binary zeros followed by a Line 346  specifies two binary zeros followed by a
346  sure you supply two digits after the initial zero if the pattern character that  sure you supply two digits after the initial zero if the pattern character that
347  follows is itself an octal digit.  follows is itself an octal digit.
348  .P  .P
349  The handling of a backslash followed by a digit other than 0 is complicated.  The escape \eo must be followed by a sequence of octal digits, enclosed in
350  Outside a character class, PCRE reads it and any following digits as a decimal  braces. An error occurs if this is not the case. This escape is a recent
351  number. If the number is less than 10, or if there have been at least that many  addition to Perl; it provides way of specifying character code points as octal
352    numbers greater than 0777, and it also allows octal numbers and back references
353    to be unambiguously specified.
354    .P
355    For greater clarity and unambiguity, it is best to avoid following \e by a
356    digit greater than zero. Instead, use \eo{} or \ex{} to specify character
357    numbers, and \eg{} to specify back references. The following paragraphs
358    describe the old, ambiguous syntax.
359    .P
360    The handling of a backslash followed by a digit other than 0 is complicated,
361    and Perl has changed in recent releases, causing PCRE also to change. Outside a
362    character class, PCRE reads the digit and any following digits as a decimal
363    number. If the number is less than 8, or if there have been at least that many
364  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
365  taken as a \fIback reference\fP. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
366  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
# Line 250  following the discussion of Line 373  following the discussion of
373  parenthesized subpatterns.  parenthesized subpatterns.
374  .\"  .\"
375  .P  .P
376  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number following \e is greater than
377  have not been that many capturing subpatterns, PCRE re-reads up to three octal  7 and there have not been that many capturing subpatterns, PCRE handles \e8 and
378  digits following the backslash, and uses them to generate a data character. Any  \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
379  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  octal digits following the backslash, using them to generate a data character.
380  character specified in octal must be less than \e400. In UTF-8 mode, values up  Any subsequent digits stand for themselves. For example:
 to \e777 are permitted. For example:  
381  .sp  .sp
382    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
383  .\" JOIN  .\" JOIN
384    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
385              previous capturing subpatterns              previous capturing subpatterns
# Line 272  to \e777 are permitted. For example: Line 394  to \e777 are permitted. For example:
394              character with octal code 113              character with octal code 113
395  .\" JOIN  .\" JOIN
396    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
397              the byte consisting entirely of 1 bits              the value 255 (decimal)
398  .\" JOIN  .\" JOIN
399    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or the two
400              followed by the two characters "8" and "1"              characters "8" and "1"
401  .sp  .sp
402  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater that are specified using this syntax
403  zero, because no more than three octal digits are ever read.  must not be introduced by a leading zero, because no more than three octal
404    digits are ever read.
405    .P
406    By default, after \ex that is not followed by {, from zero to two hexadecimal
407    digits are read (letters can be in upper or lower case). Any number of
408    hexadecimal digits may appear between \ex{ and }. If a character other than
409    a hexadecimal digit appears between \ex{ and }, or if there is no terminating
410    }, an error occurs.
411    .P
412    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
413    as just described only when it is followed by two hexadecimal digits.
414    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
415    code points greater than 256 is provided by \eu, which must be followed by
416    four hexadecimal digits; otherwise it matches a literal "u" character.
417  .P  .P
418    Characters whose value is less than 256 can be defined by either of the two
419    syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
420    way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
421    \eu00dc in JavaScript mode).
422    .
423    .
424    .SS "Constraints on character values"
425    .rs
426    .sp
427    Characters that are specified using octal or hexadecimal numbers are
428    limited to certain values, as follows:
429    .sp
430      8-bit non-UTF mode    less than 0x100
431      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
432      16-bit non-UTF mode   less than 0x10000
433      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
434      32-bit non-UTF mode   less than 0x100000000
435      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
436    .sp
437    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
438    "surrogate" codepoints), and 0xffef.
439    .
440    .
441    .SS "Escape sequences in character classes"
442    .rs
443    .sp
444  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
445  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
446  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
447  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
448  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
449  meanings  inside a character class. Like other unrecognized escape sequences, they are
450  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
451  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
452  (see below).  sequences have different meanings.
453  .\"  .
454    .
455    .SS "Unsupported escape sequences"
456    .rs
457    .sp
458    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
459    handler and used to modify the case of following characters. By default, PCRE
460    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
461    option is set, \eU matches a "U" character, and \eu can be used to define a
462    character by code point, as described in the previous section.
463  .  .
464  .  .
465  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 309  parenthesized subpatterns. Line 479  parenthesized subpatterns.
479  .\"  .\"
480  .  .
481  .  .
482    .SS "Absolute and relative subroutine calls"
483    .rs
484    .sp
485    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
486    a number enclosed either in angle brackets or single quotes, is an alternative
487    syntax for referencing a subpattern as a "subroutine". Details are discussed
488    .\" HTML <a href="#onigurumasubroutines">
489    .\" </a>
490    later.
491    .\"
492    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
493    synonymous. The former is a back reference; the latter is a
494    .\" HTML <a href="#subpatternsassubroutines">
495    .\" </a>
496    subroutine
497    .\"
498    call.
499    .
500    .
501    .\" HTML <a name="genericchartypes"></a>
502  .SS "Generic character types"  .SS "Generic character types"
503  .rs  .rs
504  .sp  .sp
505  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
506  .sp  .sp
507    \ed     any decimal digit    \ed     any decimal digit
508    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
509    \eh     any horizontal whitespace character    \eh     any horizontal white space character
510    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
511    \es     any whitespace character    \es     any white space character
512    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
513    \ev     any vertical whitespace character    \ev     any vertical white space character
514    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
515    \ew     any "word" character    \ew     any "word" character
516    \eW     any "non-word" character    \eW     any "non-word" character
517  .sp  .sp
518  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
519  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
520  .P  .\" HTML <a href="#fullstopdot">
521  These character type sequences can appear both inside and outside character  .\" </a>
522    the "." metacharacter
523    .\"
524    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
525    PCRE does not support this.
526    .P
527    Each pair of lower and upper case escape sequences partitions the complete set
528    of characters into two disjoint sets. Any given character matches one, and only
529    one, of each pair. The sequences can appear both inside and outside character
530  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
531  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
532  there is no character to match.  there is no character to match.
533  .P  .P
534  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es did not used to match the VT character (code
535  This makes it different from the the POSIX "space" class. The \es characters  11), which made it different from the the POSIX "space" class. However, Perl
536  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is  added VT at release 5.18, and PCRE followed suit at release 8.34. The default
537  included in a Perl script, \es may match the VT character. In PCRE, it never  \es characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
538  does.  (32), which are defined as white space in the "C" locale. This list may vary if
539  .P  locale-specific matching is taking place; in particular, in some locales the
540  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  "non-breaking space" character (\exA0) is recognized as white space.
541  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
542  character property support is available. These sequences retain their original  A "word" character is an underscore or any character that is a letter or digit.
543  meanings from before UTF-8 support was available, mainly for efficiency  By default, the definition of letters and digits is controlled by PCRE's
544  reasons.  low-valued character tables, and may vary if locale-specific matching is taking
545  .P  place (see
546  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\" HTML <a href="pcreapi.html#localesupport">
547  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\" </a>
548  The horizontal space characters are:  "Locale support"
549    .\"
550    in the
551    .\" HREF
552    \fBpcreapi\fP
553    .\"
554    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
555    or "french" in Windows, some character codes greater than 127 are used for
556    accented letters, and these are then matched by \ew. The use of locales with
557    Unicode is discouraged.
558    .P
559    By default, characters whose code points are greater than 127 never match \ed,
560    \es, or \ew, and always match \eD, \eS, and \eW, although this may vary for
561    characters in the range 128-255 when locale-specific matching is happening.
562    These escape sequences retain their original meanings from before Unicode
563    support was available, mainly for efficiency reasons. If PCRE is compiled with
564    Unicode property support, and the PCRE_UCP option is set, the behaviour is
565    changed so that Unicode properties are used to determine character types, as
566    follows:
567    .sp
568      \ed  any character that matches \ep{Nd} (decimal digit)
569      \es  any character that matches \ep{Z} or \eh or \ev
570      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
571    .sp
572    The upper case escapes match the inverse sets of characters. Note that \ed
573    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
574    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
575    \eB because they are defined in terms of \ew and \eW. Matching these sequences
576    is noticeably slower when PCRE_UCP is set.
577    .P
578    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
579    release 5.10. In contrast to the other sequences, which match only ASCII
580    characters by default, these always match certain high-valued code points,
581    whether or not PCRE_UCP is set. The horizontal space characters are:
582  .sp  .sp
583    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
584    U+0020     Space    U+0020     Space
585    U+00A0     Non-break space    U+00A0     Non-break space
586    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 372  The horizontal space characters are: Line 602  The horizontal space characters are:
602  .sp  .sp
603  The vertical space characters are:  The vertical space characters are:
604  .sp  .sp
605    U+000A     Linefeed    U+000A     Linefeed (LF)
606    U+000B     Vertical tab    U+000B     Vertical tab (VT)
607    U+000C     Formfeed    U+000C     Form feed (FF)
608    U+000D     Carriage return    U+000D     Carriage return (CR)
609    U+0085     Next line    U+0085     Next line (NEL)
610    U+2028     Line separator    U+2028     Line separator
611    U+2029     Paragraph separator    U+2029     Paragraph separator
612  .P  .sp
613  A "word" character is an underscore or any character less than 256 that is a  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
614  letter or digit. The definition of letters and digits is controlled by PCRE's  relevant.
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
615  .  .
616  .  .
617  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 403  is discouraged. Line 619  is discouraged.
619  .rs  .rs
620  .sp  .sp
621  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
622  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
623  equivalent to the following:  following:
624  .sp  .sp
625    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
626  .sp  .sp
# Line 415  below. Line 631  below.
631  .\"  .\"
632  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
633  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
634  U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next  U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
635  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
636  cannot be split.  cannot be split.
637  .P  .P
638  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
639  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
640  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
641  recognized.  recognized.
642  .P  .P
643  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
644  complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF  complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
645  either at compile time or when the pattern is matched. This can be made the  either at compile time or when the pattern is matched. (BSR is an abbrevation
646  default when PCRE is built; if this is the case, the other behaviour can be  for "backslash R".) This can be made the default when PCRE is built; if this is
647  requested via the PCRE_BSR_UNICODE option. It is also possible to specify these  the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
648  settings by starting a pattern string with one of the following sequences:  It is also possible to specify these settings by starting a pattern string with
649    one of the following sequences:
650  .sp  .sp
651    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
652    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
653  .sp  .sp
654  These override the default and the options given to \fBpcre_compile()\fP, but  These override the default and the options given to the compiling function, but
655  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  they can themselves be overridden by options given to a matching function. Note
656  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
657  very start of a pattern, and that they must be in upper case. If more than one  at the very start of a pattern, and that they must be in upper case. If more
658  of them is present, the last one is used.  than one of them is present, the last one is used. They can be combined with a
659  .P  change of newline convention; for example, a pattern can start with:
660  Inside a character class, \eR matches the letter "R".  .sp
661      (*ANY)(*BSR_ANYCRLF)
662    .sp
663    They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
664    (*UCP) special sequences. Inside a character class, \eR is treated as an
665    unrecognized escape sequence, and so matches the letter "R" by default, but
666    causes an error if PCRE_EXTRA is set.
667  .  .
668  .  .
669  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 449  Inside a character class, \eR matches th Line 672  Inside a character class, \eR matches th
672  .sp  .sp
673  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
674  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
675  When not in UTF-8 mode, these sequences are of course limited to testing  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
676  characters whose codepoints are less than 256, but they do work in this mode.  characters whose codepoints are less than 256, but they do work in this mode.
677  The extra escape sequences are:  The extra escape sequences are:
678  .sp  .sp
679    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
680    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
681    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
682  .sp  .sp
683  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
684  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
685  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
686  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
687  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
688    .\" </a>
689    next section).
690    .\"
691    Other Perl properties such as "InMusicalSymbols" are not currently supported by
692    PCRE. Note that \eP{Any} does not match any characters, so always causes a
693    match failure.
694  .P  .P
695  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
696  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 475  Those that are not part of an identified Line 704  Those that are not part of an identified
704  .P  .P
705  Arabic,  Arabic,
706  Armenian,  Armenian,
707    Avestan,
708  Balinese,  Balinese,
709    Bamum,
710    Batak,
711  Bengali,  Bengali,
712  Bopomofo,  Bopomofo,
713    Brahmi,
714  Braille,  Braille,
715  Buginese,  Buginese,
716  Buhid,  Buhid,
717  Canadian_Aboriginal,  Canadian_Aboriginal,
718    Carian,
719    Chakma,
720    Cham,
721  Cherokee,  Cherokee,
722  Common,  Common,
723  Coptic,  Coptic,
# Line 490  Cypriot, Line 726  Cypriot,
726  Cyrillic,  Cyrillic,
727  Deseret,  Deseret,
728  Devanagari,  Devanagari,
729    Egyptian_Hieroglyphs,
730  Ethiopic,  Ethiopic,
731  Georgian,  Georgian,
732  Glagolitic,  Glagolitic,
# Line 502  Hangul, Line 739  Hangul,
739  Hanunoo,  Hanunoo,
740  Hebrew,  Hebrew,
741  Hiragana,  Hiragana,
742    Imperial_Aramaic,
743  Inherited,  Inherited,
744    Inscriptional_Pahlavi,
745    Inscriptional_Parthian,
746    Javanese,
747    Kaithi,
748  Kannada,  Kannada,
749  Katakana,  Katakana,
750    Kayah_Li,
751  Kharoshthi,  Kharoshthi,
752  Khmer,  Khmer,
753  Lao,  Lao,
754  Latin,  Latin,
755    Lepcha,
756  Limbu,  Limbu,
757  Linear_B,  Linear_B,
758    Lisu,
759    Lycian,
760    Lydian,
761  Malayalam,  Malayalam,
762    Mandaic,
763    Meetei_Mayek,
764    Meroitic_Cursive,
765    Meroitic_Hieroglyphs,
766    Miao,
767  Mongolian,  Mongolian,
768  Myanmar,  Myanmar,
769  New_Tai_Lue,  New_Tai_Lue,
# Line 519  Nko, Line 771  Nko,
771  Ogham,  Ogham,
772  Old_Italic,  Old_Italic,
773  Old_Persian,  Old_Persian,
774    Old_South_Arabian,
775    Old_Turkic,
776    Ol_Chiki,
777  Oriya,  Oriya,
778  Osmanya,  Osmanya,
779  Phags_Pa,  Phags_Pa,
780  Phoenician,  Phoenician,
781    Rejang,
782  Runic,  Runic,
783    Samaritan,
784    Saurashtra,
785    Sharada,
786  Shavian,  Shavian,
787  Sinhala,  Sinhala,
788    Sora_Sompeng,
789    Sundanese,
790  Syloti_Nagri,  Syloti_Nagri,
791  Syriac,  Syriac,
792  Tagalog,  Tagalog,
793  Tagbanwa,  Tagbanwa,
794  Tai_Le,  Tai_Le,
795    Tai_Tham,
796    Tai_Viet,
797    Takri,
798  Tamil,  Tamil,
799  Telugu,  Telugu,
800  Thaana,  Thaana,
# Line 538  Thai, Line 802  Thai,
802  Tibetan,  Tibetan,
803  Tifinagh,  Tifinagh,
804  Ugaritic,  Ugaritic,
805    Vai,
806  Yi.  Yi.
807  .P  .P
808  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
809  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
810  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
811  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
812  .P  .P
813  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
814  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 604  the Lu, Ll, or Lt property, in other wor Line 869  the Lu, Ll, or Lt property, in other wor
869  a modifier or "other".  a modifier or "other".
870  .P  .P
871  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
872  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
873  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
874  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
875    PCRE_NO_UTF32_CHECK in the
876  .\" HREF  .\" HREF
877  \fBpcreapi\fP  \fBpcreapi\fP
878  .\"  .\"
879  page).  page). Perl does not support the Cs property.
880  .P  .P
881  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
882  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
883  properties with "Is".  properties with "Is".
884  .P  .P
# Line 621  Instead, this property is assumed for an Line 887  Instead, this property is assumed for an
887  Unicode table.  Unicode table.
888  .P  .P
889  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
890  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
891    the behaviour of current versions of Perl.
892  .P  .P
893  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
894  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
895  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
896    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
897    PCRE_UCP option or by starting the pattern with (*UCP).
898    .
899    .
900    .SS Extended grapheme clusters
901    .rs
902  .sp  .sp
903  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
904  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
905  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
906  .\" </a>  .\" </a>
907  (see below).  (see below).
908  .\"  .\"
909  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
910  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
911  non-UTF-8 mode \eX matches any one character.  .sp
912  .P    (?>\ePM\epM*)
913  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
914  a structure that contains data for over fifteen thousand characters. That is  That is, it matched a character without the "mark" property, followed by zero
915  why the traditional escape sequences such as \ed and \ew do not use Unicode  or more characters with the "mark" property. Characters with the "mark"
916  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
917    .P
918    This simple definition was extended in Unicode to include more complicated
919    kinds of composite character by giving each character a grapheme breaking
920    property, and creating rules that use these properties to define the boundaries
921    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
922    one of these clusters.
923    .P
924    \eX always matches at least one character. Then it decides whether to add
925    additional characters according to the following rules for ending a cluster:
926    .P
927    1. End at the end of the subject string.
928    .P
929    2. Do not end between CR and LF; otherwise end after any control character.
930    .P
931    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
932    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
933    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
934    character; an LVT or T character may be follwed only by a T character.
935    .P
936    4. Do not end before extending characters or spacing marks. Characters with
937    the "mark" property always have the "extend" grapheme breaking property.
938    .P
939    5. Do not end after prepend characters.
940    .P
941    6. Otherwise, end the cluster.
942    .
943    .
944    .\" HTML <a name="extraprops"></a>
945    .SS PCRE's additional properties
946    .rs
947    .sp
948    As well as the standard Unicode properties described above, PCRE supports four
949    more that make it possible to convert traditional escape sequences such as \ew
950    and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
951    properties internally when PCRE_UCP is set. However, they may also be used
952    explicitly. These properties are:
953    .sp
954      Xan   Any alphanumeric character
955      Xps   Any POSIX space character
956      Xsp   Any Perl space character
957      Xwd   Any Perl "word" character
958    .sp
959    Xan matches characters that have either the L (letter) or the N (number)
960    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
961    carriage return, and any other character that has the Z (separator) property.
962    Xsp is the same as Xps; it used to exclude vertical tab, for Perl
963    compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
964    matches the same characters as Xan, plus underscore.
965    .P
966    There is another non-standard property, Xuc, which matches any character that
967    can be represented by a Universal Character Name in C++ and other programming
968    languages. These are the characters $, @, ` (grave accent), and all characters
969    with Unicode code points greater than or equal to U+00A0, except for the
970    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
971    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
972    where H is a hexadecimal digit. Note that the Xuc property does not match these
973    sequences but the characters that they represent.)
974  .  .
975  .  .
976  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
977  .SS "Resetting the match start"  .SS "Resetting the match start"
978  .rs  .rs
979  .sp  .sp
980  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
981  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
982  .sp  .sp
983    foo\eKbar    foo\eKbar
984  .sp  .sp
# Line 673  For example, when the pattern Line 1000  For example, when the pattern
1000    (foo)\eKbar    (foo)\eKbar
1001  .sp  .sp
1002  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
1003    .P
1004    Perl documents that the use of \eK within assertions is "not well defined". In
1005    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
1006    ignored in negative assertions.
1007  .  .
1008  .  .
1009  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 697  The backslashed assertions are: Line 1028  The backslashed assertions are:
1028    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
1029    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
1030  .sp  .sp
1031  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
1032  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
1033    default it matches the corresponding literal character (for example, \eB
1034    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1035    escape sequence" error is generated instead.
1036  .P  .P
1037  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
1038  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
1039  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
1040  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
1041    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1042    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1043    of word" or "end of word" metasequence. However, whatever follows \eb normally
1044    determines which it is. For example, the fragment \eba matches "a" at the start
1045    of a word.
1046  .P  .P
1047  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1048  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 737  regular expression. Line 1076  regular expression.
1076  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1077  .rs  .rs
1078  .sp  .sp
1079    The circumflex and dollar metacharacters are zero-width assertions. That is,
1080    they test for a particular condition being true without consuming any
1081    characters from the subject string.
1082    .P
1083  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1084  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
1085  at the start of the subject string. If the \fIstartoffset\fP argument of  the start of the subject string. If the \fIstartoffset\fP argument of
1086  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1087  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1088  meaning  meaning
# Line 756  constrained to match only at the start o Line 1099  constrained to match only at the start o
1099  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1100  to be anchored.)  to be anchored.)
1101  .P  .P
1102  A dollar character is an assertion that is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1103  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1104  at the end of the string (by default). Dollar need not be the last character of  the end of the string (by default). Note, however, that it does not actually
1105  the pattern if a number of alternatives are involved, but it should be the last  match the newline. Dollar need not be the last character of the pattern if a
1106  item in any branch in which it appears. Dollar has no special meaning in a  number of alternatives are involved, but it should be the last item in any
1107  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1108  .P  .P
1109  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1110  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
# Line 787  end of the subject in both modes, and if Line 1130  end of the subject in both modes, and if
1130  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1131  .  .
1132  .  .
1133  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1134    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1135  .rs  .rs
1136  .sp  .sp
1137  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
1138  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
1139  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1140  .P  .P
1141  When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
1142  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 809  to match it. Line 1153  to match it.
1153  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
1154  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
1155  special meaning in a character class.  special meaning in a character class.
1156  .  .P
1157  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1158  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1159  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1160  .sp  name; PCRE does not support this.
1161  Outside a character class, the escape sequence \eC matches any one byte, both  .
1162  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1163  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1164  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1165  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1166  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1167    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1168    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1169    a 32-bit unit. Unlike a dot, \eC always
1170    matches line-ending characters. The feature is provided in Perl in order to
1171    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1172    used. Because \eC breaks up characters into individual data units, matching one
1173    unit with \eC in a UTF mode means that the rest of the string may start with a
1174    malformed UTF character. This has undefined results, because PCRE assumes that
1175    it is dealing with valid UTF strings (and by default it checks this at the
1176    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1177    PCRE_NO_UTF32_CHECK option is used).
1178  .P  .P
1179  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1180  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1181  .\" </a>  .\" </a>
1182  (described below),  (described below)
1183  .\"  .\"
1184  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
1185  the lookbehind.  the lookbehind.
1186    .P
1187    In general, the \eC escape sequence is best avoided. However, one
1188    way of using it that avoids the problem of malformed UTF characters is to use a
1189    lookahead to check the length of the next character, as in this pattern, which
1190    could be used with a UTF-8 string (ignore white space and line breaks):
1191    .sp
1192      (?| (?=[\ex00-\ex7f])(\eC) |
1193          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1194          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1195          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1196    .sp
1197    A group that starts with (?| resets the capturing parentheses numbers in each
1198    alternative (see
1199    .\" HTML <a href="#dupsubpatternnumber">
1200    .\" </a>
1201    "Duplicate Subpattern Numbers"
1202    .\"
1203    below). The assertions at the start of each branch check the next UTF-8
1204    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1205    character's individual bytes are then captured by the appropriate number of
1206    groups.
1207  .  .
1208  .  .
1209  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 835  the lookbehind. Line 1211  the lookbehind.
1211  .rs  .rs
1212  .sp  .sp
1213  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1214  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
1215  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1216  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
1217  escaped with a backslash.  a member of the class, it should be the first data character in the class
1218  .P  (after an initial circumflex, if present) or escaped with a backslash.
1219  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1220  character may occupy more than one byte. A matched character must be in the set  A character class matches a single character in the subject. In a UTF mode, the
1221  of characters defined by the class, unless the first character in the class  character may be more than one data unit long. A matched character must be in
1222  definition is a circumflex, in which case the subject character must not be in  the set of characters defined by the class, unless the first character in the
1223  the set defined by the class. If a circumflex is actually required as a member  class definition is a circumflex, in which case the subject character must not
1224  of the class, ensure it is not the first character, or escape it with a  be in the set defined by the class. If a circumflex is actually required as a
1225    member of the class, ensure it is not the first character, or escape it with a
1226  backslash.  backslash.
1227  .P  .P
1228  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1229  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1230  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1231  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1232  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1233  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1234  string.  string.
1235  .P  .P
1236  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1237  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  can be included in a class as a literal string of data units, or by using the
1238    \ex{ escaping mechanism.
1239  .P  .P
1240  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
1241  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
1242  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1243  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1244  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1245  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1246  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1247  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1248  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1249  UTF-8 support.  well as with UTF support.
1250  .P  .P
1251  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1252  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 879  The minus (hyphen) character can be used Line 1257  The minus (hyphen) character can be used
1257  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
1258  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
1259  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
1260  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class, or
1261    immediately after a range. For example, [b-d-z] matches letters in the range b
1262    to d, a hyphen character, or z.
1263  .P  .P
1264  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
1265  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
# Line 889  the end of range, so [W-\e]46] is interp Line 1269  the end of range, so [W-\e]46] is interp
1269  followed by two other characters. The octal or hexadecimal representation of  followed by two other characters. The octal or hexadecimal representation of
1270  "]" can also be used to end a range.  "]" can also be used to end a range.
1271  .P  .P
1272    An error is generated if a POSIX character class (see below) or an escape
1273    sequence other than one that defines a single character appears at a point
1274    where a range ending character is expected. For example, [z-\exff] is valid,
1275    but [A-\ed] and [A-[:digit:]] are not.
1276    .P
1277  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
1278  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1279  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1280  .P  .P
1281  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1282  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1283  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1284  tables for a French locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1285  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1286  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1287  property support.  property support.
1288  .P  .P
1289  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1290  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1291  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1292  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1293  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1294  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1295    .\" HTML <a href="#genericchartypes">
1296    .\" </a>
1297    "Generic character types"
1298    .\"
1299    above. The escape sequence \eb has a different meaning inside a character
1300    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1301    are not special inside a character class. Like any other unrecognized escape
1302    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1303    default, but cause an error if the PCRE_EXTRA option is set.
1304    .P
1305    A circumflex can conveniently be used with the upper case character types to
1306    specify a more restricted set of characters than the matching lower case type.
1307    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1308    whereas [\ew] includes underscore. A positive character class should be read as
1309    "something OR something OR ..." and a negative class as "NOT something AND NOT
1310    something AND NOT ...".
1311  .P  .P
1312  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1313  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 927  this notation. For example, Line 1327  this notation. For example,
1327    [01[:alpha:]%]    [01[:alpha:]%]
1328  .sp  .sp
1329  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1330  are  are:
1331  .sp  .sp
1332    alnum    letters and digits    alnum    letters and digits
1333    alpha    letters    alpha    letters
# Line 938  are Line 1338  are
1338    graph    printing characters, excluding space    graph    printing characters, excluding space
1339    lower    lower case letters    lower    lower case letters
1340    print    printing characters, including space    print    printing characters, including space
1341    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1342    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1343    upper    upper case letters    upper    upper case letters
1344    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1345    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1346  .sp  .sp
1347  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1348  space (32). Notice that this list includes the VT character (code 11). This  and space (32). If locale-specific matching is taking place, there may be
1349  makes "space" different to \es, which does not include VT (for Perl  additional space characters. "Space" used to be different to \es, which did not
1350  compatibility).  include VT, for Perl compatibility. However, Perl changed at release 5.18, and
1351    PCRE followed at release 8.34. "Space" and \es now match the same set of
1352    characters.
1353  .P  .P
1354  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1355  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
# Line 959  matches "1", "2", or any non-digit. PCRE Line 1361  matches "1", "2", or any non-digit. PCRE
1361  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1362  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1363  .P  .P
1364  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, characters with values greater than 128 do not match any of the
1365  the POSIX character classes.  POSIX character classes. However, if the PCRE_UCP option is passed to
1366    \fBpcre_compile()\fP, some of the classes are changed so that Unicode character
1367    properties are used. This is achieved by replacing certain POSIX classes by
1368    other sequences, as follows:
1369    .sp
1370      [:alnum:]  becomes  \ep{Xan}
1371      [:alpha:]  becomes  \ep{L}
1372      [:blank:]  becomes  \eh
1373      [:digit:]  becomes  \ep{Nd}
1374      [:lower:]  becomes  \ep{Ll}
1375      [:space:]  becomes  \ep{Xps}
1376      [:upper:]  becomes  \ep{Lu}
1377      [:word:]   becomes  \ep{Xwd}
1378    .sp
1379    Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1380    classes are handled specially in UCP mode:
1381    .TP 10
1382    [:graph:]
1383    This matches characters that have glyphs that mark the page when printed. In
1384    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1385    properties, except for:
1386    .sp
1387      U+061C           Arabic Letter Mark
1388      U+180E           Mongolian Vowel Separator
1389      U+2066 - U+2069  Various "isolate"s
1390    .sp
1391    .TP 10
1392    [:print:]
1393    This matches the same characters as [:graph:] plus space characters that are
1394    not controls, that is, characters with the Zs property.
1395    .TP 10
1396    [:punct:]
1397    This matches all characters that have the Unicode P (punctuation) property,
1398    plus those characters whose code points are less than 128 that have the S
1399    (Symbol) property.
1400    .P
1401    The other POSIX classes are unchanged, and match only characters with code
1402    points less than 128.
1403  .  .
1404  .  .
1405  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1007  The PCRE-specific options PCRE_DUPNAMES, Line 1446  The PCRE-specific options PCRE_DUPNAMES,
1446  changed in the same way as the Perl-compatible options by using the characters  changed in the same way as the Perl-compatible options by using the characters
1447  J, U and X respectively.  J, U and X respectively.
1448  .P  .P
1449  When an option change occurs at top level (that is, not inside subpattern  When one of these option changes occurs at top level (that is, not inside
1450  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1451  If the change is placed right at the start of a pattern, PCRE extracts it into  that follows. If the change is placed right at the start of a pattern, PCRE
1452  the global options (and it will therefore show up in data extracted by the  extracts it into the global options (and it will therefore show up in data
1453  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1454  .P  .P
1455  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1456  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1457  .sp  .sp
1458    (a(?i)b)c    (a(?i)b)c
1459  .sp  .sp
# Line 1029  matches "ab", "aB", "c", and "C", even t Line 1468  matches "ab", "aB", "c", and "C", even t
1468  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1469  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1470  behaviour otherwise.  behaviour otherwise.
1471    .P
1472    \fBNote:\fP There are other PCRE-specific options that can be set by the
1473    application when the compiling or matching functions are called. In some cases
1474    the pattern can contain special leading sequences such as (*CRLF) to override
1475    what the application has set or what has been defaulted. Details are given in
1476    the section entitled
1477    .\" HTML <a href="#newlineseq">
1478    .\" </a>
1479    "Newline sequences"
1480    .\"
1481    above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1482    sequences that can be used to set UTF and Unicode property modes; they are
1483    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1484    options, respectively. The (*UTF) sequence is a generic version that can be
1485    used with any of the libraries. However, the application can set the
1486    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1487  .  .
1488  .  .
1489  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1042  Turning part of a pattern into a subpatt Line 1497  Turning part of a pattern into a subpatt
1497  .sp  .sp
1498    cat(aract|erpillar|)    cat(aract|erpillar|)
1499  .sp  .sp
1500  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1501  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1502  .sp  .sp
1503  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1504  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1505  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of the
1506  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1507  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1508  .P  .P
1509  For example, if the string "the red king" is matched against the pattern  Opening parentheses are counted from left to right (starting from 1) to obtain
1510    numbers for the capturing subpatterns. For example, if the string "the red
1511    king" is matched against the pattern
1512  .sp  .sp
1513    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1514  .sp  .sp
# Line 1083  is reached, an option setting in one bra Line 1540  is reached, an option setting in one bra
1540  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1541  .  .
1542  .  .
1543    .\" HTML <a name="dupsubpatternnumber"></a>
1544  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1545  .rs  .rs
1546  .sp  .sp
# Line 1099  at captured substring number one, whiche Line 1557  at captured substring number one, whiche
1557  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1558  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1559  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1560  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1561  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1562  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1563  .sp  .sp
1564    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1565    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1566    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1567  .sp  .sp
1568  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1569  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1570    or "defdef":
1571    .sp
1572      /(?|(abc)|(def))\e1/
1573    .sp
1574    In contrast, a subroutine call to a numbered subpattern always refers to the
1575    first one in the pattern with the given number. The following pattern matches
1576    "abcabc" or "defabc":
1577    .sp
1578      /(?|(abc)|(def))(?1)/
1579    .sp
1580    If a
1581    .\" HTML <a href="#conditions">
1582    .\" </a>
1583    condition test
1584    .\"
1585    for a subpattern's having matched refers to a non-unique number, the test is
1586    true if any of the subpatterns of that number have matched.
1587  .P  .P
1588  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1589  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1124  if an expression is modified, the number Line 1598  if an expression is modified, the number
1598  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1599  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1600  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1601  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1602    have different names, but PCRE does not.
1603  .P  .P
1604  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1605  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1606  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1607  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1608  .\" </a>  .\" </a>
1609  backreferences,  back references,
1610  .\"  .\"
1611  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1612  .\" </a>  .\" </a>
# Line 1144  conditions, Line 1619  conditions,
1619  .\"  .\"
1620  can be made by name as well as by number.  can be made by name as well as by number.
1621  .P  .P
1622  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores, but must
1623  capturing parentheses are still allocated numbers as well as names, exactly as  start with a non-digit. Named capturing parentheses are still allocated numbers
1624  if the names were not present. The PCRE API provides function calls for  as well as names, exactly as if the names were not present. The PCRE API
1625  extracting the name-to-number translation table from a compiled pattern. There  provides function calls for extracting the name-to-number translation table
1626  is also a convenience function for extracting a captured substring by name.  from a compiled pattern. There is also a convenience function for extracting a
1627    captured substring by name.
1628  .P  .P
1629  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1630  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1631  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1632  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1633  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1634  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1635    name, and in both cases you want to extract the abbreviation. This pattern
1636    (ignoring the line breaks) does the job:
1637  .sp  .sp
1638    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1639    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1169  subpattern, as described in the previous Line 1647  subpattern, as described in the previous
1647  .P  .P
1648  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1649  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1650  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1651  make a reference to a non-unique named subpattern from elsewhere in the  .P
1652  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1653  details of the interfaces for handling named subpatterns, see the  the pattern, the subpatterns to which the name refers are checked in the order
1654    in which they appear in the overall pattern. The first one that is set is used
1655    for the reference. For example, this pattern matches both "foofoo" and
1656    "barbar" but not "foobar" or "barfoo":
1657    .sp
1658      (?:(?<n>foo)|(?<n>bar))\k<n>
1659    .sp
1660    .P
1661    If you make a subroutine call to a non-unique named subpattern, the one that
1662    corresponds to the first occurrence of the name is used. In the absence of
1663    duplicate numbers (see the previous section) this is the one with the lowest
1664    number.
1665    .P
1666    If you use a named reference in a condition
1667    test (see the
1668    .\"
1669    .\" HTML <a href="#conditions">
1670    .\" </a>
1671    section about conditions
1672    .\"
1673    below), either to check whether a subpattern has matched, or to check for
1674    recursion, all subpatterns with the same name are tested. If the condition is
1675    true for any one of them, the overall condition is true. This is the same
1676    behaviour as testing by number. For further details of the interfaces for
1677    handling named subpatterns, see the
1678  .\" HREF  .\" HREF
1679  \fBpcreapi\fP  \fBpcreapi\fP
1680  .\"  .\"
1681  documentation.  documentation.
1682    .P
1683    \fBWarning:\fP You cannot use different names to distinguish between two
1684    subpatterns with the same number because PCRE uses only the numbers when
1685    matching. For this reason, an error is given at compile time if different names
1686    are given to subpatterns with the same number. However, you can always give the
1687    same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1688    set.
1689  .  .
1690  .  .
1691  .SH REPETITION  .SH REPETITION
# Line 1188  items: Line 1697  items:
1697    a literal data character    a literal data character
1698    the dot metacharacter    the dot metacharacter
1699    the \eC escape sequence    the \eC escape sequence
1700    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1701    the \eR escape sequence    the \eR escape sequence
1702    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1703    a character class    a character class
1704    a back reference (see next section)    a back reference (see next section)
1705    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1706      a subroutine call to a subpattern (recursive or otherwise)
1707  .sp  .sp
1708  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1709  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1218  where a quantifier is not allowed, or on Line 1728  where a quantifier is not allowed, or on
1728  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1729  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1730  .P  .P
1731  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1732  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1733  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1734  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1735  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1736  .P  .P
1737  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1738  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1739    subpatterns that are referenced as
1740    .\" HTML <a href="#subpatternsassubroutines">
1741    .\" </a>
1742    subroutines
1743    .\"
1744    from elsewhere in the pattern (but see also the section entitled
1745    .\" HTML <a href="#subdefine">
1746    .\" </a>
1747    "Defining subpatterns for use by reference only"
1748    .\"
1749    below). Items other than subpatterns that have a {0} quantifier are omitted
1750    from the compiled pattern.
1751  .P  .P
1752  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1753  abbreviations:  abbreviations:
# Line 1296  In cases where it is known that the subj Line 1818  In cases where it is known that the subj
1818  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1819  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1820  .P  .P
1821  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1822  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1823  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1824  succeeds. Consider, for example:  succeeds. Consider, for example:
1825  .sp  .sp
# Line 1306  succeeds. Consider, for example: Line 1828  succeeds. Consider, for example:
1828  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1829  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1830  .P  .P
1831    Another case where implicit anchoring is not applied is when the leading .* is
1832    inside an atomic group. Once again, a match at the start may fail where a later
1833    one succeeds. Consider this pattern:
1834    .sp
1835      (?>.*?a)b
1836    .sp
1837    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1838    (*PRUNE) and (*SKIP) also disable this optimization.
1839    .P
1840  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1841  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1842  .sp  .sp
# Line 1450  no such problem when named parentheses a Line 1981  no such problem when named parentheses a
1981  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1982  .P  .P
1983  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1984  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1985  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1986  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1987  .sp  .sp
1988    (ring), \e1    (ring), \e1
1989    (ring), \eg1    (ring), \eg1
# Line 1466  example: Line 1997  example:
1997    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1998  .sp  .sp
1999  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
2000  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
2001  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
2002  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
2003  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
2004  .P  .P
2005  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
2006  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1508  after the reference. Line 2039  after the reference.
2039  .P  .P
2040  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
2041  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
2042  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
2043  .sp  .sp
2044    (a|(bc))\e2    (a|(bc))\e2
2045  .sp  .sp
2046  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
2047  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
2048  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
2049  with a digit character, some delimiter must be used to terminate the back  .P
2050  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
2051  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
2052    If the pattern continues with a digit character, some delimiter must be used to
2053    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2054    white space. Otherwise, the \eg{ syntax or an empty comment (see
2055  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
2056  .\" </a>  .\" </a>
2057  "Comments"  "Comments"
2058  .\"  .\"
2059  below) can be used.  below) can be used.
2060  .P  .
2061    .SS "Recursive back references"
2062    .rs
2063    .sp
2064  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
2065  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
2066  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1537  to the previous iteration. In order for Line 2074  to the previous iteration. In order for
2074  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
2075  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
2076  minimum of zero.  minimum of zero.
2077    .P
2078    Back references of this type cause the group that they reference to be treated
2079    as an
2080    .\" HTML <a href="#atomicgroup">
2081    .\" </a>
2082    atomic group.
2083    .\"
2084    Once the whole group has been matched, a subsequent matching failure cannot
2085    cause backtracking into the middle of the group.
2086  .  .
2087  .  .
2088  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1556  those that look ahead of the current pos Line 2102  those that look ahead of the current pos
2102  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,
2103  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.
2104  .P  .P
2105  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2106  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
2107  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2108  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2109  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2110  because it does not make sense for negative assertions.  .P
2111    For compatibility with Perl, assertion subpatterns may be repeated; though
2112    it makes no sense to assert the same thing several times, the side effect of
2113    capturing parentheses may occasionally be useful. In practice, there only three
2114    cases:
2115    .sp
2116    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2117    However, it may contain internal capturing parenthesized groups that are called
2118    from elsewhere via the
2119    .\" HTML <a href="#subpatternsassubroutines">
2120    .\" </a>
2121    subroutine mechanism.
2122    .\"
2123    .sp
2124    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2125    were {0,1}. At run time, the rest of the pattern match is tried with and
2126    without the assertion, the order depending on the greediness of the quantifier.
2127    .sp
2128    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2129    The assertion is obeyed just once when encountered during matching.
2130  .  .
2131  .  .
2132  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1590  lookbehind assertion is needed to achiev Line 2155  lookbehind assertion is needed to achiev
2155  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
2156  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
2157  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
2158    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2159  .  .
2160  .  .
2161  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1614  is permitted, but Line 2180  is permitted, but
2180  .sp  .sp
2181  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2182  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
2183  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
2184  match the same length of string. An assertion such as  length of string. An assertion such as
2185  .sp  .sp
2186    (?<=ab(c|de))    (?<=ab(c|de))
2187  .sp  .sp
2188  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
2189  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
2190    branches:
2191  .sp  .sp
2192    (?<=abc|abde)    (?<=abc|abde)
2193  .sp  .sp
2194  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2195  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2196  .\" </a>  .\" </a>
2197  (see above)  (see above)
2198  .\"  .\"
2199  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
2200  fixed-length.  restriction.
2201  .P  .P
2202  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2203  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
2204  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2205  assertion fails.  assertion fails.
2206  .P  .P
2207  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
2208  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
2209  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
2210  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2211    permitted.
2212    .P
2213    .\" HTML <a href="#subpatternsassubroutines">
2214    .\" </a>
2215    "Subroutine"
2216    .\"
2217    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2218    as the subpattern matches a fixed-length string.
2219    .\" HTML <a href="#recursion">
2220    .\" </a>
2221    Recursion,
2222    .\"
2223    however, is not supported.
2224  .P  .P
2225  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2226  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
2227  pattern such as  strings. Consider a simple pattern such as
2228  .sp  .sp
2229    abcd$    abcd$
2230  .sp  .sp
# Line 1708  characters that are not "999". Line 2288  characters that are not "999".
2288  .sp  .sp
2289  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2290  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2291  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2292  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2293  .sp  .sp
2294    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2295    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2296  .sp  .sp
2297  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2298  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2299  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2300    itself contain nested subpatterns of any form, including conditional
2301    subpatterns; the restriction to two alternatives applies only at the level of
2302    the condition. This pattern fragment is an example where the alternatives are
2303    complex:
2304    .sp
2305      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2306    .sp
2307  .P  .P
2308  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2309  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1725  recursion, a pseudo-condition called DEF Line 2312  recursion, a pseudo-condition called DEF
2312  .rs  .rs
2313  .sp  .sp
2314  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2315  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2316  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2317  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2318  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2319  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2320  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2321    section about duplicate subpattern numbers),
2322    .\"
2323    the condition is true if any of them have matched. An alternative notation is
2324    to precede the digits with a plus or minus sign. In this case, the subpattern
2325    number is relative rather than absolute. The most recently opened parentheses
2326    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2327    loops it can also make sense to refer to subsequent groups. The next
2328    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2329    zero in any of these forms is not used; it provokes a compile-time error.)
2330  .P  .P
2331  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2332  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1741  three parts for ease of discussion: Line 2337  three parts for ease of discussion:
2337  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2338  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2339  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2340  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2341  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2342  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2343  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2344  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1760  This makes the fragment independent of t Line 2356  This makes the fragment independent of t
2356  .sp  .sp
2357  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2358  subpattern by name. For compatibility with earlier versions of PCRE, which had  subpattern by name. For compatibility with earlier versions of PCRE, which had
2359  this facility before Perl, the syntax (?(name)...) is also recognized. However,  this facility before Perl, the syntax (?(name)...) is also recognized.
 there is a possible ambiguity with this syntax, because subpattern names may  
 consist entirely of digits. PCRE looks first for a named subpattern; if it  
 cannot find one and the name consists entirely of digits, PCRE looks for a  
 subpattern of that number, which must be greater than zero. Using subpattern  
 names that consist entirely of digits is not recommended.  
2360  .P  .P
2361  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2362  .sp  .sp
2363    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2364  .sp  .sp
2365    If the name used in a condition of this kind is a duplicate, the test is
2366    applied to all subpatterns of the same name, and is true if any one of them has
2367    matched.
2368  .  .
2369  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2370  .rs  .rs
# Line 1782  letter R, for example: Line 2376  letter R, for example:
2376  .sp  .sp
2377    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2378  .sp  .sp
2379  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2380  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2381  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2382    applied to all subpatterns of the same name, and is true if any one of them is
2383    the most recent recursion.
2384  .P  .P
2385  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2386  patterns are described below.  .\" HTML <a href="#recursion">
2387    .\" </a>
2388    The syntax for recursive patterns
2389    .\"
2390    is described below.
2391  .  .
2392    .\" HTML <a name="subdefine"></a>
2393  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2394  .rs  .rs
2395  .sp  .sp
# Line 1796  If the condition is the string (DEFINE), Line 2397  If the condition is the string (DEFINE),
2397  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
2398  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2399  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
2400  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2401  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2402  written like this (ignore whitespace and line breaks):  .\" </a>
2403    subroutines
2404    .\"
2405    is described below.) For example, a pattern to match an IPv4 address such as
2406    "192.168.23.245" could be written like this (ignore white space and line
2407    breaks):
2408  .sp  .sp
2409    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2410    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1806  written like this (ignore whitespace and Line 2412  written like this (ignore whitespace and
2412  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2413  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2414  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2415  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2416  .P  pattern uses references to the named group to match the four dot-separated
2417  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2418  .  .
2419  .SS "Assertion conditions"  .SS "Assertion conditions"
2420  .rs  .rs
# Line 1835  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2439  dd-aaa-dd or dd-dd-dd, where aaa are let
2439  .SH COMMENTS  .SH COMMENTS
2440  .rs  .rs
2441  .sp  .sp
2442  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2443  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2444  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2445    subpattern name or number. The characters that make up a comment play no part
2446    in the pattern matching.
2447  .P  .P
2448  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2449  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2450  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2451    this case continues to immediately after the next newline character or
2452    character sequence in the pattern. Which characters are interpreted as newlines
2453    is controlled by the options passed to a compiling function or by a special
2454    sequence at the start of the pattern, as described in the section entitled
2455    .\" HTML <a href="#newlines">
2456    .\" </a>
2457    "Newline conventions"
2458    .\"
2459    above. Note that the end of this type of comment is a literal newline sequence
2460    in the pattern; escape sequences that happen to represent a newline do not
2461    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2462    default newline convention is in force:
2463    .sp
2464      abc #comment \en still comment
2465    .sp
2466    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2467    a newline in the pattern. The sequence \en is still literal at this stage, so
2468    it does not terminate the comment. Only an actual character with the code value
2469    0x0a (the default newline) does so.
2470  .  .
2471  .  .
2472  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1867  recursively to the pattern in which it a Line 2492  recursively to the pattern in which it a
2492  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2493  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2494  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2495  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2496  .P  .P
2497  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
2498  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
2499  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2500    .\" HTML <a href="#subpatternsassubroutines">
2501    .\" </a>
2502    non-recursive subroutine
2503    .\"
2504  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
2505  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2506  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2507  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2508  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2509  .sp  .sp
2510    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2511  .sp  .sp
2512  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2513  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2514  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2515  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2516    to avoid backtracking into sequences of non-parentheses.
2517  .P  .P
2518  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
2519  pattern, so instead you could use this:  pattern, so instead you could use this:
2520  .sp  .sp
2521    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2522  .sp  .sp
2523  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2524  them instead of the whole pattern.  them instead of the whole pattern.
2525  .P  .P
2526  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2527  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. Instead of (?1) in the
2528  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2529  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2530  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2531  .P  .P
2532  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2533  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2534  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2535  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2536    .\" </a>
2537    non-recursive subroutine
2538    .\"
2539    calls, as described in the next section.
2540  .P  .P
2541  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2542  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2543  could rewrite the above example as follows:  could rewrite the above example as follows:
2544  .sp  .sp
2545    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2546  .sp  .sp
2547  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2548  used.  used.
2549  .P  .P
2550  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2551  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2552  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2553  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2554  .sp  .sp
2555    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2556  .sp  .sp
2557  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2558  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2559  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2560  before failure can be reported.  before failure can be reported.
2561  .P  .P
2562  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2563  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2564  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2565  .\" HREF  .\" HREF
2566  \fBpcrecallout\fP  \fBpcrecallout\fP
2567  .\"  .\"
# Line 1942  documentation). If the pattern above is Line 2569  documentation). If the pattern above is
2569  .sp  .sp
2570    (ab(cd)ef)    (ab(cd)ef)
2571  .sp  .sp
2572  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2573  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2574  .sp  matched at the top level, its final captured value is unset, even if it was
2575    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2576       ^                        ^  .P
2577       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2578  .sp  obtain extra memory to store data during a recursion, which it does by using
2579  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2580  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2581  .P  .P
2582  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2583  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1967  different alternatives for the recursive Line 2591  different alternatives for the recursive
2591  is the actual recursive call.  is the actual recursive call.
2592  .  .
2593  .  .
2594    .\" HTML <a name="recursiondifference"></a>
2595    .SS "Differences in recursion processing between PCRE and Perl"
2596    .rs
2597    .sp
2598    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2599    (like Python, but unlike Perl), a recursive subpattern call is always treated
2600    as an atomic group. That is, once it has matched some of the subject string, it
2601    is never re-entered, even if it contains untried alternatives and there is a
2602    subsequent matching failure. This can be illustrated by the following pattern,
2603    which purports to match a palindromic string that contains an odd number of
2604    characters (for example, "a", "aba", "abcba", "abcdcba"):
2605    .sp
2606      ^(.|(.)(?1)\e2)$
2607    .sp
2608    The idea is that it either matches a single character, or two identical
2609    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2610    it does not if the pattern is longer than three characters. Consider the
2611    subject string "abcba":
2612    .P
2613    At the top level, the first character is matched, but as it is not at the end
2614    of the string, the first alternative fails; the second alternative is taken
2615    and the recursion kicks in. The recursive call to subpattern 1 successfully
2616    matches the next character ("b"). (Note that the beginning and end of line
2617    tests are not part of the recursion).
2618    .P
2619    Back at the top level, the next character ("c") is compared with what
2620    subpattern 2 matched, which was "a". This fails. Because the recursion is
2621    treated as an atomic group, there are now no backtracking points, and so the
2622    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2623    try the second alternative.) However, if the pattern is written with the
2624    alternatives in the other order, things are different:
2625    .sp
2626      ^((.)(?1)\e2|.)$
2627    .sp
2628    This time, the recursing alternative is tried first, and continues to recurse
2629    until it runs out of characters, at which point the recursion fails. But this
2630    time we do have another alternative to try at the higher level. That is the big
2631    difference: in the previous case the remaining alternative is at a deeper
2632    recursion level, which PCRE cannot use.
2633    .P
2634    To change the pattern so that it matches all palindromic strings, not just
2635    those with an odd number of characters, it is tempting to change the pattern to
2636    this:
2637    .sp
2638      ^((.)(?1)\e2|.?)$
2639    .sp
2640    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2641    deeper recursion has matched a single character, it cannot be entered again in
2642    order to match an empty string. The solution is to separate the two cases, and
2643    write out the odd and even cases as alternatives at the higher level:
2644    .sp
2645      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2646    .sp
2647    If you want to match typical palindromic phrases, the pattern has to ignore all
2648    non-word characters, which can be done like this:
2649    .sp
2650      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2651    .sp
2652    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2653    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2654    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2655    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2656    more) to match typical phrases, and Perl takes so long that you think it has
2657    gone into a loop.
2658    .P
2659    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2660    string does not start with a palindrome that is shorter than the entire string.
2661    For example, although "abcba" is correctly matched, if the subject is "ababa",
2662    PCRE finds the palindrome "aba" at the start, then fails at top level because
2663    the end of the string does not follow. Once again, it cannot jump back into the
2664    recursion to try other alternatives, so the entire match fails.
2665    .P
2666    The second way in which PCRE and Perl differ in their recursion processing is
2667    in the handling of captured values. In Perl, when a subpattern is called
2668    recursively or as a subpattern (see the next section), it has no access to any
2669    values that were captured outside the recursion, whereas in PCRE these values
2670    can be referenced. Consider this pattern:
2671    .sp
2672      ^(.)(\e1|a(?2))
2673    .sp
2674    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2675    then in the second group, when the back reference \e1 fails to match "b", the
2676    second alternative matches "a" and then recurses. In the recursion, \e1 does
2677    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2678    match because inside the recursive call \e1 cannot access the externally set
2679    value.
2680    .
2681    .
2682  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2683  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2684  .rs  .rs
2685  .sp  .sp
2686  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
2687  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
2688  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2689  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2690  relative, as in these examples:  relative, as in these examples:
2691  .sp  .sp
# Line 1993  matches "sense and sensibility" and "res Line 2705  matches "sense and sensibility" and "res
2705  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
2706  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2707  .P  .P
2708  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2709  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
2710  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
2711  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2712  .P  subroutine call revert to their previous values afterwards.
2713  When a subpattern is used as a subroutine, processing options such as  .P
2714  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2715  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2716    different calls. For example, consider this pattern:
2717  .sp  .sp
2718    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2719  .sp  .sp
# Line 2008  It matches "abcabc". It does not match " Line 2721  It matches "abcabc". It does not match "
2721  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2722  .  .
2723  .  .
2724    .\" HTML <a name="onigurumasubroutines"></a>
2725    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2726    .rs
2727    .sp
2728    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2729    a number enclosed either in angle brackets or single quotes, is an alternative
2730    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2731    are two of the examples used above, rewritten using this syntax:
2732    .sp
2733      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2734      (sens|respons)e and \eg'1'ibility
2735    .sp
2736    PCRE supports an extension to Oniguruma: if a number is preceded by a
2737    plus or a minus sign it is taken as a relative reference. For example:
2738    .sp
2739      (abc)(?i:\eg<-1>)
2740    .sp
2741    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2742    synonymous. The former is a back reference; the latter is a subroutine call.
2743    .
2744    .
2745  .SH CALLOUTS  .SH CALLOUTS
2746  .rs  .rs
2747  .sp  .sp
# Line 2018  same pair of parentheses when there is a Line 2752  same pair of parentheses when there is a
2752  .P  .P
2753  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2754  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
2755  function by putting its entry point in the global variable \fIpcre_callout\fP.  function by putting its entry point in the global variable \fIpcre_callout\fP
2756    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2757  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2758  .P  .P
2759  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
# Line 2028  For example, this pattern has two callou Line 2763  For example, this pattern has two callou
2763  .sp  .sp
2764    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2765  .sp  .sp
2766  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2767  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2768  255.  255. If there is a conditional group in the pattern whose condition is an
2769  .P  assertion, an additional callout is inserted just before the condition. An
2770  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  explicit callout may also be set at this position, as in this example:
2771  set), the external function is called. It is provided with the number of the  .sp
2772  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2773  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2774  may cause matching to proceed, to backtrack, or to fail altogether. A complete  Note that this applies only to assertion conditions, not to other types of
2775  description of the interface to the callout function is given in the  condition.
2776    .P
2777    During matching, when PCRE reaches a callout point, the external function is
2778    called. It is provided with the number of the callout, the position in the
2779    pattern, and, optionally, one item of data originally supplied by the caller of
2780    the matching function. The callout function may cause matching to proceed, to
2781    backtrack, or to fail altogether.
2782    .P
2783    By default, PCRE implements a number of optimizations at compile time and
2784    matching time, and one side-effect is that sometimes callouts are skipped. If
2785    you need all possible callouts to happen, you need to set options that disable
2786    the relevant optimizations. More details, and a complete description of the
2787    interface to the callout function, are given in the
2788  .\" HREF  .\" HREF
2789  \fBpcrecallout\fP  \fBpcrecallout\fP
2790  .\"  .\"
2791  documentation.  documentation.
2792  .  .
2793  .  .
2794    .\" HTML <a name="backtrackcontrol"></a>
2795  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2796  .rs  .rs
2797  .sp  .sp
2798  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2799  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2800  or removal in a future version of Perl". It goes on to say: "Their usage in  change or removal in a future version of Perl". It goes on to say: "Their usage
2801  production code should be noted to avoid problems during upgrades." The same  in production code should be noted to avoid problems during upgrades." The same
2802  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2803  .P  .P
 Since these verbs are specifically related to backtracking, they can be used  
 only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  
 backtracking algorithm. They cause an error if encountered by  
 \fBpcre_dfa_exec()\fP.  
 .P  
2804  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2805  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2806  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2807  form is just (*VERB). Any number of these verbs may occur in a pattern. There  differently depending on whether or not a name is present. A name is any
2808  are two kinds:  sequence of characters that does not include a closing parenthesis. The maximum
2809    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2810    libraries. If the name is empty, that is, if the closing parenthesis
2811    immediately follows the colon, the effect is as if the colon were not there.
2812    Any number of these verbs may occur in a pattern.
2813    .P
2814    Since these verbs are specifically related to backtracking, most of them can be
2815    used only when the pattern is to be matched using one of the traditional
2816    matching functions, because these use a backtracking algorithm. With the
2817    exception of (*FAIL), which behaves like a failing negative assertion, the
2818    backtracking control verbs cause an error if encountered by a DFA matching
2819    function.
2820    .P
2821    The behaviour of these verbs in
2822    .\" HTML <a href="#btrepeat">
2823    .\" </a>
2824    repeated groups,
2825    .\"
2826    .\" HTML <a href="#btassert">
2827    .\" </a>
2828    assertions,
2829    .\"
2830    and in
2831    .\" HTML <a href="#btsub">
2832    .\" </a>
2833    subpatterns called as subroutines
2834    .\"
2835    (whether or not recursively) is documented below.
2836    .
2837    .
2838    .\" HTML <a name="nooptimize"></a>
2839    .SS "Optimizations that affect backtracking verbs"
2840    .rs
2841    .sp
2842    PCRE contains some optimizations that are used to speed up matching by running
2843    some checks at the start of each match attempt. For example, it may know the
2844    minimum length of matching subject, or that a particular character must be
2845    present. When one of these optimizations bypasses the running of a match, any
2846    included backtracking verbs will not, of course, be processed. You can suppress
2847    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2848    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2849    pattern with (*NO_START_OPT). There is more discussion of this option in the
2850    section entitled
2851    .\" HTML <a href="pcreapi.html#execoptions">
2852    .\" </a>
2853    "Option bits for \fBpcre_exec()\fP"
2854    .\"
2855    in the
2856    .\" HREF
2857    \fBpcreapi\fP
2858    .\"
2859    documentation.
2860    .P
2861    Experiments with Perl suggest that it too has similar optimizations, sometimes
2862    leading to anomalous results.
2863    .
2864  .  .
2865  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2866  .rs  .rs
2867  .sp  .sp
2868  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2869    followed by a name.
2870  .sp  .sp
2871     (*ACCEPT)     (*ACCEPT)
2872  .sp  .sp
2873  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2874  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2875  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2876  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2877  captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2878    .P
2879    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2880    example:
2881  .sp  .sp
2882    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2883  .sp  .sp
2884  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2885  captured.  the outer parentheses.
2886  .sp  .sp
2887    (*FAIL) or (*F)    (*FAIL) or (*F)
2888  .sp  .sp
2889  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
2890  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
2891  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2892  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 2095  callout feature, as for example in this Line 2897  callout feature, as for example in this
2897  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2898  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2899  .  .
2900    .
2901    .SS "Recording which path was taken"
2902    .rs
2903    .sp
2904    There is one verb whose main purpose is to track how a match was arrived at,
2905    though it also has a secondary use in conjunction with advancing the match
2906    starting point (see (*SKIP) below).
2907    .sp
2908      (*MARK:NAME) or (*:NAME)
2909    .sp
2910    A name is always required with this verb. There may be as many instances of
2911    (*MARK) as you like in a pattern, and their names do not have to be unique.
2912    .P
2913    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2914    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2915    caller as described in the section entitled
2916    .\" HTML <a href="pcreapi.html#extradata">
2917    .\" </a>
2918    "Extra data for \fBpcre_exec()\fP"
2919    .\"
2920    in the
2921    .\" HREF
2922    \fBpcreapi\fP
2923    .\"
2924    documentation. Here is an example of \fBpcretest\fP output, where the /K
2925    modifier requests the retrieval and outputting of (*MARK) data:
2926    .sp
2927        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2928      data> XY
2929       0: XY
2930      MK: A
2931      XZ
2932       0: XZ
2933      MK: B
2934    .sp
2935    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2936    indicates which of the two alternatives matched. This is a more efficient way
2937    of obtaining this information than putting each alternative in its own
2938    capturing parentheses.
2939    .P
2940    If a verb with a name is encountered in a positive assertion that is true, the
2941    name is recorded and passed back if it is the last-encountered. This does not
2942    happen for negative assertions or failing positive assertions.
2943    .P
2944    After a partial match or a failed match, the last encountered name in the
2945    entire match process is returned. For example:
2946    .sp
2947        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2948      data> XP
2949      No match, mark = B
2950    .sp
2951    Note that in this unanchored example the mark is retained from the match
2952    attempt that started at the letter "X" in the subject. Subsequent match
2953    attempts starting at "P" and then with an empty string do not get as far as the
2954    (*MARK) item, but nevertheless do not reset it.
2955    .P
2956    If you are interested in (*MARK) values after failed matches, you should
2957    probably set the PCRE_NO_START_OPTIMIZE option
2958    .\" HTML <a href="#nooptimize">
2959    .\" </a>
2960    (see above)
2961    .\"
2962    to ensure that the match is always attempted.
2963    .
2964    .
2965  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2966  .rs  .rs
2967  .sp  .sp
2968  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2969  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2970  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2971    the verb. However, when one of these verbs appears inside an atomic group or an
2972    assertion that is true, its effect is confined to that group, because once the
2973    group has been matched, there is never any backtracking into it. In this
2974    situation, backtracking can "jump back" to the left of the entire atomic group
2975    or assertion. (Remember also, as stated above, that this localization also
2976    applies in subroutine calls.)
2977    .P
2978    These verbs differ in exactly what kind of failure occurs when backtracking
2979    reaches them. The behaviour described below is what happens when the verb is
2980    not in a subroutine or an assertion. Subsequent sections cover these special
2981    cases.
2982  .sp  .sp
2983    (*COMMIT)    (*COMMIT)
2984  .sp  .sp
2985  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2986  does not match. Even if the pattern is unanchored, no further attempts to find  outright if there is a later matching failure that causes backtracking to reach
2987  a match by advancing the start point take place. Once (*COMMIT) has been  it. Even if the pattern is unanchored, no further attempts to find a match by
2988  passed, \fBpcre_exec()\fP is committed to finding a match at the current  advancing the starting point take place. If (*COMMIT) is the only backtracking
2989  starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2990    committed to finding a match at the current starting point, or not at all. For
2991    example:
2992  .sp  .sp
2993    a+(*COMMIT)b    a+(*COMMIT)b
2994  .sp  .sp
2995  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2996  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2997  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2998    (*PRUNE)  match failure.
2999  .sp  .P
3000  This verb causes the match to fail at the current position if the rest of the  If there is more than one backtracking verb in a pattern, a different one that
3001  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3002  advance to the next starting character then happens. Backtracking can occur as  match does not always guarantee that a match must be at this starting point.
3003  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .P
3004  if there is no match to the right, backtracking cannot cross (*PRUNE).  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
3005  In simple cases, the use of (*PRUNE) is just an alternative to an atomic  unless PCRE's start-of-match optimizations are turned off, as shown in this
3006  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot  \fBpcretest\fP example:
3007  be expressed in any other way.  .sp
3008        re> /(*COMMIT)abc/
3009      data> xyzabc
3010       0: abc
3011      xyzabc\eY
3012      No match
3013    .sp
3014    PCRE knows that any match must start with "a", so the optimization skips along
3015    the subject to "a" before running the first match attempt, which succeeds. When
3016    the optimization is disabled by the \eY escape in the second subject, the match
3017    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
3018    starting points.
3019    .sp
3020      (*PRUNE) or (*PRUNE:NAME)
3021    .sp
3022    This verb causes the match to fail at the current starting position in the
3023    subject if there is a later matching failure that causes backtracking to reach
3024    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3025    starting character then happens. Backtracking can occur as usual to the left of
3026    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3027    if there is no match to the right, backtracking cannot cross (*PRUNE). In
3028    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3029    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3030    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3031    as (*COMMIT).
3032    .P
3033    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3034    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3035    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3036  .sp  .sp
3037    (*SKIP)    (*SKIP)
3038  .sp  .sp
3039  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
3040  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
3041  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3042  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
3043    successful match. Consider:
3044  .sp  .sp
3045    a+(*SKIP)b    a+(*SKIP)b
3046  .sp  .sp
3047  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
3048  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
3049  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
3050  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
3051  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
3052  instead of skipping on to "c".  instead of skipping on to "c".
3053  .sp  .sp
3054    (*THEN)    (*SKIP:NAME)
3055  .sp  .sp
3056  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. When it is
3057  not match. That is, it cancels pending backtracking, but only within the  triggered, the previous path through the pattern is searched for the most
3058  current alternation. Its name comes from the observation that it can be used  recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3059  for a pattern-based if-then-else block:  is to the subject position that corresponds to that (*MARK) instead of to where
3060    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3061    (*SKIP) is ignored.
3062    .P
3063    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3064    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3065    .sp
3066      (*THEN) or (*THEN:NAME)
3067    .sp
3068    This verb causes a skip to the next innermost alternative when backtracking
3069    reaches it. That is, it cancels any further backtracking within the current
3070    alternative. Its name comes from the observation that it can be used for a
3071    pattern-based if-then-else block:
3072  .sp  .sp
3073    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3074  .sp  .sp
3075  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
3076  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
3077  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. If that
3078  is used outside of any alternation, it acts exactly like (*PRUNE).  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3079    more alternatives, so there is a backtrack to whatever came before the entire
3080    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3081    .P
3082    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3083    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3084    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3085    .P
3086    A subpattern that does not contain a | character is just a part of the
3087    enclosing alternative; it is not a nested alternation with only one
3088    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3089    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3090    pattern fragments that do not contain any | characters at this level:
3091    .sp
3092      A (B(*THEN)C) | D
3093    .sp
3094    If A and B are matched, but there is a failure in C, matching does not
3095    backtrack into A; instead it moves to the next alternative, that is, D.
3096    However, if the subpattern containing (*THEN) is given an alternative, it
3097    behaves differently:
3098    .sp
3099      A (B(*THEN)C | (*FAIL)) | D
3100    .sp
3101    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3102    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3103    because there are no more alternatives to try. In this case, matching does now
3104    backtrack into A.
3105    .P
3106    Note that a conditional subpattern is not considered as having two
3107    alternatives, because only one is ever used. In other words, the | character in
3108    a conditional subpattern has a different meaning. Ignoring white space,
3109    consider:
3110    .sp
3111      ^.*? (?(?=a) a | b(*THEN)c )
3112    .sp
3113    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3114    it initially matches zero characters. The condition (?=a) then fails, the
3115    character "b" is matched, but "c" is not. At this point, matching does not
3116    backtrack to .*? as might perhaps be expected from the presence of the |
3117    character. The conditional subpattern is part of the single alternative that
3118    comprises the whole pattern, and so the match fails. (If there was a backtrack
3119    into .*?, allowing it to match "b", the match would succeed.)
3120    .P
3121    The verbs just described provide four different "strengths" of control when
3122    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3123    next alternative. (*PRUNE) comes next, failing the match at the current
3124    starting position, but allowing an advance to the next character (for an
3125    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3126    than one character. (*COMMIT) is the strongest, causing the entire match to
3127    fail.
3128    .
3129    .
3130    .SS "More than one backtracking verb"
3131    .rs
3132    .sp
3133    If more than one backtracking verb is present in a pattern, the one that is
3134    backtracked onto first acts. For example, consider this pattern, where A, B,
3135    etc. are complex pattern fragments:
3136    .sp
3137      (A(*COMMIT)B(*THEN)C|ABD)
3138    .sp
3139    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3140    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3141    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3142    not always the same as Perl's. It means that if two or more backtracking verbs
3143    appear in succession, all the the last of them has no effect. Consider this
3144    example:
3145    .sp
3146      ...(*COMMIT)(*PRUNE)...
3147    .sp
3148    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3149    it to be triggered, and its action is taken. There can never be a backtrack
3150    onto (*COMMIT).
3151    .
3152    .
3153    .\" HTML <a name="btrepeat"></a>
3154    .SS "Backtracking verbs in repeated groups"
3155    .rs
3156    .sp
3157    PCRE differs from Perl in its handling of backtracking verbs in repeated
3158    groups. For example, consider:
3159    .sp
3160      /(a(*COMMIT)b)+ac/
3161    .sp
3162    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3163    the second repeat of the group acts.
3164    .
3165    .
3166    .\" HTML <a name="btassert"></a>
3167    .SS "Backtracking verbs in assertions"
3168    .rs
3169    .sp
3170    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3171    .P
3172    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3173    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3174    fail without any further processing.
3175    .P
3176    The other backtracking verbs are not treated specially if they appear in a
3177    positive assertion. In particular, (*THEN) skips to the next alternative in the
3178    innermost enclosing group that has alternations, whether or not this is within
3179    the assertion.
3180    .P
3181    Negative assertions are, however, different, in order to ensure that changing a
3182    positive assertion into a negative assertion changes its result. Backtracking
3183    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3184    without considering any further alternative branches in the assertion.
3185    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3186    within the assertion (the normal behaviour), but if the assertion does not have
3187    such an alternative, (*THEN) behaves like (*PRUNE).
3188    .
3189    .
3190    .\" HTML <a name="btsub"></a>
3191    .SS "Backtracking verbs in subroutines"
3192    .rs
3193    .sp
3194    These behaviours occur whether or not the subpattern is called recursively.
3195    Perl's treatment of subroutines is different in some cases.
3196    .P
3197    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3198    an immediate backtrack.
3199    .P
3200    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3201    succeed without any further processing. Matching then continues after the
3202    subroutine call.
3203    .P
3204    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3205    the subroutine match to fail.
3206    .P
3207    (*THEN) skips to the next alternative in the innermost enclosing group within
3208    the subpattern that has alternatives. If there is no such group within the
3209    subpattern, (*THEN) causes the subroutine match to fail.
3210  .  .
3211  .  .
3212  .SH "SEE ALSO"  .SH "SEE ALSO"
3213  .rs  .rs
3214  .sp  .sp
3215  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3216    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3217  .  .
3218  .  .
3219  .SH AUTHOR  .SH AUTHOR
# Line 2177  Cambridge CB2 3QH, England. Line 3230  Cambridge CB2 3QH, England.
3230  .rs  .rs
3231  .sp  .sp
3232  .nf  .nf
3233  Last updated: 11 September 2007  Last updated: 12 November 2013
3234  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3235  .fi  .fi

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