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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "09 November 2013" "PCRE 8.34"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 20  have copious examples. Jeffrey Friedl's Line 20  have copious examples. Jeffrey Friedl's
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  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_POSSESSIFY 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    .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  .SH "NEWLINE CONVENTIONS"  .\" 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 76  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"
# Line 98  below. A change of \eR setting can be co Line 164  below. A change of \eR setting can be co
164  convention.  convention.
165  .  .
166  .  .
167    .SS "Setting match and recursion limits"
168    .rs
169    .sp
170    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
171    internal \fBmatch()\fP function is called and on the maximum depth of
172    recursive calls. These facilities are provided to catch runaway matches that
173    are provoked by patterns with huge matching trees (a typical example is a
174    pattern with nested unlimited repeats) and to avoid running out of system stack
175    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
176    gives an error return. The limits can also be set by items at the start of the
177    pattern of the form
178    .sp
179      (*LIMIT_MATCH=d)
180      (*LIMIT_RECURSION=d)
181    .sp
182    where d is any number of decimal digits. However, the value of the setting must
183    be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
184    for it to have any effect. In other words, the pattern writer can lower the
185    limits set by the programmer, but not raise them. If there is more than one
186    setting of one of these limits, the lower value is used.
187    .
188    .
189    .SH "EBCDIC CHARACTER CODES"
190    .rs
191    .sp
192    PCRE can be compiled to run in an environment that uses EBCDIC as its character
193    code rather than ASCII or Unicode (typically a mainframe system). In the
194    sections below, character code values are ASCII or Unicode; in an EBCDIC
195    environment these characters may have different code values, and there are no
196    code points greater than 255.
197    .
198    .
199  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
200  .rs  .rs
201  .sp  .sp
# Line 109  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 161  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 171  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 190  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 199  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 241  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 256  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 278  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 326  syntax for referencing a subpattern as a Line 490  syntax for referencing a subpattern as a
490  later.  later.
491  .\"  .\"
492  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
493  synonymous. The former is a back reference; the latter is a subroutine call.  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 \es
537  included in a Perl script, \es may match the VT character. In PCRE, it never  characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space (32).
538  does.  .P
539  .P  A "word" character is an underscore or any character that is a letter or digit.
540  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, the definition of letters and digits is controlled by PCRE's
541  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  low-valued character tables, and may vary if locale-specific matching is taking
542  character property support is available. These sequences retain their original  place (see
543  meanings from before UTF-8 support was available, mainly for efficiency  .\" HTML <a href="pcreapi.html#localesupport">
544  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" </a>
545  and \eW.  "Locale support"
546  .P  .\"
547  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  in the
548  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\" HREF
549  The horizontal space characters are:  \fBpcreapi\fP
550    .\"
551    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
552    or "french" in Windows, some character codes greater than 128 are used for
553    accented letters, and these are then matched by \ew. The use of locales with
554    Unicode is discouraged.
555    .P
556    By default, in a UTF mode, characters with values greater than 128 never match
557    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
558    their original meanings from before UTF support was available, mainly for
559    efficiency reasons. However, if PCRE is compiled with Unicode property support,
560    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
561    properties are used to determine character types, as follows:
562    .sp
563      \ed  any character that matches \ep{Nd} (decimal digit)
564      \es  any character that matches \ep{Z} or \eh or \ev
565      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
566    .sp
567    The upper case escapes match the inverse sets of characters. Note that \ed
568    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
569    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
570    \eB because they are defined in terms of \ew and \eW. Matching these sequences
571    is noticeably slower when PCRE_UCP is set.
572    .P
573    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
574    release 5.10. In contrast to the other sequences, which match only ASCII
575    characters by default, these always match certain high-valued codepoints,
576    whether or not PCRE_UCP is set. The horizontal space characters are:
577  .sp  .sp
578    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
579    U+0020     Space    U+0020     Space
580    U+00A0     Non-break space    U+00A0     Non-break space
581    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 393  The horizontal space characters are: Line 597  The horizontal space characters are:
597  .sp  .sp
598  The vertical space characters are:  The vertical space characters are:
599  .sp  .sp
600    U+000A     Linefeed    U+000A     Linefeed (LF)
601    U+000B     Vertical tab    U+000B     Vertical tab (VT)
602    U+000C     Formfeed    U+000C     Form feed (FF)
603    U+000D     Carriage return    U+000D     Carriage return (CR)
604    U+0085     Next line    U+0085     Next line (NEL)
605    U+2028     Line separator    U+2028     Line separator
606    U+2029     Paragraph separator    U+2029     Paragraph separator
607  .P  .sp
608  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
609  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.  
610  .  .
611  .  .
612  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 424  is discouraged. Line 614  is discouraged.
614  .rs  .rs
615  .sp  .sp
616  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
617  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
618  equivalent to the following:  following:
619  .sp  .sp
620    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
621  .sp  .sp
# Line 436  below. Line 626  below.
626  .\"  .\"
627  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
628  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,
629  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
630  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
631  cannot be split.  cannot be split.
632  .P  .P
633  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
634  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).
635  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
636  recognized.  recognized.
# Line 456  one of the following sequences: Line 646  one of the following sequences:
646    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
647    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
648  .sp  .sp
649  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
650  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
651  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
652  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
653  of them is present, the last one is used. They can be combined with a change of  than one of them is present, the last one is used. They can be combined with a
654  newline convention, for example, a pattern can start with:  change of newline convention; for example, a pattern can start with:
655  .sp  .sp
656    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
657  .sp  .sp
658  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
659    (*UCP) special sequences. Inside a character class, \eR is treated as an
660    unrecognized escape sequence, and so matches the letter "R" by default, but
661    causes an error if PCRE_EXTRA is set.
662  .  .
663  .  .
664  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 474  Inside a character class, \eR matches th Line 667  Inside a character class, \eR matches th
667  .sp  .sp
668  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
669  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
670  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
671  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.
672  The extra escape sequences are:  The extra escape sequences are:
673  .sp  .sp
674    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
675    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
676    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
677  .sp  .sp
678  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
679  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
680  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
681  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
682  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
683    .\" </a>
684    next section).
685    .\"
686    Other Perl properties such as "InMusicalSymbols" are not currently supported by
687    PCRE. Note that \eP{Any} does not match any characters, so always causes a
688    match failure.
689  .P  .P
690  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
691  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 500  Those that are not part of an identified Line 699  Those that are not part of an identified
699  .P  .P
700  Arabic,  Arabic,
701  Armenian,  Armenian,
702    Avestan,
703  Balinese,  Balinese,
704    Bamum,
705    Batak,
706  Bengali,  Bengali,
707  Bopomofo,  Bopomofo,
708    Brahmi,
709  Braille,  Braille,
710  Buginese,  Buginese,
711  Buhid,  Buhid,
712  Canadian_Aboriginal,  Canadian_Aboriginal,
713    Carian,
714    Chakma,
715    Cham,
716  Cherokee,  Cherokee,
717  Common,  Common,
718  Coptic,  Coptic,
# Line 515  Cypriot, Line 721  Cypriot,
721  Cyrillic,  Cyrillic,
722  Deseret,  Deseret,
723  Devanagari,  Devanagari,
724    Egyptian_Hieroglyphs,
725  Ethiopic,  Ethiopic,
726  Georgian,  Georgian,
727  Glagolitic,  Glagolitic,
# Line 527  Hangul, Line 734  Hangul,
734  Hanunoo,  Hanunoo,
735  Hebrew,  Hebrew,
736  Hiragana,  Hiragana,
737    Imperial_Aramaic,
738  Inherited,  Inherited,
739    Inscriptional_Pahlavi,
740    Inscriptional_Parthian,
741    Javanese,
742    Kaithi,
743  Kannada,  Kannada,
744  Katakana,  Katakana,
745    Kayah_Li,
746  Kharoshthi,  Kharoshthi,
747  Khmer,  Khmer,
748  Lao,  Lao,
749  Latin,  Latin,
750    Lepcha,
751  Limbu,  Limbu,
752  Linear_B,  Linear_B,
753    Lisu,
754    Lycian,
755    Lydian,
756  Malayalam,  Malayalam,
757    Mandaic,
758    Meetei_Mayek,
759    Meroitic_Cursive,
760    Meroitic_Hieroglyphs,
761    Miao,
762  Mongolian,  Mongolian,
763  Myanmar,  Myanmar,
764  New_Tai_Lue,  New_Tai_Lue,
# Line 544  Nko, Line 766  Nko,
766  Ogham,  Ogham,
767  Old_Italic,  Old_Italic,
768  Old_Persian,  Old_Persian,
769    Old_South_Arabian,
770    Old_Turkic,
771    Ol_Chiki,
772  Oriya,  Oriya,
773  Osmanya,  Osmanya,
774  Phags_Pa,  Phags_Pa,
775  Phoenician,  Phoenician,
776    Rejang,
777  Runic,  Runic,
778    Samaritan,
779    Saurashtra,
780    Sharada,
781  Shavian,  Shavian,
782  Sinhala,  Sinhala,
783    Sora_Sompeng,
784    Sundanese,
785  Syloti_Nagri,  Syloti_Nagri,
786  Syriac,  Syriac,
787  Tagalog,  Tagalog,
788  Tagbanwa,  Tagbanwa,
789  Tai_Le,  Tai_Le,
790    Tai_Tham,
791    Tai_Viet,
792    Takri,
793  Tamil,  Tamil,
794  Telugu,  Telugu,
795  Thaana,  Thaana,
# Line 563  Thai, Line 797  Thai,
797  Tibetan,  Tibetan,
798  Tifinagh,  Tifinagh,
799  Ugaritic,  Ugaritic,
800    Vai,
801  Yi.  Yi.
802  .P  .P
803  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
804  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
805  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
806  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
807  .P  .P
808  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
809  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 629  the Lu, Ll, or Lt property, in other wor Line 864  the Lu, Ll, or Lt property, in other wor
864  a modifier or "other".  a modifier or "other".
865  .P  .P
866  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
867  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
868  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
869  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
870    PCRE_NO_UTF32_CHECK in the
871  .\" HREF  .\" HREF
872  \fBpcreapi\fP  \fBpcreapi\fP
873  .\"  .\"
874  page).  page). Perl does not support the Cs property.
875  .P  .P
876  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})
877  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
878  properties with "Is".  properties with "Is".
879  .P  .P
# Line 646  Instead, this property is assumed for an Line 882  Instead, this property is assumed for an
882  Unicode table.  Unicode table.
883  .P  .P
884  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
885  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
886    the behaviour of current versions of Perl.
887  .P  .P
888  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
889  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
890  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
891    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
892    PCRE_UCP option or by starting the pattern with (*UCP).
893    .
894    .
895    .SS Extended grapheme clusters
896    .rs
897  .sp  .sp
898  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
899  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  
900  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
901  .\" </a>  .\" </a>
902  (see below).  (see below).
903  .\"  .\"
904  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
905  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
906  non-UTF-8 mode \eX matches any one character.  .sp
907  .P    (?>\ePM\epM*)
908  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
909  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
910  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"
911  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
912    .P
913    This simple definition was extended in Unicode to include more complicated
914    kinds of composite character by giving each character a grapheme breaking
915    property, and creating rules that use these properties to define the boundaries
916    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
917    one of these clusters.
918    .P
919    \eX always matches at least one character. Then it decides whether to add
920    additional characters according to the following rules for ending a cluster:
921    .P
922    1. End at the end of the subject string.
923    .P
924    2. Do not end between CR and LF; otherwise end after any control character.
925    .P
926    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
927    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
928    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
929    character; an LVT or T character may be follwed only by a T character.
930    .P
931    4. Do not end before extending characters or spacing marks. Characters with
932    the "mark" property always have the "extend" grapheme breaking property.
933    .P
934    5. Do not end after prepend characters.
935    .P
936    6. Otherwise, end the cluster.
937    .
938    .
939    .\" HTML <a name="extraprops"></a>
940    .SS PCRE's additional properties
941    .rs
942    .sp
943    As well as the standard Unicode properties described above, PCRE supports four
944    more that make it possible to convert traditional escape sequences such as \ew
945    and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
946    properties internally when PCRE_UCP is set. However, they may also be used
947    explicitly. These properties are:
948    .sp
949      Xan   Any alphanumeric character
950      Xps   Any POSIX space character
951      Xsp   Any Perl space character
952      Xwd   Any Perl "word" character
953    .sp
954    Xan matches characters that have either the L (letter) or the N (number)
955    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
956    carriage return, and any other character that has the Z (separator) property.
957    Xsp is the same as Xps; it used to exclude vertical tab, for Perl
958    compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
959    matches the same characters as Xan, plus underscore.
960    .P
961    There is another non-standard property, Xuc, which matches any character that
962    can be represented by a Universal Character Name in C++ and other programming
963    languages. These are the characters $, @, ` (grave accent), and all characters
964    with Unicode code points greater than or equal to U+00A0, except for the
965    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
966    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
967    where H is a hexadecimal digit. Note that the Xuc property does not match these
968    sequences but the characters that they represent.)
969  .  .
970  .  .
971  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
972  .SS "Resetting the match start"  .SS "Resetting the match start"
973  .rs  .rs
974  .sp  .sp
975  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
976  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:  
977  .sp  .sp
978    foo\eKbar    foo\eKbar
979  .sp  .sp
# Line 698  For example, when the pattern Line 995  For example, when the pattern
995    (foo)\eKbar    (foo)\eKbar
996  .sp  .sp
997  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
998    .P
999    Perl documents that the use of \eK within assertions is "not well defined". In
1000    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
1001    ignored in negative assertions.
1002  .  .
1003  .  .
1004  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 722  The backslashed assertions are: Line 1023  The backslashed assertions are:
1023    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
1024    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
1025  .sp  .sp
1026  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
1027  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
1028    default it matches the corresponding literal character (for example, \eB
1029    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1030    escape sequence" error is generated instead.
1031  .P  .P
1032  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
1033  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
1034  \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
1035  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
1036    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1037    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1038    of word" or "end of word" metasequence. However, whatever follows \eb normally
1039    determines which it is. For example, the fragment \eba matches "a" at the start
1040    of a word.
1041  .P  .P
1042  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1043  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 762  regular expression. Line 1071  regular expression.
1071  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1072  .rs  .rs
1073  .sp  .sp
1074    The circumflex and dollar metacharacters are zero-width assertions. That is,
1075    they test for a particular condition being true without consuming any
1076    characters from the subject string.
1077    .P
1078  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1079  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
1080  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
1081  \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
1082  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1083  meaning  meaning
# Line 781  constrained to match only at the start o Line 1094  constrained to match only at the start o
1094  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1095  to be anchored.)  to be anchored.)
1096  .P  .P
1097  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
1098  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
1099  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
1100  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
1101  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
1102  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1103  .P  .P
1104  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
1105  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 812  end of the subject in both modes, and if Line 1125  end of the subject in both modes, and if
1125  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1126  .  .
1127  .  .
1128  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1129    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1130  .rs  .rs
1131  .sp  .sp
1132  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
1133  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
1134  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1135  .P  .P
1136  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
1137  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 834  to match it. Line 1148  to match it.
1148  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
1149  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
1150  special meaning in a character class.  special meaning in a character class.
1151  .  .P
1152  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1153  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1154  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1155  .sp  name; PCRE does not support this.
1156  Outside a character class, the escape sequence \eC matches any one byte, both  .
1157  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1158  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1159  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1160  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1161  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1162    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1163    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1164    a 32-bit unit. Unlike a dot, \eC always
1165    matches line-ending characters. The feature is provided in Perl in order to
1166    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1167    used. Because \eC breaks up characters into individual data units, matching one
1168    unit with \eC in a UTF mode means that the rest of the string may start with a
1169    malformed UTF character. This has undefined results, because PCRE assumes that
1170    it is dealing with valid UTF strings (and by default it checks this at the
1171    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1172    PCRE_NO_UTF32_CHECK option is used).
1173  .P  .P
1174  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1175  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1176  .\" </a>  .\" </a>
1177  (described below),  (described below)
1178  .\"  .\"
1179  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
1180  the lookbehind.  the lookbehind.
1181    .P
1182    In general, the \eC escape sequence is best avoided. However, one
1183    way of using it that avoids the problem of malformed UTF characters is to use a
1184    lookahead to check the length of the next character, as in this pattern, which
1185    could be used with a UTF-8 string (ignore white space and line breaks):
1186    .sp
1187      (?| (?=[\ex00-\ex7f])(\eC) |
1188          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1189          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1190          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1191    .sp
1192    A group that starts with (?| resets the capturing parentheses numbers in each
1193    alternative (see
1194    .\" HTML <a href="#dupsubpatternnumber">
1195    .\" </a>
1196    "Duplicate Subpattern Numbers"
1197    .\"
1198    below). The assertions at the start of each branch check the next UTF-8
1199    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1200    character's individual bytes are then captured by the appropriate number of
1201    groups.
1202  .  .
1203  .  .
1204  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 860  the lookbehind. Line 1206  the lookbehind.
1206  .rs  .rs
1207  .sp  .sp
1208  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1209  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.
1210  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
1211  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
1212  escaped with a backslash.  a member of the class, it should be the first data character in the class
1213  .P  (after an initial circumflex, if present) or escaped with a backslash.
1214  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1215  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
1216  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
1217  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
1218  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
1219  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
1220    member of the class, ensure it is not the first character, or escape it with a
1221  backslash.  backslash.
1222  .P  .P
1223  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1224  [^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
1225  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1226  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
1227  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
1228  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
1229  string.  string.
1230  .P  .P
1231  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)
1232  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
1233    \ex{ escaping mechanism.
1234  .P  .P
1235  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
1236  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
1237  "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
1238  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
1239  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
1240  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1241  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1242  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
1243  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
1244  UTF-8 support.  well as with UTF support.
1245  .P  .P
1246  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
1247  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 904  The minus (hyphen) character can be used Line 1252  The minus (hyphen) character can be used
1252  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,
1253  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
1254  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
1255  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
1256    immediately after a range. For example, [b-d-z] matches letters in the range b
1257    to d, a hyphen character, or z.
1258  .P  .P
1259  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
1260  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 914  the end of range, so [W-\e]46] is interp Line 1264  the end of range, so [W-\e]46] is interp
1264  followed by two other characters. The octal or hexadecimal representation of  followed by two other characters. The octal or hexadecimal representation of
1265  "]" can also be used to end a range.  "]" can also be used to end a range.
1266  .P  .P
1267    An error is generated if a POSIX character class (see below) or an escape
1268    sequence other than one that defines a single character appears at a point
1269    where a range ending character is expected. For example, [z-\exff] is valid,
1270    but [A-\ed] and [A-[:digit:]] are not.
1271    .P
1272  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
1273  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1274  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}].  
1275  .P  .P
1276  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
1277  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
1278  [][\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
1279  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
1280  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
1281  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
1282  property support.  property support.
1283  .P  .P
1284  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,
1285  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
1286  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1287  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
1288  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
1289  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1290    .\" HTML <a href="#genericchartypes">
1291    .\" </a>
1292    "Generic character types"
1293    .\"
1294    above. The escape sequence \eb has a different meaning inside a character
1295    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1296    are not special inside a character class. Like any other unrecognized escape
1297    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1298    default, but cause an error if the PCRE_EXTRA option is set.
1299    .P
1300    A circumflex can conveniently be used with the upper case character types to
1301    specify a more restricted set of characters than the matching lower case type.
1302    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1303    whereas [\ew] includes underscore. A positive character class should be read as
1304    "something OR something OR ..." and a negative class as "NOT something AND NOT
1305    something AND NOT ...".
1306  .P  .P
1307  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1308  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 952  this notation. For example, Line 1322  this notation. For example,
1322    [01[:alpha:]%]    [01[:alpha:]%]
1323  .sp  .sp
1324  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1325  are  are:
1326  .sp  .sp
1327    alnum    letters and digits    alnum    letters and digits
1328    alpha    letters    alpha    letters
# Line 963  are Line 1333  are
1333    graph    printing characters, excluding space    graph    printing characters, excluding space
1334    lower    lower case letters    lower    lower case letters
1335    print    printing characters, including space    print    printing characters, including space
1336    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1337    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1338    upper    upper case letters    upper    upper case letters
1339    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1340    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1341  .sp  .sp
1342  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1343  space (32). Notice that this list includes the VT character (code 11). This  space (32). "Space" used to be different to \es, which did not include VT, for
1344  makes "space" different to \es, which does not include VT (for Perl  Perl compatibility. However, Perl changed at release 5.18, and PCRE followed at
1345  compatibility).  release 8.34. "Space" and \es now match the same set of characters.
1346  .P  .P
1347  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
1348  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 984  matches "1", "2", or any non-digit. PCRE Line 1354  matches "1", "2", or any non-digit. PCRE
1354  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
1355  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1356  .P  .P
1357  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1358  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1359    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1360    character properties are used. This is achieved by replacing certain POSIX
1361    classes by other sequences, as follows:
1362    .sp
1363      [:alnum:]  becomes  \ep{Xan}
1364      [:alpha:]  becomes  \ep{L}
1365      [:blank:]  becomes  \eh
1366      [:digit:]  becomes  \ep{Nd}
1367      [:lower:]  becomes  \ep{Ll}
1368      [:space:]  becomes  \ep{Xps}
1369      [:upper:]  becomes  \ep{Lu}
1370      [:word:]   becomes  \ep{Xwd}
1371    .sp
1372    Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1373    classes are handled specially in UCP mode:
1374    .TP 10
1375    [:graph:]
1376    This matches characters that have glyphs that mark the page when printed. In
1377    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1378    properties, except for:
1379    .sp
1380      U+061C           Arabic Letter Mark
1381      U+180E           Mongolian Vowel Separator
1382      U+2066 - U+2069  Various "isolate"s
1383    .sp
1384    .TP 10
1385    [:print:]
1386    This matches the same characters as [:graph:] plus space characters that are
1387    not controls, that is, characters with the Zs property.
1388    .TP 10
1389    [:punct:]
1390    This matches all characters that have the Unicode P (punctuation) property,
1391    plus those characters whose code points are less than 128 that have the S
1392    (Symbol) property.
1393    .P
1394    The other POSIX classes are unchanged, and match only characters with code
1395    points less than 128.
1396  .  .
1397  .  .
1398  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1032  The PCRE-specific options PCRE_DUPNAMES, Line 1439  The PCRE-specific options PCRE_DUPNAMES,
1439  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
1440  J, U and X respectively.  J, U and X respectively.
1441  .P  .P
1442  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
1443  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1444  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
1445  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
1446  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1447  .P  .P
1448  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1449  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
1450  .sp  .sp
1451    (a(?i)b)c    (a(?i)b)c
1452  .sp  .sp
# Line 1056  option settings happen at compile time. Line 1463  option settings happen at compile time.
1463  behaviour otherwise.  behaviour otherwise.
1464  .P  .P
1465  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1466  application when the compile or match functions are called. In some cases the  application when the compiling or matching functions are called. In some cases
1467  pattern can contain special leading sequences to override what the application  the pattern can contain special leading sequences such as (*CRLF) to override
1468  has set or what has been defaulted. Details are given in the section entitled  what the application has set or what has been defaulted. Details are given in
1469    the section entitled
1470  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1471  .\" </a>  .\" </a>
1472  "Newline sequences"  "Newline sequences"
1473  .\"  .\"
1474  above.  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1475    sequences that can be used to set UTF and Unicode property modes; they are
1476    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1477    options, respectively. The (*UTF) sequence is a generic version that can be
1478    used with any of the libraries. However, the application can set the
1479    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1480  .  .
1481  .  .
1482  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1077  Turning part of a pattern into a subpatt Line 1490  Turning part of a pattern into a subpatt
1490  .sp  .sp
1491    cat(aract|erpillar|)    cat(aract|erpillar|)
1492  .sp  .sp
1493  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1494  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1495  .sp  .sp
1496  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
1497  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
1498  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
1499  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1500  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1501  .P  .P
1502  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
1503    numbers for the capturing subpatterns. For example, if the string "the red
1504    king" is matched against the pattern
1505  .sp  .sp
1506    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1507  .sp  .sp
# Line 1118  is reached, an option setting in one bra Line 1533  is reached, an option setting in one bra
1533  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1534  .  .
1535  .  .
1536    .\" HTML <a name="dupsubpatternnumber"></a>
1537  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1538  .rs  .rs
1539  .sp  .sp
# Line 1134  at captured substring number one, whiche Line 1550  at captured substring number one, whiche
1550  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
1551  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1552  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
1553  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
1554  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1555  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.  
1556  .sp  .sp
1557    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1558    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1559    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1560  .sp  .sp
1561  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
1562  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1563    or "defdef":
1564    .sp
1565      /(?|(abc)|(def))\e1/
1566    .sp
1567    In contrast, a subroutine call to a numbered subpattern always refers to the
1568    first one in the pattern with the given number. The following pattern matches
1569    "abcabc" or "defabc":
1570    .sp
1571      /(?|(abc)|(def))(?1)/
1572    .sp
1573    If a
1574    .\" HTML <a href="#conditions">
1575    .\" </a>
1576    condition test
1577    .\"
1578    for a subpattern's having matched refers to a non-unique number, the test is
1579    true if any of the subpatterns of that number have matched.
1580  .P  .P
1581  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1582  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1159  if an expression is modified, the number Line 1591  if an expression is modified, the number
1591  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1592  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
1593  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
1594  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1595    have different names, but PCRE does not.
1596  .P  .P
1597  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
1598  (?'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
1599  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1600  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1601  .\" </a>  .\" </a>
1602  backreferences,  back references,
1603  .\"  .\"
1604  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1605  .\" </a>  .\" </a>
# Line 1179  conditions, Line 1612  conditions,
1612  .\"  .\"
1613  can be made by name as well as by number.  can be made by name as well as by number.
1614  .P  .P
1615  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores, but must
1616  capturing parentheses are still allocated numbers as well as names, exactly as  start with a non-digit. Named capturing parentheses are still allocated numbers
1617  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
1618  extracting the name-to-number translation table from a compiled pattern. There  provides function calls for extracting the name-to-number translation table
1619  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
1620    captured substring by name.
1621  .P  .P
1622  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
1623  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
1624  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
1625  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
1626  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
1627  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
1628    name, and in both cases you want to extract the abbreviation. This pattern
1629    (ignoring the line breaks) does the job:
1630  .sp  .sp
1631    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1632    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1204  subpattern, as described in the previous Line 1640  subpattern, as described in the previous
1640  .P  .P
1641  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1642  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
1643  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1644  make a reference to a non-unique named subpattern from elsewhere in the  .P
1645  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
1646  details of the interfaces for handling named subpatterns, see the  the pattern, the subpatterns to which the name refers are checked in the order
1647    in which they appear in the overall pattern. The first one that is set is used
1648    for the reference. For example, this pattern matches both "foofoo" and
1649    "barbar" but not "foobar" or "barfoo":
1650    .sp
1651      (?:(?<n>foo)|(?<n>bar))\k<n>
1652    .sp
1653    .P
1654    If you make a subroutine call to a non-unique named subpattern, the one that
1655    corresponds to the first occurrence of the name is used. In the absence of
1656    duplicate numbers (see the previous section) this is the one with the lowest
1657    number.
1658    .P
1659    If you use a named reference in a condition
1660    test (see the
1661    .\"
1662    .\" HTML <a href="#conditions">
1663    .\" </a>
1664    section about conditions
1665    .\"
1666    below), either to check whether a subpattern has matched, or to check for
1667    recursion, all subpatterns with the same name are tested. If the condition is
1668    true for any one of them, the overall condition is true. This is the same
1669    behaviour as testing by number. For further details of the interfaces for
1670    handling named subpatterns, see the
1671  .\" HREF  .\" HREF
1672  \fBpcreapi\fP  \fBpcreapi\fP
1673  .\"  .\"
1674  documentation.  documentation.
1675  .P  .P
1676  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1677  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1678  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1679    are given to subpatterns with the same number. However, you can always give the
1680    same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1681    set.
1682  .  .
1683  .  .
1684  .SH REPETITION  .SH REPETITION
# Line 1227  items: Line 1690  items:
1690    a literal data character    a literal data character
1691    the dot metacharacter    the dot metacharacter
1692    the \eC escape sequence    the \eC escape sequence
1693    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1694    the \eR escape sequence    the \eR escape sequence
1695    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1696    a character class    a character class
1697    a back reference (see next section)    a back reference (see next section)
1698    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1699      a subroutine call to a subpattern (recursive or otherwise)
1700  .sp  .sp
1701  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1702  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1257  where a quantifier is not allowed, or on Line 1721  where a quantifier is not allowed, or on
1721  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
1722  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1723  .P  .P
1724  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
1725  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
1726  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,
1727  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
1728  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1729  .P  .P
1730  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
1731  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
# Line 1270  subpatterns that are referenced as Line 1734  subpatterns that are referenced as
1734  .\" </a>  .\" </a>
1735  subroutines  subroutines
1736  .\"  .\"
1737  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1738  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1739    .\" </a>
1740    "Defining subpatterns for use by reference only"
1741    .\"
1742    below). Items other than subpatterns that have a {0} quantifier are omitted
1743    from the compiled pattern.
1744  .P  .P
1745  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1746  abbreviations:  abbreviations:
# Line 1342  In cases where it is known that the subj Line 1811  In cases where it is known that the subj
1811  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1812  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1813  .P  .P
1814  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1815  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1816  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
1817  succeeds. Consider, for example:  succeeds. Consider, for example:
1818  .sp  .sp
# Line 1352  succeeds. Consider, for example: Line 1821  succeeds. Consider, for example:
1821  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
1822  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1823  .P  .P
1824    Another case where implicit anchoring is not applied is when the leading .* is
1825    inside an atomic group. Once again, a match at the start may fail where a later
1826    one succeeds. Consider this pattern:
1827    .sp
1828      (?>.*?a)b
1829    .sp
1830    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1831    (*PRUNE) and (*SKIP) also disable this optimization.
1832    .P
1833  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1834  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1835  .sp  .sp
# Line 1496  no such problem when named parentheses a Line 1974  no such problem when named parentheses a
1974  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1975  .P  .P
1976  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
1977  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
1978  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
1979  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1980  .sp  .sp
1981    (ring), \e1    (ring), \e1
1982    (ring), \eg1    (ring), \eg1
# Line 1512  example: Line 1990  example:
1990    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1991  .sp  .sp
1992  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
1993  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.
1994  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
1995  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
1996  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1997  .P  .P
1998  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1999  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1554  after the reference. Line 2032  after the reference.
2032  .P  .P
2033  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
2034  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
2035  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
2036  .sp  .sp
2037    (a|(bc))\e2    (a|(bc))\e2
2038  .sp  .sp
2039  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
2040  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
2041  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
2042  with a digit character, some delimiter must be used to terminate the back  .P
2043  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
2044  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
2045    If the pattern continues with a digit character, some delimiter must be used to
2046    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2047    white space. Otherwise, the \eg{ syntax or an empty comment (see
2048  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
2049  .\" </a>  .\" </a>
2050  "Comments"  "Comments"
2051  .\"  .\"
2052  below) can be used.  below) can be used.
2053  .P  .
2054    .SS "Recursive back references"
2055    .rs
2056    .sp
2057  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
2058  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.
2059  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1583  to the previous iteration. In order for Line 2067  to the previous iteration. In order for
2067  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
2068  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
2069  minimum of zero.  minimum of zero.
2070    .P
2071    Back references of this type cause the group that they reference to be treated
2072    as an
2073    .\" HTML <a href="#atomicgroup">
2074    .\" </a>
2075    atomic group.
2076    .\"
2077    Once the whole group has been matched, a subsequent matching failure cannot
2078    cause backtracking into the middle of the group.
2079  .  .
2080  .  .
2081  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1602  those that look ahead of the current pos Line 2095  those that look ahead of the current pos
2095  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,
2096  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.
2097  .P  .P
2098  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2099  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
2100  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2101  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2102  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2103  because it does not make sense for negative assertions.  .P
2104    For compatibility with Perl, assertion subpatterns may be repeated; though
2105    it makes no sense to assert the same thing several times, the side effect of
2106    capturing parentheses may occasionally be useful. In practice, there only three
2107    cases:
2108    .sp
2109    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2110    However, it may contain internal capturing parenthesized groups that are called
2111    from elsewhere via the
2112    .\" HTML <a href="#subpatternsassubroutines">
2113    .\" </a>
2114    subroutine mechanism.
2115    .\"
2116    .sp
2117    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2118    were {0,1}. At run time, the rest of the pattern match is tried with and
2119    without the assertion, the order depending on the greediness of the quantifier.
2120    .sp
2121    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2122    The assertion is obeyed just once when encountered during matching.
2123  .  .
2124  .  .
2125  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1636  lookbehind assertion is needed to achiev Line 2148  lookbehind assertion is needed to achiev
2148  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
2149  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
2150  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.
2151    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2152  .  .
2153  .  .
2154  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1660  is permitted, but Line 2173  is permitted, but
2173  .sp  .sp
2174  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2175  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
2176  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
2177  match the same length of string. An assertion such as  length of string. An assertion such as
2178  .sp  .sp
2179    (?<=ab(c|de))    (?<=ab(c|de))
2180  .sp  .sp
2181  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
2182  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
2183    branches:
2184  .sp  .sp
2185    (?<=abc|abde)    (?<=abc|abde)
2186  .sp  .sp
2187  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2188  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2189  .\" </a>  .\" </a>
2190  (see above)  (see above)
2191  .\"  .\"
2192  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
2193  fixed-length.  restriction.
2194  .P  .P
2195  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2196  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
2197  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2198  assertion fails.  assertion fails.
2199  .P  .P
2200  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
2201  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
2202  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
2203  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2204    permitted.
2205    .P
2206    .\" HTML <a href="#subpatternsassubroutines">
2207    .\" </a>
2208    "Subroutine"
2209    .\"
2210    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2211    as the subpattern matches a fixed-length string.
2212    .\" HTML <a href="#recursion">
2213    .\" </a>
2214    Recursion,
2215    .\"
2216    however, is not supported.
2217  .P  .P
2218  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2219  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
2220  pattern such as  strings. Consider a simple pattern such as
2221  .sp  .sp
2222    abcd$    abcd$
2223  .sp  .sp
# Line 1754  characters that are not "999". Line 2281  characters that are not "999".
2281  .sp  .sp
2282  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2283  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2284  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2285  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2286  .sp  .sp
2287    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2288    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2289  .sp  .sp
2290  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2291  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
2292  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2293    itself contain nested subpatterns of any form, including conditional
2294    subpatterns; the restriction to two alternatives applies only at the level of
2295    the condition. This pattern fragment is an example where the alternatives are
2296    complex:
2297    .sp
2298      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2299    .sp
2300  .P  .P
2301  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2302  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1771  recursion, a pseudo-condition called DEF Line 2305  recursion, a pseudo-condition called DEF
2305  .rs  .rs
2306  .sp  .sp
2307  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
2308  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2309  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
2310  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2311  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2312  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2313  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2314    section about duplicate subpattern numbers),
2315    .\"
2316    the condition is true if any of them have matched. An alternative notation is
2317    to precede the digits with a plus or minus sign. In this case, the subpattern
2318    number is relative rather than absolute. The most recently opened parentheses
2319    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2320    loops it can also make sense to refer to subsequent groups. The next
2321    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2322    zero in any of these forms is not used; it provokes a compile-time error.)
2323  .P  .P
2324  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2325  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 1787  three parts for ease of discussion: Line 2330  three parts for ease of discussion:
2330  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2331  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
2332  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
2333  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2334  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,
2335  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
2336  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2337  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 1806  This makes the fragment independent of t Line 2349  This makes the fragment independent of t
2349  .sp  .sp
2350  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2351  subpattern by name. For compatibility with earlier versions of PCRE, which had  subpattern by name. For compatibility with earlier versions of PCRE, which had
2352  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.  
2353  .P  .P
2354  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2355  .sp  .sp
2356    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2357  .sp  .sp
2358    If the name used in a condition of this kind is a duplicate, the test is
2359    applied to all subpatterns of the same name, and is true if any one of them has
2360    matched.
2361  .  .
2362  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2363  .rs  .rs
# Line 1828  letter R, for example: Line 2369  letter R, for example:
2369  .sp  .sp
2370    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2371  .sp  .sp
2372  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
2373  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
2374  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2375    applied to all subpatterns of the same name, and is true if any one of them is
2376    the most recent recursion.
2377  .P  .P
2378  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2379  patterns are described below.  .\" HTML <a href="#recursion">
2380    .\" </a>
2381    The syntax for recursive patterns
2382    .\"
2383    is described below.
2384  .  .
2385    .\" HTML <a name="subdefine"></a>
2386  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2387  .rs  .rs
2388  .sp  .sp
# Line 1842  If the condition is the string (DEFINE), Line 2390  If the condition is the string (DEFINE),
2390  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
2391  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2392  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
2393  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2394  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2395  written like this (ignore whitespace and line breaks):  .\" </a>
2396    subroutines
2397    .\"
2398    is described below.) For example, a pattern to match an IPv4 address such as
2399    "192.168.23.245" could be written like this (ignore white space and line
2400    breaks):
2401  .sp  .sp
2402    (?(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) )
2403    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1852  written like this (ignore whitespace and Line 2405  written like this (ignore whitespace and
2405  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
2406  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2407  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
2408  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2409  .P  pattern uses references to the named group to match the four dot-separated
2410  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.  
2411  .  .
2412  .SS "Assertion conditions"  .SS "Assertion conditions"
2413  .rs  .rs
# Line 1881  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2432  dd-aaa-dd or dd-dd-dd, where aaa are let
2432  .SH COMMENTS  .SH COMMENTS
2433  .rs  .rs
2434  .sp  .sp
2435  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
2436  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,
2437  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
2438    subpattern name or number. The characters that make up a comment play no part
2439    in the pattern matching.
2440  .P  .P
2441  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
2442  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2443  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2444    this case continues to immediately after the next newline character or
2445    character sequence in the pattern. Which characters are interpreted as newlines
2446    is controlled by the options passed to a compiling function or by a special
2447    sequence at the start of the pattern, as described in the section entitled
2448    .\" HTML <a href="#newlines">
2449    .\" </a>
2450    "Newline conventions"
2451    .\"
2452    above. Note that the end of this type of comment is a literal newline sequence
2453    in the pattern; escape sequences that happen to represent a newline do not
2454    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2455    default newline convention is in force:
2456    .sp
2457      abc #comment \en still comment
2458    .sp
2459    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2460    a newline in the pattern. The sequence \en is still literal at this stage, so
2461    it does not terminate the comment. Only an actual character with the code value
2462    0x0a (the default newline) does so.
2463  .  .
2464  .  .
2465  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1913  recursively to the pattern in which it a Line 2485  recursively to the pattern in which it a
2485  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2486  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2487  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2488  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.
2489  .P  .P
2490  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
2491  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
2492  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
2493    .\" HTML <a href="#subpatternsassubroutines">
2494    .\" </a>
2495    non-recursive subroutine
2496    .\"
2497  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
2498  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2499  .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  
2500  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2501  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2502  .sp  .sp
2503    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2504  .sp  .sp
2505  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2506  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
2507  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2508  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2509    to avoid backtracking into sequences of non-parentheses.
2510  .P  .P
2511  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
2512  pattern, so instead you could use this:  pattern, so instead you could use this:
2513  .sp  .sp
2514    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2515  .sp  .sp
2516  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
2517  them instead of the whole pattern.  them instead of the whole pattern.
2518  .P  .P
2519  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
2520  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
2521  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
2522  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2523  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.  
2524  .P  .P
2525  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2526  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2527  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2528  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2529    .\" </a>
2530    non-recursive subroutine
2531    .\"
2532    calls, as described in the next section.
2533  .P  .P
2534  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2535  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
2536  could rewrite the above example as follows:  could rewrite the above example as follows:
2537  .sp  .sp
2538    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2539  .sp  .sp
2540  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
2541  used.  used.
2542  .P  .P
2543  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2544  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
2545  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
2546  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2547  .sp  .sp
2548    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2549  .sp  .sp
2550  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,
2551  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
2552  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
2553  before failure can be reported.  before failure can be reported.
2554  .P  .P
2555  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
2556  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
2557  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  
2558  .\" HREF  .\" HREF
2559  \fBpcrecallout\fP  \fBpcrecallout\fP
2560  .\"  .\"
# Line 1988  documentation). If the pattern above is Line 2562  documentation). If the pattern above is
2562  .sp  .sp
2563    (ab(cd)ef)    (ab(cd)ef)
2564  .sp  .sp
2565  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
2566  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
2567  .sp  matched at the top level, its final captured value is unset, even if it was
2568    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2569       ^                        ^  .P
2570       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2571  .sp  obtain extra memory to store data during a recursion, which it does by using
2572  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
2573  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.  
2574  .P  .P
2575  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.
2576  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2013  different alternatives for the recursive Line 2584  different alternatives for the recursive
2584  is the actual recursive call.  is the actual recursive call.
2585  .  .
2586  .  .
2587    .\" HTML <a name="recursiondifference"></a>
2588    .SS "Differences in recursion processing between PCRE and Perl"
2589    .rs
2590    .sp
2591    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2592    (like Python, but unlike Perl), a recursive subpattern call is always treated
2593    as an atomic group. That is, once it has matched some of the subject string, it
2594    is never re-entered, even if it contains untried alternatives and there is a
2595    subsequent matching failure. This can be illustrated by the following pattern,
2596    which purports to match a palindromic string that contains an odd number of
2597    characters (for example, "a", "aba", "abcba", "abcdcba"):
2598    .sp
2599      ^(.|(.)(?1)\e2)$
2600    .sp
2601    The idea is that it either matches a single character, or two identical
2602    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2603    it does not if the pattern is longer than three characters. Consider the
2604    subject string "abcba":
2605    .P
2606    At the top level, the first character is matched, but as it is not at the end
2607    of the string, the first alternative fails; the second alternative is taken
2608    and the recursion kicks in. The recursive call to subpattern 1 successfully
2609    matches the next character ("b"). (Note that the beginning and end of line
2610    tests are not part of the recursion).
2611    .P
2612    Back at the top level, the next character ("c") is compared with what
2613    subpattern 2 matched, which was "a". This fails. Because the recursion is
2614    treated as an atomic group, there are now no backtracking points, and so the
2615    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2616    try the second alternative.) However, if the pattern is written with the
2617    alternatives in the other order, things are different:
2618    .sp
2619      ^((.)(?1)\e2|.)$
2620    .sp
2621    This time, the recursing alternative is tried first, and continues to recurse
2622    until it runs out of characters, at which point the recursion fails. But this
2623    time we do have another alternative to try at the higher level. That is the big
2624    difference: in the previous case the remaining alternative is at a deeper
2625    recursion level, which PCRE cannot use.
2626    .P
2627    To change the pattern so that it matches all palindromic strings, not just
2628    those with an odd number of characters, it is tempting to change the pattern to
2629    this:
2630    .sp
2631      ^((.)(?1)\e2|.?)$
2632    .sp
2633    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2634    deeper recursion has matched a single character, it cannot be entered again in
2635    order to match an empty string. The solution is to separate the two cases, and
2636    write out the odd and even cases as alternatives at the higher level:
2637    .sp
2638      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2639    .sp
2640    If you want to match typical palindromic phrases, the pattern has to ignore all
2641    non-word characters, which can be done like this:
2642    .sp
2643      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2644    .sp
2645    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2646    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2647    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2648    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2649    more) to match typical phrases, and Perl takes so long that you think it has
2650    gone into a loop.
2651    .P
2652    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2653    string does not start with a palindrome that is shorter than the entire string.
2654    For example, although "abcba" is correctly matched, if the subject is "ababa",
2655    PCRE finds the palindrome "aba" at the start, then fails at top level because
2656    the end of the string does not follow. Once again, it cannot jump back into the
2657    recursion to try other alternatives, so the entire match fails.
2658    .P
2659    The second way in which PCRE and Perl differ in their recursion processing is
2660    in the handling of captured values. In Perl, when a subpattern is called
2661    recursively or as a subpattern (see the next section), it has no access to any
2662    values that were captured outside the recursion, whereas in PCRE these values
2663    can be referenced. Consider this pattern:
2664    .sp
2665      ^(.)(\e1|a(?2))
2666    .sp
2667    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2668    then in the second group, when the back reference \e1 fails to match "b", the
2669    second alternative matches "a" and then recurses. In the recursion, \e1 does
2670    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2671    match because inside the recursive call \e1 cannot access the externally set
2672    value.
2673    .
2674    .
2675  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2676  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2677  .rs  .rs
2678  .sp  .sp
2679  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
2680  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
2681  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2682  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2683  relative, as in these examples:  relative, as in these examples:
2684  .sp  .sp
# Line 2039  matches "sense and sensibility" and "res Line 2698  matches "sense and sensibility" and "res
2698  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
2699  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2700  .P  .P
2701  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2702  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
2703  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
2704  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2705  .P  subroutine call revert to their previous values afterwards.
2706  When a subpattern is used as a subroutine, processing options such as  .P
2707  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2708  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2709    different calls. For example, consider this pattern:
2710  .sp  .sp
2711    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2712  .sp  .sp
# Line 2085  same pair of parentheses when there is a Line 2745  same pair of parentheses when there is a
2745  .P  .P
2746  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
2747  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
2748  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
2749    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2750  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2751  .P  .P
2752  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 2095  For example, this pattern has two callou Line 2756  For example, this pattern has two callou
2756  .sp  .sp
2757    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2758  .sp  .sp
2759  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
2760  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2761  255.  255. If there is a conditional group in the pattern whose condition is an
2762  .P  assertion, an additional callout is inserted just before the condition. An
2763  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:
2764  set), the external function is called. It is provided with the number of the  .sp
2765  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2766  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2767  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
2768  description of the interface to the callout function is given in the  condition.
2769    .P
2770    During matching, when PCRE reaches a callout point, the external function is
2771    called. It is provided with the number of the callout, the position in the
2772    pattern, and, optionally, one item of data originally supplied by the caller of
2773    the matching function. The callout function may cause matching to proceed, to
2774    backtrack, or to fail altogether.
2775    .P
2776    By default, PCRE implements a number of optimizations at compile time and
2777    matching time, and one side-effect is that sometimes callouts are skipped. If
2778    you need all possible callouts to happen, you need to set options that disable
2779    the relevant optimizations. More details, and a complete description of the
2780    interface to the callout function, are given in the
2781  .\" HREF  .\" HREF
2782  \fBpcrecallout\fP  \fBpcrecallout\fP
2783  .\"  .\"
2784  documentation.  documentation.
2785  .  .
2786  .  .
2787    .\" HTML <a name="backtrackcontrol"></a>
2788  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2789  .rs  .rs
2790  .sp  .sp
2791  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2792  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2793  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
2794  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
2795  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2796  .P  .P
2797    The new verbs make use of what was previously invalid syntax: an opening
2798    parenthesis followed by an asterisk. They are generally of the form
2799    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2800    differently depending on whether or not a name is present. A name is any
2801    sequence of characters that does not include a closing parenthesis. The maximum
2802    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2803    libraries. If the name is empty, that is, if the closing parenthesis
2804    immediately follows the colon, the effect is as if the colon were not there.
2805    Any number of these verbs may occur in a pattern.
2806    .P
2807  Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
2808  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  used only when the pattern is to be matched using one of the traditional
2809  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, because these use a backtracking algorithm. With the
2810  failing negative assertion, they cause an error if encountered by  exception of (*FAIL), which behaves like a failing negative assertion, the
2811  \fBpcre_dfa_exec()\fP.  backtracking control verbs cause an error if encountered by a DFA matching
2812    function.
2813  .P  .P
2814  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2815  parenthesis followed by an asterisk. In Perl, they are generally of the form  .\" HTML <a href="#btrepeat">
2816  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  .\" </a>
2817  form is just (*VERB). Any number of these verbs may occur in a pattern. There  repeated groups,
2818  are two kinds:  .\"
2819    .\" HTML <a href="#btassert">
2820    .\" </a>
2821    assertions,
2822    .\"
2823    and in
2824    .\" HTML <a href="#btsub">
2825    .\" </a>
2826    subpatterns called as subroutines
2827    .\"
2828    (whether or not recursively) is documented below.
2829    .
2830    .
2831    .\" HTML <a name="nooptimize"></a>
2832    .SS "Optimizations that affect backtracking verbs"
2833    .rs
2834    .sp
2835    PCRE contains some optimizations that are used to speed up matching by running
2836    some checks at the start of each match attempt. For example, it may know the
2837    minimum length of matching subject, or that a particular character must be
2838    present. When one of these optimizations bypasses the running of a match, any
2839    included backtracking verbs will not, of course, be processed. You can suppress
2840    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2841    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2842    pattern with (*NO_START_OPT). There is more discussion of this option in the
2843    section entitled
2844    .\" HTML <a href="pcreapi.html#execoptions">
2845    .\" </a>
2846    "Option bits for \fBpcre_exec()\fP"
2847    .\"
2848    in the
2849    .\" HREF
2850    \fBpcreapi\fP
2851    .\"
2852    documentation.
2853    .P
2854    Experiments with Perl suggest that it too has similar optimizations, sometimes
2855    leading to anomalous results.
2856    .
2857  .  .
2858  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2859  .rs  .rs
2860  .sp  .sp
2861  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2862    followed by a name.
2863  .sp  .sp
2864     (*ACCEPT)     (*ACCEPT)
2865  .sp  .sp
2866  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
2867  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2868  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2869  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
2870  captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2871    .P
2872    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2873    example:
2874  .sp  .sp
2875    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2876  .sp  .sp
2877  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
2878  captured.  the outer parentheses.
2879  .sp  .sp
2880    (*FAIL) or (*F)    (*FAIL) or (*F)
2881  .sp  .sp
2882  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
2883  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
2884  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2885  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 2163  callout feature, as for example in this Line 2890  callout feature, as for example in this
2890  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
2891  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2892  .  .
2893    .
2894    .SS "Recording which path was taken"
2895    .rs
2896    .sp
2897    There is one verb whose main purpose is to track how a match was arrived at,
2898    though it also has a secondary use in conjunction with advancing the match
2899    starting point (see (*SKIP) below).
2900    .sp
2901      (*MARK:NAME) or (*:NAME)
2902    .sp
2903    A name is always required with this verb. There may be as many instances of
2904    (*MARK) as you like in a pattern, and their names do not have to be unique.
2905    .P
2906    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2907    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2908    caller as described in the section entitled
2909    .\" HTML <a href="pcreapi.html#extradata">
2910    .\" </a>
2911    "Extra data for \fBpcre_exec()\fP"
2912    .\"
2913    in the
2914    .\" HREF
2915    \fBpcreapi\fP
2916    .\"
2917    documentation. Here is an example of \fBpcretest\fP output, where the /K
2918    modifier requests the retrieval and outputting of (*MARK) data:
2919    .sp
2920        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2921      data> XY
2922       0: XY
2923      MK: A
2924      XZ
2925       0: XZ
2926      MK: B
2927    .sp
2928    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2929    indicates which of the two alternatives matched. This is a more efficient way
2930    of obtaining this information than putting each alternative in its own
2931    capturing parentheses.
2932    .P
2933    If a verb with a name is encountered in a positive assertion that is true, the
2934    name is recorded and passed back if it is the last-encountered. This does not
2935    happen for negative assertions or failing positive assertions.
2936    .P
2937    After a partial match or a failed match, the last encountered name in the
2938    entire match process is returned. For example:
2939    .sp
2940        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2941      data> XP
2942      No match, mark = B
2943    .sp
2944    Note that in this unanchored example the mark is retained from the match
2945    attempt that started at the letter "X" in the subject. Subsequent match
2946    attempts starting at "P" and then with an empty string do not get as far as the
2947    (*MARK) item, but nevertheless do not reset it.
2948    .P
2949    If you are interested in (*MARK) values after failed matches, you should
2950    probably set the PCRE_NO_START_OPTIMIZE option
2951    .\" HTML <a href="#nooptimize">
2952    .\" </a>
2953    (see above)
2954    .\"
2955    to ensure that the match is always attempted.
2956    .
2957    .
2958  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2959  .rs  .rs
2960  .sp  .sp
2961  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2962  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
2963  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
2964    the verb. However, when one of these verbs appears inside an atomic group or an
2965    assertion that is true, its effect is confined to that group, because once the
2966    group has been matched, there is never any backtracking into it. In this
2967    situation, backtracking can "jump back" to the left of the entire atomic group
2968    or assertion. (Remember also, as stated above, that this localization also
2969    applies in subroutine calls.)
2970    .P
2971    These verbs differ in exactly what kind of failure occurs when backtracking
2972    reaches them. The behaviour described below is what happens when the verb is
2973    not in a subroutine or an assertion. Subsequent sections cover these special
2974    cases.
2975  .sp  .sp
2976    (*COMMIT)    (*COMMIT)
2977  .sp  .sp
2978  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
2979  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
2980  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
2981  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
2982  starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2983    committed to finding a match at the current starting point, or not at all. For
2984    example:
2985  .sp  .sp
2986    a+(*COMMIT)b    a+(*COMMIT)b
2987  .sp  .sp
2988  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
2989  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
2990  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2991    (*PRUNE)  match failure.
2992  .sp  .P
2993  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
2994  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2995  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.
2996  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .P
2997  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,
2998  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
2999  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot  \fBpcretest\fP example:
3000  be expressed in any other way.  .sp
3001        re> /(*COMMIT)abc/
3002      data> xyzabc
3003       0: abc
3004      xyzabc\eY
3005      No match
3006    .sp
3007    PCRE knows that any match must start with "a", so the optimization skips along
3008    the subject to "a" before running the first match attempt, which succeeds. When
3009    the optimization is disabled by the \eY escape in the second subject, the match
3010    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
3011    starting points.
3012    .sp
3013      (*PRUNE) or (*PRUNE:NAME)
3014    .sp
3015    This verb causes the match to fail at the current starting position in the
3016    subject if there is a later matching failure that causes backtracking to reach
3017    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3018    starting character then happens. Backtracking can occur as usual to the left of
3019    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3020    if there is no match to the right, backtracking cannot cross (*PRUNE). In
3021    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3022    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3023    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3024    as (*COMMIT).
3025    .P
3026    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3027    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3028    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3029  .sp  .sp
3030    (*SKIP)    (*SKIP)
3031  .sp  .sp
3032  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
3033  "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,
3034  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3035  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
3036    successful match. Consider:
3037  .sp  .sp
3038    a+(*SKIP)b    a+(*SKIP)b
3039  .sp  .sp
3040  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
3041  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
3042  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
3043  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
3044  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
3045  instead of skipping on to "c".  instead of skipping on to "c".
3046  .sp  .sp
3047    (*THEN)    (*SKIP:NAME)
3048  .sp  .sp
3049  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
3050  not match. That is, it cancels pending backtracking, but only within the  triggered, the previous path through the pattern is searched for the most
3051  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
3052  for a pattern-based if-then-else block:  is to the subject position that corresponds to that (*MARK) instead of to where
3053    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3054    (*SKIP) is ignored.
3055    .P
3056    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3057    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3058    .sp
3059      (*THEN) or (*THEN:NAME)
3060    .sp
3061    This verb causes a skip to the next innermost alternative when backtracking
3062    reaches it. That is, it cancels any further backtracking within the current
3063    alternative. Its name comes from the observation that it can be used for a
3064    pattern-based if-then-else block:
3065  .sp  .sp
3066    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3067  .sp  .sp
3068  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
3069  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
3070  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. If that
3071  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
3072    more alternatives, so there is a backtrack to whatever came before the entire
3073    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3074    .P
3075    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3076    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3077    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3078    .P
3079    A subpattern that does not contain a | character is just a part of the
3080    enclosing alternative; it is not a nested alternation with only one
3081    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3082    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3083    pattern fragments that do not contain any | characters at this level:
3084    .sp
3085      A (B(*THEN)C) | D
3086    .sp
3087    If A and B are matched, but there is a failure in C, matching does not
3088    backtrack into A; instead it moves to the next alternative, that is, D.
3089    However, if the subpattern containing (*THEN) is given an alternative, it
3090    behaves differently:
3091    .sp
3092      A (B(*THEN)C | (*FAIL)) | D
3093    .sp
3094    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3095    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3096    because there are no more alternatives to try. In this case, matching does now
3097    backtrack into A.
3098    .P
3099    Note that a conditional subpattern is not considered as having two
3100    alternatives, because only one is ever used. In other words, the | character in
3101    a conditional subpattern has a different meaning. Ignoring white space,
3102    consider:
3103    .sp
3104      ^.*? (?(?=a) a | b(*THEN)c )
3105    .sp
3106    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3107    it initially matches zero characters. The condition (?=a) then fails, the
3108    character "b" is matched, but "c" is not. At this point, matching does not
3109    backtrack to .*? as might perhaps be expected from the presence of the |
3110    character. The conditional subpattern is part of the single alternative that
3111    comprises the whole pattern, and so the match fails. (If there was a backtrack
3112    into .*?, allowing it to match "b", the match would succeed.)
3113    .P
3114    The verbs just described provide four different "strengths" of control when
3115    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3116    next alternative. (*PRUNE) comes next, failing the match at the current
3117    starting position, but allowing an advance to the next character (for an
3118    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3119    than one character. (*COMMIT) is the strongest, causing the entire match to
3120    fail.
3121    .
3122    .
3123    .SS "More than one backtracking verb"
3124    .rs
3125    .sp
3126    If more than one backtracking verb is present in a pattern, the one that is
3127    backtracked onto first acts. For example, consider this pattern, where A, B,
3128    etc. are complex pattern fragments:
3129    .sp
3130      (A(*COMMIT)B(*THEN)C|ABD)
3131    .sp
3132    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3133    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3134    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3135    not always the same as Perl's. It means that if two or more backtracking verbs
3136    appear in succession, all the the last of them has no effect. Consider this
3137    example:
3138    .sp
3139      ...(*COMMIT)(*PRUNE)...
3140    .sp
3141    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3142    it to be triggered, and its action is taken. There can never be a backtrack
3143    onto (*COMMIT).
3144    .
3145    .
3146    .\" HTML <a name="btrepeat"></a>
3147    .SS "Backtracking verbs in repeated groups"
3148    .rs
3149    .sp
3150    PCRE differs from Perl in its handling of backtracking verbs in repeated
3151    groups. For example, consider:
3152    .sp
3153      /(a(*COMMIT)b)+ac/
3154    .sp
3155    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3156    the second repeat of the group acts.
3157    .
3158    .
3159    .\" HTML <a name="btassert"></a>
3160    .SS "Backtracking verbs in assertions"
3161    .rs
3162    .sp
3163    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3164    .P
3165    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3166    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3167    fail without any further processing.
3168    .P
3169    The other backtracking verbs are not treated specially if they appear in a
3170    positive assertion. In particular, (*THEN) skips to the next alternative in the
3171    innermost enclosing group that has alternations, whether or not this is within
3172    the assertion.
3173    .P
3174    Negative assertions are, however, different, in order to ensure that changing a
3175    positive assertion into a negative assertion changes its result. Backtracking
3176    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3177    without considering any further alternative branches in the assertion.
3178    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3179    within the assertion (the normal behaviour), but if the assertion does not have
3180    such an alternative, (*THEN) behaves like (*PRUNE).
3181    .
3182    .
3183    .\" HTML <a name="btsub"></a>
3184    .SS "Backtracking verbs in subroutines"
3185    .rs
3186    .sp
3187    These behaviours occur whether or not the subpattern is called recursively.
3188    Perl's treatment of subroutines is different in some cases.
3189    .P
3190    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3191    an immediate backtrack.
3192    .P
3193    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3194    succeed without any further processing. Matching then continues after the
3195    subroutine call.
3196    .P
3197    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3198    the subroutine match to fail.
3199    .P
3200    (*THEN) skips to the next alternative in the innermost enclosing group within
3201    the subpattern that has alternatives. If there is no such group within the
3202    subpattern, (*THEN) causes the subroutine match to fail.
3203  .  .
3204  .  .
3205  .SH "SEE ALSO"  .SH "SEE ALSO"
3206  .rs  .rs
3207  .sp  .sp
3208  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3209    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3210  .  .
3211  .  .
3212  .SH AUTHOR  .SH AUTHOR
# Line 2245  Cambridge CB2 3QH, England. Line 3223  Cambridge CB2 3QH, England.
3223  .rs  .rs
3224  .sp  .sp
3225  .nf  .nf
3226  Last updated: 18 March 2009  Last updated: 09 November 2013
3227  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3228  .fi  .fi

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