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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    backslash. All other characters (in particular, those whose codepoints are
274    greater than 127) are treated as literals.
275    .P
276    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
277  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
278  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
279  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
280  .P  .P
281  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
282  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 292  Perl, $ and @ cause variable interpolati
292    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
293  .sp  .sp
294  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
295    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
296    by \eE later in the pattern, the literal interpretation continues to the end of
297    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
298    a character class, this causes an error, because the character class is not
299    terminated.
300  .  .
301  .  .
302  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 199  The \eQ...\eE sequence is recognized bot Line 306  The \eQ...\eE sequence is recognized bot
306  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
307  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
308  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
309  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
310  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:  
311  .sp  .sp
312    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
313    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
314    \ee        escape (hex 1B)    \ee        escape (hex 1B)
315    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
316    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
317    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
318    \et        tab (hex 09)    \et        tab (hex 09)
319    \eddd      character with octal code ddd, or backreference    \e0dd      character with octal code 0dd
320      \eddd      character with octal code ddd, or back reference
321      \eo{ddd..} character with octal code ddd..
322    \exhh      character with hex code hh    \exhh      character with hex code hh
323    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
324      \euhhhh    character with hex code hhhh (JavaScript mode only)
325  .sp  .sp
326  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
327  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
328  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),
329  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
330  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
331  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.
332  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
333  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
334  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,
335  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
336  point, which is 10FFFF.  bytes. In this mode, all values are valid after \ec. If the next character is a
337  .P  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
338  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
339  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
340  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}.  
341  .P  .P
342  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
343  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 345  specifies two binary zeros followed by a
345  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
346  follows is itself an octal digit.  follows is itself an octal digit.
347  .P  .P
348  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
349  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
350  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
351    numbers greater than 0777, and it also allows octal numbers and back references
352    to be unambiguously specified.
353    .P
354    For greater clarity and unambiguity, it is best to avoid following \e by a
355    digit greater than zero. Instead, use \eo{} or \ex{} to specify character
356    numbers, and \eg{} to specify back references. The following paragraphs
357    describe the old, ambiguous syntax.
358    .P
359    The handling of a backslash followed by a digit other than 0 is complicated,
360    and Perl has changed in recent releases, causing PCRE also to change. Outside a
361    character class, PCRE reads the digit and any following digits as a decimal
362    number. If the number is less than 8, or if there have been at least that many
363  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
364  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
365  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
# Line 256  following the discussion of Line 372  following the discussion of
372  parenthesized subpatterns.  parenthesized subpatterns.
373  .\"  .\"
374  .P  .P
375  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
376  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
377  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
378  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.
379  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:  
380  .sp  .sp
381    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
382  .\" JOIN  .\" JOIN
383    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
384              previous capturing subpatterns              previous capturing subpatterns
# Line 278  to \e777 are permitted. For example: Line 393  to \e777 are permitted. For example:
393              character with octal code 113              character with octal code 113
394  .\" JOIN  .\" JOIN
395    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
396              the byte consisting entirely of 1 bits              the value 255 (decimal)
397  .\" JOIN  .\" JOIN
398    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or the two
399              followed by the two characters "8" and "1"              characters "8" and "1"
400  .sp  .sp
401  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
402  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
403    digits are ever read.
404    .P
405    By default, after \ex that is not followed by {, from zero to two hexadecimal
406    digits are read (letters can be in upper or lower case). Any number of
407    hexadecimal digits may appear between \ex{ and }. If a character other than
408    a hexadecimal digit appears between \ex{ and }, or if there is no terminating
409    }, an error occurs.
410    .P
411    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
412    as just described only when it is followed by two hexadecimal digits.
413    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
414    code points greater than 256 is provided by \eu, which must be followed by
415    four hexadecimal digits; otherwise it matches a literal "u" character.
416  .P  .P
417    Characters whose value is less than 256 can be defined by either of the two
418    syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
419    way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
420    \eu00dc in JavaScript mode).
421    .
422    .
423    .SS "Constraints on character values"
424    .rs
425    .sp
426    Characters that are specified using octal or hexadecimal numbers are
427    limited to certain values, as follows:
428    .sp
429      8-bit non-UTF mode    less than 0x100
430      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
431      16-bit non-UTF mode   less than 0x10000
432      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
433      32-bit non-UTF mode   less than 0x100000000
434      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
435    .sp
436    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
437    "surrogate" codepoints), and 0xffef.
438    .
439    .
440    .SS "Escape sequences in character classes"
441    .rs
442    .sp
443  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
444  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
445  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
446  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
447  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
448  meanings  inside a character class. Like other unrecognized escape sequences, they are
449  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
450  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
451  (see below).  sequences have different meanings.
452  .\"  .
453    .
454    .SS "Unsupported escape sequences"
455    .rs
456    .sp
457    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
458    handler and used to modify the case of following characters. By default, PCRE
459    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
460    option is set, \eU matches a "U" character, and \eu can be used to define a
461    character by code point, as described in the previous section.
462  .  .
463  .  .
464  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 326  syntax for referencing a subpattern as a Line 489  syntax for referencing a subpattern as a
489  later.  later.
490  .\"  .\"
491  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
492  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a
493    .\" HTML <a href="#subpatternsassubroutines">
494    .\" </a>
495    subroutine
496    .\"
497    call.
498  .  .
499  .  .
500    .\" HTML <a name="genericchartypes"></a>
501  .SS "Generic character types"  .SS "Generic character types"
502  .rs  .rs
503  .sp  .sp
504  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:  
505  .sp  .sp
506    \ed     any decimal digit    \ed     any decimal digit
507    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
508    \eh     any horizontal whitespace character    \eh     any horizontal white space character
509    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
510    \es     any whitespace character    \es     any white space character
511    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
512    \ev     any vertical whitespace character    \ev     any vertical white space character
513    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
514    \ew     any "word" character    \ew     any "word" character
515    \eW     any "non-word" character    \eW     any "non-word" character
516  .sp  .sp
517  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.
518  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
519  .P  .\" HTML <a href="#fullstopdot">
520  These character type sequences can appear both inside and outside character  .\" </a>
521    the "." metacharacter
522    .\"
523    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
524    PCRE does not support this.
525    .P
526    Each pair of lower and upper case escape sequences partitions the complete set
527    of characters into two disjoint sets. Any given character matches one, and only
528    one, of each pair. The sequences can appear both inside and outside character
529  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
530  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
531  there is no character to match.  there is no character to match.
532  .P  .P
533  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
534  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
535  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
536  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).
537  does.  .P
538  .P  A "word" character is an underscore or any character that is a letter or digit.
539  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
540  \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
541  character property support is available. These sequences retain their original  place (see
542  meanings from before UTF-8 support was available, mainly for efficiency  .\" HTML <a href="pcreapi.html#localesupport">
543  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" </a>
544  and \eW.  "Locale support"
545  .P  .\"
546  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  in the
547  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\" HREF
548  The horizontal space characters are:  \fBpcreapi\fP
549    .\"
550    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
551    or "french" in Windows, some character codes greater than 128 are used for
552    accented letters, and these are then matched by \ew. The use of locales with
553    Unicode is discouraged.
554    .P
555    By default, in a UTF mode, characters with values greater than 128 never match
556    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
557    their original meanings from before UTF support was available, mainly for
558    efficiency reasons. However, if PCRE is compiled with Unicode property support,
559    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
560    properties are used to determine character types, as follows:
561    .sp
562      \ed  any character that matches \ep{Nd} (decimal digit)
563      \es  any character that matches \ep{Z} or \eh or \ev
564      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
565    .sp
566    The upper case escapes match the inverse sets of characters. Note that \ed
567    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
568    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
569    \eB because they are defined in terms of \ew and \eW. Matching these sequences
570    is noticeably slower when PCRE_UCP is set.
571    .P
572    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
573    release 5.10. In contrast to the other sequences, which match only ASCII
574    characters by default, these always match certain high-valued codepoints,
575    whether or not PCRE_UCP is set. The horizontal space characters are:
576  .sp  .sp
577    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
578    U+0020     Space    U+0020     Space
579    U+00A0     Non-break space    U+00A0     Non-break space
580    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 393  The horizontal space characters are: Line 596  The horizontal space characters are:
596  .sp  .sp
597  The vertical space characters are:  The vertical space characters are:
598  .sp  .sp
599    U+000A     Linefeed    U+000A     Linefeed (LF)
600    U+000B     Vertical tab    U+000B     Vertical tab (VT)
601    U+000C     Formfeed    U+000C     Form feed (FF)
602    U+000D     Carriage return    U+000D     Carriage return (CR)
603    U+0085     Next line    U+0085     Next line (NEL)
604    U+2028     Line separator    U+2028     Line separator
605    U+2029     Paragraph separator    U+2029     Paragraph separator
606  .P  .sp
607  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
608  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.  
609  .  .
610  .  .
611  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 424  is discouraged. Line 613  is discouraged.
613  .rs  .rs
614  .sp  .sp
615  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
616  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
617  equivalent to the following:  following:
618  .sp  .sp
619    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
620  .sp  .sp
# Line 436  below. Line 625  below.
625  .\"  .\"
626  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
627  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,
628  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
629  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
630  cannot be split.  cannot be split.
631  .P  .P
632  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
633  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).
634  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
635  recognized.  recognized.
# Line 456  one of the following sequences: Line 645  one of the following sequences:
645    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
646    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
647  .sp  .sp
648  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
649  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
650  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
651  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
652  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
653  newline convention, for example, a pattern can start with:  change of newline convention; for example, a pattern can start with:
654  .sp  .sp
655    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
656  .sp  .sp
657  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
658    (*UCP) special sequences. Inside a character class, \eR is treated as an
659    unrecognized escape sequence, and so matches the letter "R" by default, but
660    causes an error if PCRE_EXTRA is set.
661  .  .
662  .  .
663  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 474  Inside a character class, \eR matches th Line 666  Inside a character class, \eR matches th
666  .sp  .sp
667  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
668  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
669  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
670  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.
671  The extra escape sequences are:  The extra escape sequences are:
672  .sp  .sp
673    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
674    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
675    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
676  .sp  .sp
677  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
678  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
679  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
680  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
681  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
682    .\" </a>
683    next section).
684    .\"
685    Other Perl properties such as "InMusicalSymbols" are not currently supported by
686    PCRE. Note that \eP{Any} does not match any characters, so always causes a
687    match failure.
688  .P  .P
689  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
690  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 698  Those that are not part of an identified
698  .P  .P
699  Arabic,  Arabic,
700  Armenian,  Armenian,
701    Avestan,
702  Balinese,  Balinese,
703    Bamum,
704    Batak,
705  Bengali,  Bengali,
706  Bopomofo,  Bopomofo,
707    Brahmi,
708  Braille,  Braille,
709  Buginese,  Buginese,
710  Buhid,  Buhid,
711  Canadian_Aboriginal,  Canadian_Aboriginal,
712    Carian,
713    Chakma,
714    Cham,
715  Cherokee,  Cherokee,
716  Common,  Common,
717  Coptic,  Coptic,
# Line 515  Cypriot, Line 720  Cypriot,
720  Cyrillic,  Cyrillic,
721  Deseret,  Deseret,
722  Devanagari,  Devanagari,
723    Egyptian_Hieroglyphs,
724  Ethiopic,  Ethiopic,
725  Georgian,  Georgian,
726  Glagolitic,  Glagolitic,
# Line 527  Hangul, Line 733  Hangul,
733  Hanunoo,  Hanunoo,
734  Hebrew,  Hebrew,
735  Hiragana,  Hiragana,
736    Imperial_Aramaic,
737  Inherited,  Inherited,
738    Inscriptional_Pahlavi,
739    Inscriptional_Parthian,
740    Javanese,
741    Kaithi,
742  Kannada,  Kannada,
743  Katakana,  Katakana,
744    Kayah_Li,
745  Kharoshthi,  Kharoshthi,
746  Khmer,  Khmer,
747  Lao,  Lao,
748  Latin,  Latin,
749    Lepcha,
750  Limbu,  Limbu,
751  Linear_B,  Linear_B,
752    Lisu,
753    Lycian,
754    Lydian,
755  Malayalam,  Malayalam,
756    Mandaic,
757    Meetei_Mayek,
758    Meroitic_Cursive,
759    Meroitic_Hieroglyphs,
760    Miao,
761  Mongolian,  Mongolian,
762  Myanmar,  Myanmar,
763  New_Tai_Lue,  New_Tai_Lue,
# Line 544  Nko, Line 765  Nko,
765  Ogham,  Ogham,
766  Old_Italic,  Old_Italic,
767  Old_Persian,  Old_Persian,
768    Old_South_Arabian,
769    Old_Turkic,
770    Ol_Chiki,
771  Oriya,  Oriya,
772  Osmanya,  Osmanya,
773  Phags_Pa,  Phags_Pa,
774  Phoenician,  Phoenician,
775    Rejang,
776  Runic,  Runic,
777    Samaritan,
778    Saurashtra,
779    Sharada,
780  Shavian,  Shavian,
781  Sinhala,  Sinhala,
782    Sora_Sompeng,
783    Sundanese,
784  Syloti_Nagri,  Syloti_Nagri,
785  Syriac,  Syriac,
786  Tagalog,  Tagalog,
787  Tagbanwa,  Tagbanwa,
788  Tai_Le,  Tai_Le,
789    Tai_Tham,
790    Tai_Viet,
791    Takri,
792  Tamil,  Tamil,
793  Telugu,  Telugu,
794  Thaana,  Thaana,
# Line 563  Thai, Line 796  Thai,
796  Tibetan,  Tibetan,
797  Tifinagh,  Tifinagh,
798  Ugaritic,  Ugaritic,
799    Vai,
800  Yi.  Yi.
801  .P  .P
802  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
803  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
804  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
805  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
806  .P  .P
807  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
808  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 863  the Lu, Ll, or Lt property, in other wor
863  a modifier or "other".  a modifier or "other".
864  .P  .P
865  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
866  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
867  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
868  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
869    PCRE_NO_UTF32_CHECK in the
870  .\" HREF  .\" HREF
871  \fBpcreapi\fP  \fBpcreapi\fP
872  .\"  .\"
873  page).  page). Perl does not support the Cs property.
874  .P  .P
875  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})
876  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
877  properties with "Is".  properties with "Is".
878  .P  .P
# Line 646  Instead, this property is assumed for an Line 881  Instead, this property is assumed for an
881  Unicode table.  Unicode table.
882  .P  .P
883  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
884  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
885    the behaviour of current versions of Perl.
886  .P  .P
887  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
888  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
889  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
890    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
891    PCRE_UCP option or by starting the pattern with (*UCP).
892    .
893    .
894    .SS Extended grapheme clusters
895    .rs
896  .sp  .sp
897  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
898  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  
899  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
900  .\" </a>  .\" </a>
901  (see below).  (see below).
902  .\"  .\"
903  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
904  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
905  non-UTF-8 mode \eX matches any one character.  .sp
906  .P    (?>\ePM\epM*)
907  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
908  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
909  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"
910  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
911    .P
912    This simple definition was extended in Unicode to include more complicated
913    kinds of composite character by giving each character a grapheme breaking
914    property, and creating rules that use these properties to define the boundaries
915    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
916    one of these clusters.
917    .P
918    \eX always matches at least one character. Then it decides whether to add
919    additional characters according to the following rules for ending a cluster:
920    .P
921    1. End at the end of the subject string.
922    .P
923    2. Do not end between CR and LF; otherwise end after any control character.
924    .P
925    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
926    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
927    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
928    character; an LVT or T character may be follwed only by a T character.
929    .P
930    4. Do not end before extending characters or spacing marks. Characters with
931    the "mark" property always have the "extend" grapheme breaking property.
932    .P
933    5. Do not end after prepend characters.
934    .P
935    6. Otherwise, end the cluster.
936    .
937    .
938    .\" HTML <a name="extraprops"></a>
939    .SS PCRE's additional properties
940    .rs
941    .sp
942    As well as the standard Unicode properties described above, PCRE supports four
943    more that make it possible to convert traditional escape sequences such as \ew
944    and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
945    properties internally when PCRE_UCP is set. However, they may also be used
946    explicitly. These properties are:
947    .sp
948      Xan   Any alphanumeric character
949      Xps   Any POSIX space character
950      Xsp   Any Perl space character
951      Xwd   Any Perl "word" character
952    .sp
953    Xan matches characters that have either the L (letter) or the N (number)
954    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
955    carriage return, and any other character that has the Z (separator) property.
956    Xsp is the same as Xps; it used to exclude vertical tab, for Perl
957    compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
958    matches the same characters as Xan, plus underscore.
959    .P
960    There is another non-standard property, Xuc, which matches any character that
961    can be represented by a Universal Character Name in C++ and other programming
962    languages. These are the characters $, @, ` (grave accent), and all characters
963    with Unicode code points greater than or equal to U+00A0, except for the
964    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
965    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
966    where H is a hexadecimal digit. Note that the Xuc property does not match these
967    sequences but the characters that they represent.)
968  .  .
969  .  .
970  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
971  .SS "Resetting the match start"  .SS "Resetting the match start"
972  .rs  .rs
973  .sp  .sp
974  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
975  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:  
976  .sp  .sp
977    foo\eKbar    foo\eKbar
978  .sp  .sp
# Line 698  For example, when the pattern Line 994  For example, when the pattern
994    (foo)\eKbar    (foo)\eKbar
995  .sp  .sp
996  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
997    .P
998    Perl documents that the use of \eK within assertions is "not well defined". In
999    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
1000    ignored in negative assertions.
1001  .  .
1002  .  .
1003  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 722  The backslashed assertions are: Line 1022  The backslashed assertions are:
1022    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
1023    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
1024  .sp  .sp
1025  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
1026  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
1027    default it matches the corresponding literal character (for example, \eB
1028    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1029    escape sequence" error is generated instead.
1030  .P  .P
1031  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
1032  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
1033  \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
1034  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
1035    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1036    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1037    of word" or "end of word" metasequence. However, whatever follows \eb normally
1038    determines which it is. For example, the fragment \eba matches "a" at the start
1039    of a word.
1040  .P  .P
1041  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1042  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 1070  regular expression.
1070  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1071  .rs  .rs
1072  .sp  .sp
1073    The circumflex and dollar metacharacters are zero-width assertions. That is,
1074    they test for a particular condition being true without consuming any
1075    characters from the subject string.
1076    .P
1077  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1078  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
1079  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
1080  \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
1081  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1082  meaning  meaning
# Line 781  constrained to match only at the start o Line 1093  constrained to match only at the start o
1093  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1094  to be anchored.)  to be anchored.)
1095  .P  .P
1096  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
1097  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
1098  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
1099  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
1100  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
1101  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1102  .P  .P
1103  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
1104  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 1124  end of the subject in both modes, and if
1124  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1125  .  .
1126  .  .
1127  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1128    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1129  .rs  .rs
1130  .sp  .sp
1131  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
1132  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
1133  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1134  .P  .P
1135  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
1136  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 1147  to match it.
1147  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
1148  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
1149  special meaning in a character class.  special meaning in a character class.
1150  .  .P
1151  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1152  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1153  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1154  .sp  name; PCRE does not support this.
1155  Outside a character class, the escape sequence \eC matches any one byte, both  .
1156  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1157  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1158  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1159  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1160  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1161    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1162    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1163    a 32-bit unit. Unlike a dot, \eC always
1164    matches line-ending characters. The feature is provided in Perl in order to
1165    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1166    used. Because \eC breaks up characters into individual data units, matching one
1167    unit with \eC in a UTF mode means that the rest of the string may start with a
1168    malformed UTF character. This has undefined results, because PCRE assumes that
1169    it is dealing with valid UTF strings (and by default it checks this at the
1170    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1171    PCRE_NO_UTF32_CHECK option is used).
1172  .P  .P
1173  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1174  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1175  .\" </a>  .\" </a>
1176  (described below),  (described below)
1177  .\"  .\"
1178  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
1179  the lookbehind.  the lookbehind.
1180    .P
1181    In general, the \eC escape sequence is best avoided. However, one
1182    way of using it that avoids the problem of malformed UTF characters is to use a
1183    lookahead to check the length of the next character, as in this pattern, which
1184    could be used with a UTF-8 string (ignore white space and line breaks):
1185    .sp
1186      (?| (?=[\ex00-\ex7f])(\eC) |
1187          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1188          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1189          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1190    .sp
1191    A group that starts with (?| resets the capturing parentheses numbers in each
1192    alternative (see
1193    .\" HTML <a href="#dupsubpatternnumber">
1194    .\" </a>
1195    "Duplicate Subpattern Numbers"
1196    .\"
1197    below). The assertions at the start of each branch check the next UTF-8
1198    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1199    character's individual bytes are then captured by the appropriate number of
1200    groups.
1201  .  .
1202  .  .
1203  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 860  the lookbehind. Line 1205  the lookbehind.
1205  .rs  .rs
1206  .sp  .sp
1207  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1208  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.
1209  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
1210  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
1211  escaped with a backslash.  a member of the class, it should be the first data character in the class
1212  .P  (after an initial circumflex, if present) or escaped with a backslash.
1213  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1214  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
1215  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
1216  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
1217  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
1218  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
1219    member of the class, ensure it is not the first character, or escape it with a
1220  backslash.  backslash.
1221  .P  .P
1222  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1223  [^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
1224  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1225  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
1226  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
1227  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
1228  string.  string.
1229  .P  .P
1230  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)
1231  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
1232    \ex{ escaping mechanism.
1233  .P  .P
1234  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
1235  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
1236  "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
1237  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
1238  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
1239  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1240  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1241  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
1242  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
1243  UTF-8 support.  well as with UTF support.
1244  .P  .P
1245  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
1246  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 1251  The minus (hyphen) character can be used
1251  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,
1252  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
1253  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
1254  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
1255    immediately after a range. For example, [b-d-z] matches letters in the range b
1256    to d, a hyphen character, or z.
1257  .P  .P
1258  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
1259  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 1263  the end of range, so [W-\e]46] is interp
1263  followed by two other characters. The octal or hexadecimal representation of  followed by two other characters. The octal or hexadecimal representation of
1264  "]" can also be used to end a range.  "]" can also be used to end a range.
1265  .P  .P
1266    An error is generated if a POSIX character class (see below) or an escape
1267    sequence other than one that defines a single character appears at a point
1268    where a range ending character is expected. For example, [z-\exff] is valid,
1269    but [A-\ed] and [A-[:digit:]] are not.
1270    .P
1271  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
1272  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1273  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}].  
1274  .P  .P
1275  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
1276  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
1277  [][\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
1278  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
1279  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
1280  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
1281  property support.  property support.
1282  .P  .P
1283  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,
1284  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
1285  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1286  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
1287  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
1288  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1289    .\" HTML <a href="#genericchartypes">
1290    .\" </a>
1291    "Generic character types"
1292    .\"
1293    above. The escape sequence \eb has a different meaning inside a character
1294    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1295    are not special inside a character class. Like any other unrecognized escape
1296    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1297    default, but cause an error if the PCRE_EXTRA option is set.
1298    .P
1299    A circumflex can conveniently be used with the upper case character types to
1300    specify a more restricted set of characters than the matching lower case type.
1301    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1302    whereas [\ew] includes underscore. A positive character class should be read as
1303    "something OR something OR ..." and a negative class as "NOT something AND NOT
1304    something AND NOT ...".
1305  .P  .P
1306  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1307  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 1321  this notation. For example,
1321    [01[:alpha:]%]    [01[:alpha:]%]
1322  .sp  .sp
1323  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1324  are  are:
1325  .sp  .sp
1326    alnum    letters and digits    alnum    letters and digits
1327    alpha    letters    alpha    letters
# Line 963  are Line 1332  are
1332    graph    printing characters, excluding space    graph    printing characters, excluding space
1333    lower    lower case letters    lower    lower case letters
1334    print    printing characters, including space    print    printing characters, including space
1335    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1336    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1337    upper    upper case letters    upper    upper case letters
1338    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1339    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1340  .sp  .sp
1341  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
1342  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
1343  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
1344  compatibility).  release 8.34. "Space" and \es now match the same set of characters.
1345  .P  .P
1346  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
1347  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 1353  matches "1", "2", or any non-digit. PCRE
1353  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
1354  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1355  .P  .P
1356  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
1357  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1358    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1359    character properties are used. This is achieved by replacing certain POSIX
1360    classes by other sequences, as follows:
1361    .sp
1362      [:alnum:]  becomes  \ep{Xan}
1363      [:alpha:]  becomes  \ep{L}
1364      [:blank:]  becomes  \eh
1365      [:digit:]  becomes  \ep{Nd}
1366      [:lower:]  becomes  \ep{Ll}
1367      [:space:]  becomes  \ep{Xps}
1368      [:upper:]  becomes  \ep{Lu}
1369      [:word:]   becomes  \ep{Xwd}
1370    .sp
1371    Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1372    classes are handled specially in UCP mode:
1373    .TP 10
1374    [:graph:]
1375    This matches characters that have glyphs that mark the page when printed. In
1376    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1377    properties, except for:
1378    .sp
1379      U+061C           Arabic Letter Mark
1380      U+180E           Mongolian Vowel Separator
1381      U+2066 - U+2069  Various "isolate"s
1382    .sp
1383    .TP 10
1384    [:print:]
1385    This matches the same characters as [:graph:] plus space characters that are
1386    not controls, that is, characters with the Zs property.
1387    .TP 10
1388    [:punct:]
1389    This matches all characters that have the Unicode P (punctuation) property,
1390    plus those characters whose code points are less than 128 that have the S
1391    (Symbol) property.
1392    .P
1393    The other POSIX classes are unchanged, and match only characters with code
1394    points less than 128.
1395  .  .
1396  .  .
1397  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1032  The PCRE-specific options PCRE_DUPNAMES, Line 1438  The PCRE-specific options PCRE_DUPNAMES,
1438  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
1439  J, U and X respectively.  J, U and X respectively.
1440  .P  .P
1441  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
1442  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1443  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
1444  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
1445  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1446  .P  .P
1447  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1448  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
1449  .sp  .sp
1450    (a(?i)b)c    (a(?i)b)c
1451  .sp  .sp
# Line 1056  option settings happen at compile time. Line 1462  option settings happen at compile time.
1462  behaviour otherwise.  behaviour otherwise.
1463  .P  .P
1464  \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
1465  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
1466  pattern can contain special leading sequences to override what the application  the pattern can contain special leading sequences such as (*CRLF) to override
1467  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
1468    the section entitled
1469  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1470  .\" </a>  .\" </a>
1471  "Newline sequences"  "Newline sequences"
1472  .\"  .\"
1473  above.  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1474    sequences that can be used to set UTF and Unicode property modes; they are
1475    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1476    options, respectively. The (*UTF) sequence is a generic version that can be
1477    used with any of the libraries. However, the application can set the
1478    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1479  .  .
1480  .  .
1481  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1077  Turning part of a pattern into a subpatt Line 1489  Turning part of a pattern into a subpatt
1489  .sp  .sp
1490    cat(aract|erpillar|)    cat(aract|erpillar|)
1491  .sp  .sp
1492  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1493  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1494  .sp  .sp
1495  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
1496  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
1497  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
1498  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1499  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1500  .P  .P
1501  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
1502    numbers for the capturing subpatterns. For example, if the string "the red
1503    king" is matched against the pattern
1504  .sp  .sp
1505    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1506  .sp  .sp
# Line 1118  is reached, an option setting in one bra Line 1532  is reached, an option setting in one bra
1532  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1533  .  .
1534  .  .
1535    .\" HTML <a name="dupsubpatternnumber"></a>
1536  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1537  .rs  .rs
1538  .sp  .sp
# Line 1134  at captured substring number one, whiche Line 1549  at captured substring number one, whiche
1549  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
1550  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1551  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
1552  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
1553  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1554  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.  
1555  .sp  .sp
1556    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1557    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1558    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1559  .sp  .sp
1560  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
1561  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1562    or "defdef":
1563    .sp
1564      /(?|(abc)|(def))\e1/
1565    .sp
1566    In contrast, a subroutine call to a numbered subpattern always refers to the
1567    first one in the pattern with the given number. The following pattern matches
1568    "abcabc" or "defabc":
1569    .sp
1570      /(?|(abc)|(def))(?1)/
1571    .sp
1572    If a
1573    .\" HTML <a href="#conditions">
1574    .\" </a>
1575    condition test
1576    .\"
1577    for a subpattern's having matched refers to a non-unique number, the test is
1578    true if any of the subpatterns of that number have matched.
1579  .P  .P
1580  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1581  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 1590  if an expression is modified, the number
1590  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1591  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
1592  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
1593  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1594    have different names, but PCRE does not.
1595  .P  .P
1596  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
1597  (?'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
1598  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1599  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1600  .\" </a>  .\" </a>
1601  backreferences,  back references,
1602  .\"  .\"
1603  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1604  .\" </a>  .\" </a>
# Line 1179  conditions, Line 1611  conditions,
1611  .\"  .\"
1612  can be made by name as well as by number.  can be made by name as well as by number.
1613  .P  .P
1614  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores, but must
1615  capturing parentheses are still allocated numbers as well as names, exactly as  start with a non-digit. Named capturing parentheses are still allocated numbers
1616  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
1617  extracting the name-to-number translation table from a compiled pattern. There  provides function calls for extracting the name-to-number translation table
1618  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
1619    captured substring by name.
1620  .P  .P
1621  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
1622  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
1623  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
1624  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
1625  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
1626  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
1627    name, and in both cases you want to extract the abbreviation. This pattern
1628    (ignoring the line breaks) does the job:
1629  .sp  .sp
1630    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1631    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1204  subpattern, as described in the previous Line 1639  subpattern, as described in the previous
1639  .P  .P
1640  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1641  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
1642  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1643  make a reference to a non-unique named subpattern from elsewhere in the  .P
1644  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
1645  details of the interfaces for handling named subpatterns, see the  the pattern, the subpatterns to which the name refers are checked in the order
1646    in which they appear in the overall pattern. The first one that is set is used
1647    for the reference. For example, this pattern matches both "foofoo" and
1648    "barbar" but not "foobar" or "barfoo":
1649    .sp
1650      (?:(?<n>foo)|(?<n>bar))\k<n>
1651    .sp
1652    .P
1653    If you make a subroutine call to a non-unique named subpattern, the one that
1654    corresponds to the first occurrence of the name is used. In the absence of
1655    duplicate numbers (see the previous section) this is the one with the lowest
1656    number.
1657    .P
1658    If you use a named reference in a condition
1659    test (see the
1660    .\"
1661    .\" HTML <a href="#conditions">
1662    .\" </a>
1663    section about conditions
1664    .\"
1665    below), either to check whether a subpattern has matched, or to check for
1666    recursion, all subpatterns with the same name are tested. If the condition is
1667    true for any one of them, the overall condition is true. This is the same
1668    behaviour as testing by number. For further details of the interfaces for
1669    handling named subpatterns, see the
1670  .\" HREF  .\" HREF
1671  \fBpcreapi\fP  \fBpcreapi\fP
1672  .\"  .\"
1673  documentation.  documentation.
1674  .P  .P
1675  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1676  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1677  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1678    are given to subpatterns with the same number. However, you can always give the
1679    same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1680    set.
1681  .  .
1682  .  .
1683  .SH REPETITION  .SH REPETITION
# Line 1227  items: Line 1689  items:
1689    a literal data character    a literal data character
1690    the dot metacharacter    the dot metacharacter
1691    the \eC escape sequence    the \eC escape sequence
1692    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1693    the \eR escape sequence    the \eR escape sequence
1694    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1695    a character class    a character class
1696    a back reference (see next section)    a back reference (see next section)
1697    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1698      a subroutine call to a subpattern (recursive or otherwise)
1699  .sp  .sp
1700  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1701  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 1720  where a quantifier is not allowed, or on
1720  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
1721  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1722  .P  .P
1723  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
1724  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
1725  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,
1726  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
1727  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1728  .P  .P
1729  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
1730  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 1733  subpatterns that are referenced as
1733  .\" </a>  .\" </a>
1734  subroutines  subroutines
1735  .\"  .\"
1736  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1737  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1738    .\" </a>
1739    "Defining subpatterns for use by reference only"
1740    .\"
1741    below). Items other than subpatterns that have a {0} quantifier are omitted
1742    from the compiled pattern.
1743  .P  .P
1744  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1745  abbreviations:  abbreviations:
# Line 1342  In cases where it is known that the subj Line 1810  In cases where it is known that the subj
1810  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1811  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1812  .P  .P
1813  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1814  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1815  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
1816  succeeds. Consider, for example:  succeeds. Consider, for example:
1817  .sp  .sp
# Line 1352  succeeds. Consider, for example: Line 1820  succeeds. Consider, for example:
1820  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
1821  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1822  .P  .P
1823    Another case where implicit anchoring is not applied is when the leading .* is
1824    inside an atomic group. Once again, a match at the start may fail where a later
1825    one succeeds. Consider this pattern:
1826    .sp
1827      (?>.*?a)b
1828    .sp
1829    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1830    (*PRUNE) and (*SKIP) also disable this optimization.
1831    .P
1832  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1833  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1834  .sp  .sp
# Line 1496  no such problem when named parentheses a Line 1973  no such problem when named parentheses a
1973  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1974  .P  .P
1975  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
1976  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
1977  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
1978  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1979  .sp  .sp
1980    (ring), \e1    (ring), \e1
1981    (ring), \eg1    (ring), \eg1
# Line 1512  example: Line 1989  example:
1989    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1990  .sp  .sp
1991  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
1992  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.
1993  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
1994  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
1995  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1996  .P  .P
1997  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1998  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 2031  after the reference.
2031  .P  .P
2032  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
2033  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
2034  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
2035  .sp  .sp
2036    (a|(bc))\e2    (a|(bc))\e2
2037  .sp  .sp
2038  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
2039  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
2040  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
2041  with a digit character, some delimiter must be used to terminate the back  .P
2042  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
2043  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
2044    If the pattern continues with a digit character, some delimiter must be used to
2045    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2046    white space. Otherwise, the \eg{ syntax or an empty comment (see
2047  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
2048  .\" </a>  .\" </a>
2049  "Comments"  "Comments"
2050  .\"  .\"
2051  below) can be used.  below) can be used.
2052  .P  .
2053    .SS "Recursive back references"
2054    .rs
2055    .sp
2056  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
2057  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.
2058  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 2066  to the previous iteration. In order for
2066  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
2067  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
2068  minimum of zero.  minimum of zero.
2069    .P
2070    Back references of this type cause the group that they reference to be treated
2071    as an
2072    .\" HTML <a href="#atomicgroup">
2073    .\" </a>
2074    atomic group.
2075    .\"
2076    Once the whole group has been matched, a subsequent matching failure cannot
2077    cause backtracking into the middle of the group.
2078  .  .
2079  .  .
2080  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1602  those that look ahead of the current pos Line 2094  those that look ahead of the current pos
2094  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,
2095  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.
2096  .P  .P
2097  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2098  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
2099  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2100  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2101  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2102  because it does not make sense for negative assertions.  .P
2103    For compatibility with Perl, assertion subpatterns may be repeated; though
2104    it makes no sense to assert the same thing several times, the side effect of
2105    capturing parentheses may occasionally be useful. In practice, there only three
2106    cases:
2107    .sp
2108    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2109    However, it may contain internal capturing parenthesized groups that are called
2110    from elsewhere via the
2111    .\" HTML <a href="#subpatternsassubroutines">
2112    .\" </a>
2113    subroutine mechanism.
2114    .\"
2115    .sp
2116    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2117    were {0,1}. At run time, the rest of the pattern match is tried with and
2118    without the assertion, the order depending on the greediness of the quantifier.
2119    .sp
2120    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2121    The assertion is obeyed just once when encountered during matching.
2122  .  .
2123  .  .
2124  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1636  lookbehind assertion is needed to achiev Line 2147  lookbehind assertion is needed to achiev
2147  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
2148  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
2149  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.
2150    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2151  .  .
2152  .  .
2153  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1660  is permitted, but Line 2172  is permitted, but
2172  .sp  .sp
2173  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2174  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
2175  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
2176  match the same length of string. An assertion such as  length of string. An assertion such as
2177  .sp  .sp
2178    (?<=ab(c|de))    (?<=ab(c|de))
2179  .sp  .sp
2180  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
2181  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
2182    branches:
2183  .sp  .sp
2184    (?<=abc|abde)    (?<=abc|abde)
2185  .sp  .sp
2186  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2187  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2188  .\" </a>  .\" </a>
2189  (see above)  (see above)
2190  .\"  .\"
2191  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
2192  fixed-length.  restriction.
2193  .P  .P
2194  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2195  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
2196  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2197  assertion fails.  assertion fails.
2198  .P  .P
2199  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
2200  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
2201  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
2202  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2203    permitted.
2204    .P
2205    .\" HTML <a href="#subpatternsassubroutines">
2206    .\" </a>
2207    "Subroutine"
2208    .\"
2209    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2210    as the subpattern matches a fixed-length string.
2211    .\" HTML <a href="#recursion">
2212    .\" </a>
2213    Recursion,
2214    .\"
2215    however, is not supported.
2216  .P  .P
2217  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2218  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
2219  pattern such as  strings. Consider a simple pattern such as
2220  .sp  .sp
2221    abcd$    abcd$
2222  .sp  .sp
# Line 1754  characters that are not "999". Line 2280  characters that are not "999".
2280  .sp  .sp
2281  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2282  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2283  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2284  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2285  .sp  .sp
2286    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2287    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2288  .sp  .sp
2289  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2290  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
2291  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2292    itself contain nested subpatterns of any form, including conditional
2293    subpatterns; the restriction to two alternatives applies only at the level of
2294    the condition. This pattern fragment is an example where the alternatives are
2295    complex:
2296    .sp
2297      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2298    .sp
2299  .P  .P
2300  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2301  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 2304  recursion, a pseudo-condition called DEF
2304  .rs  .rs
2305  .sp  .sp
2306  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
2307  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2308  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
2309  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2310  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2311  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2312  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2313    section about duplicate subpattern numbers),
2314    .\"
2315    the condition is true if any of them have matched. An alternative notation is
2316    to precede the digits with a plus or minus sign. In this case, the subpattern
2317    number is relative rather than absolute. The most recently opened parentheses
2318    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2319    loops it can also make sense to refer to subsequent groups. The next
2320    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2321    zero in any of these forms is not used; it provokes a compile-time error.)
2322  .P  .P
2323  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2324  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 2329  three parts for ease of discussion:
2329  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2330  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
2331  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
2332  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2333  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,
2334  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
2335  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2336  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 2348  This makes the fragment independent of t
2348  .sp  .sp
2349  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2350  subpattern by name. For compatibility with earlier versions of PCRE, which had  subpattern by name. For compatibility with earlier versions of PCRE, which had
2351  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.  
2352  .P  .P
2353  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2354  .sp  .sp
2355    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2356  .sp  .sp
2357    If the name used in a condition of this kind is a duplicate, the test is
2358    applied to all subpatterns of the same name, and is true if any one of them has
2359    matched.
2360  .  .
2361  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2362  .rs  .rs
# Line 1828  letter R, for example: Line 2368  letter R, for example:
2368  .sp  .sp
2369    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2370  .sp  .sp
2371  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
2372  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
2373  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2374    applied to all subpatterns of the same name, and is true if any one of them is
2375    the most recent recursion.
2376  .P  .P
2377  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2378  patterns are described below.  .\" HTML <a href="#recursion">
2379    .\" </a>
2380    The syntax for recursive patterns
2381    .\"
2382    is described below.
2383  .  .
2384    .\" HTML <a name="subdefine"></a>
2385  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2386  .rs  .rs
2387  .sp  .sp
# Line 1842  If the condition is the string (DEFINE), Line 2389  If the condition is the string (DEFINE),
2389  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
2390  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2391  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
2392  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2393  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2394  written like this (ignore whitespace and line breaks):  .\" </a>
2395    subroutines
2396    .\"
2397    is described below.) For example, a pattern to match an IPv4 address such as
2398    "192.168.23.245" could be written like this (ignore white space and line
2399    breaks):
2400  .sp  .sp
2401    (?(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) )
2402    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1852  written like this (ignore whitespace and Line 2404  written like this (ignore whitespace and
2404  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
2405  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2406  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
2407  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2408  .P  pattern uses references to the named group to match the four dot-separated
2409  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.  
2410  .  .
2411  .SS "Assertion conditions"  .SS "Assertion conditions"
2412  .rs  .rs
# Line 1881  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2431  dd-aaa-dd or dd-dd-dd, where aaa are let
2431  .SH COMMENTS  .SH COMMENTS
2432  .rs  .rs
2433  .sp  .sp
2434  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
2435  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,
2436  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
2437    subpattern name or number. The characters that make up a comment play no part
2438    in the pattern matching.
2439  .P  .P
2440  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
2441  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2442  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2443    this case continues to immediately after the next newline character or
2444    character sequence in the pattern. Which characters are interpreted as newlines
2445    is controlled by the options passed to a compiling function or by a special
2446    sequence at the start of the pattern, as described in the section entitled
2447    .\" HTML <a href="#newlines">
2448    .\" </a>
2449    "Newline conventions"
2450    .\"
2451    above. Note that the end of this type of comment is a literal newline sequence
2452    in the pattern; escape sequences that happen to represent a newline do not
2453    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2454    default newline convention is in force:
2455    .sp
2456      abc #comment \en still comment
2457    .sp
2458    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2459    a newline in the pattern. The sequence \en is still literal at this stage, so
2460    it does not terminate the comment. Only an actual character with the code value
2461    0x0a (the default newline) does so.
2462  .  .
2463  .  .
2464  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1913  recursively to the pattern in which it a Line 2484  recursively to the pattern in which it a
2484  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2485  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2486  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2487  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.
2488  .P  .P
2489  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
2490  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
2491  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
2492    .\" HTML <a href="#subpatternsassubroutines">
2493    .\" </a>
2494    non-recursive subroutine
2495    .\"
2496  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
2497  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2498  .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  
2499  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2500  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2501  .sp  .sp
2502    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2503  .sp  .sp
2504  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2505  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
2506  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2507  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2508    to avoid backtracking into sequences of non-parentheses.
2509  .P  .P
2510  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
2511  pattern, so instead you could use this:  pattern, so instead you could use this:
2512  .sp  .sp
2513    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2514  .sp  .sp
2515  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
2516  them instead of the whole pattern.  them instead of the whole pattern.
2517  .P  .P
2518  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
2519  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
2520  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
2521  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2522  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.  
2523  .P  .P
2524  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2525  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2526  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2527  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2528    .\" </a>
2529    non-recursive subroutine
2530    .\"
2531    calls, as described in the next section.
2532  .P  .P
2533  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2534  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
2535  could rewrite the above example as follows:  could rewrite the above example as follows:
2536  .sp  .sp
2537    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2538  .sp  .sp
2539  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
2540  used.  used.
2541  .P  .P
2542  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2543  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
2544  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
2545  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2546  .sp  .sp
2547    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2548  .sp  .sp
2549  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,
2550  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
2551  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
2552  before failure can be reported.  before failure can be reported.
2553  .P  .P
2554  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
2555  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
2556  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  
2557  .\" HREF  .\" HREF
2558  \fBpcrecallout\fP  \fBpcrecallout\fP
2559  .\"  .\"
# Line 1988  documentation). If the pattern above is Line 2561  documentation). If the pattern above is
2561  .sp  .sp
2562    (ab(cd)ef)    (ab(cd)ef)
2563  .sp  .sp
2564  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
2565  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
2566  .sp  matched at the top level, its final captured value is unset, even if it was
2567    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2568       ^                        ^  .P
2569       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2570  .sp  obtain extra memory to store data during a recursion, which it does by using
2571  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
2572  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.  
2573  .P  .P
2574  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.
2575  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 2583  different alternatives for the recursive
2583  is the actual recursive call.  is the actual recursive call.
2584  .  .
2585  .  .
2586    .\" HTML <a name="recursiondifference"></a>
2587    .SS "Differences in recursion processing between PCRE and Perl"
2588    .rs
2589    .sp
2590    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2591    (like Python, but unlike Perl), a recursive subpattern call is always treated
2592    as an atomic group. That is, once it has matched some of the subject string, it
2593    is never re-entered, even if it contains untried alternatives and there is a
2594    subsequent matching failure. This can be illustrated by the following pattern,
2595    which purports to match a palindromic string that contains an odd number of
2596    characters (for example, "a", "aba", "abcba", "abcdcba"):
2597    .sp
2598      ^(.|(.)(?1)\e2)$
2599    .sp
2600    The idea is that it either matches a single character, or two identical
2601    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2602    it does not if the pattern is longer than three characters. Consider the
2603    subject string "abcba":
2604    .P
2605    At the top level, the first character is matched, but as it is not at the end
2606    of the string, the first alternative fails; the second alternative is taken
2607    and the recursion kicks in. The recursive call to subpattern 1 successfully
2608    matches the next character ("b"). (Note that the beginning and end of line
2609    tests are not part of the recursion).
2610    .P
2611    Back at the top level, the next character ("c") is compared with what
2612    subpattern 2 matched, which was "a". This fails. Because the recursion is
2613    treated as an atomic group, there are now no backtracking points, and so the
2614    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2615    try the second alternative.) However, if the pattern is written with the
2616    alternatives in the other order, things are different:
2617    .sp
2618      ^((.)(?1)\e2|.)$
2619    .sp
2620    This time, the recursing alternative is tried first, and continues to recurse
2621    until it runs out of characters, at which point the recursion fails. But this
2622    time we do have another alternative to try at the higher level. That is the big
2623    difference: in the previous case the remaining alternative is at a deeper
2624    recursion level, which PCRE cannot use.
2625    .P
2626    To change the pattern so that it matches all palindromic strings, not just
2627    those with an odd number of characters, it is tempting to change the pattern to
2628    this:
2629    .sp
2630      ^((.)(?1)\e2|.?)$
2631    .sp
2632    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2633    deeper recursion has matched a single character, it cannot be entered again in
2634    order to match an empty string. The solution is to separate the two cases, and
2635    write out the odd and even cases as alternatives at the higher level:
2636    .sp
2637      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2638    .sp
2639    If you want to match typical palindromic phrases, the pattern has to ignore all
2640    non-word characters, which can be done like this:
2641    .sp
2642      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2643    .sp
2644    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2645    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2646    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2647    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2648    more) to match typical phrases, and Perl takes so long that you think it has
2649    gone into a loop.
2650    .P
2651    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2652    string does not start with a palindrome that is shorter than the entire string.
2653    For example, although "abcba" is correctly matched, if the subject is "ababa",
2654    PCRE finds the palindrome "aba" at the start, then fails at top level because
2655    the end of the string does not follow. Once again, it cannot jump back into the
2656    recursion to try other alternatives, so the entire match fails.
2657    .P
2658    The second way in which PCRE and Perl differ in their recursion processing is
2659    in the handling of captured values. In Perl, when a subpattern is called
2660    recursively or as a subpattern (see the next section), it has no access to any
2661    values that were captured outside the recursion, whereas in PCRE these values
2662    can be referenced. Consider this pattern:
2663    .sp
2664      ^(.)(\e1|a(?2))
2665    .sp
2666    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2667    then in the second group, when the back reference \e1 fails to match "b", the
2668    second alternative matches "a" and then recurses. In the recursion, \e1 does
2669    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2670    match because inside the recursive call \e1 cannot access the externally set
2671    value.
2672    .
2673    .
2674  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2675  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2676  .rs  .rs
2677  .sp  .sp
2678  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
2679  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
2680  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2681  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2682  relative, as in these examples:  relative, as in these examples:
2683  .sp  .sp
# Line 2039  matches "sense and sensibility" and "res Line 2697  matches "sense and sensibility" and "res
2697  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
2698  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2699  .P  .P
2700  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2701  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
2702  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
2703  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2704  .P  subroutine call revert to their previous values afterwards.
2705  When a subpattern is used as a subroutine, processing options such as  .P
2706  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2707  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2708    different calls. For example, consider this pattern:
2709  .sp  .sp
2710    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2711  .sp  .sp
# Line 2085  same pair of parentheses when there is a Line 2744  same pair of parentheses when there is a
2744  .P  .P
2745  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
2746  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
2747  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
2748    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2749  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2750  .P  .P
2751  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 2755  For example, this pattern has two callou
2755  .sp  .sp
2756    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2757  .sp  .sp
2758  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
2759  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2760  255.  255. If there is a conditional group in the pattern whose condition is an
2761  .P  assertion, an additional callout is inserted just before the condition. An
2762  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:
2763  set), the external function is called. It is provided with the number of the  .sp
2764  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2765  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2766  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
2767  description of the interface to the callout function is given in the  condition.
2768    .P
2769    During matching, when PCRE reaches a callout point, the external function is
2770    called. It is provided with the number of the callout, the position in the
2771    pattern, and, optionally, one item of data originally supplied by the caller of
2772    the matching function. The callout function may cause matching to proceed, to
2773    backtrack, or to fail altogether.
2774    .P
2775    By default, PCRE implements a number of optimizations at compile time and
2776    matching time, and one side-effect is that sometimes callouts are skipped. If
2777    you need all possible callouts to happen, you need to set options that disable
2778    the relevant optimizations. More details, and a complete description of the
2779    interface to the callout function, are given in the
2780  .\" HREF  .\" HREF
2781  \fBpcrecallout\fP  \fBpcrecallout\fP
2782  .\"  .\"
2783  documentation.  documentation.
2784  .  .
2785  .  .
2786    .\" HTML <a name="backtrackcontrol"></a>
2787  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2788  .rs  .rs
2789  .sp  .sp
2790  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2791  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2792  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
2793  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
2794  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2795  .P  .P
2796    The new verbs make use of what was previously invalid syntax: an opening
2797    parenthesis followed by an asterisk. They are generally of the form
2798    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2799    differently depending on whether or not a name is present. A name is any
2800    sequence of characters that does not include a closing parenthesis. The maximum
2801    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2802    libraries. If the name is empty, that is, if the closing parenthesis
2803    immediately follows the colon, the effect is as if the colon were not there.
2804    Any number of these verbs may occur in a pattern.
2805    .P
2806  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
2807  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
2808  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, because these use a backtracking algorithm. With the
2809  failing negative assertion, they cause an error if encountered by  exception of (*FAIL), which behaves like a failing negative assertion, the
2810  \fBpcre_dfa_exec()\fP.  backtracking control verbs cause an error if encountered by a DFA matching
2811    function.
2812  .P  .P
2813  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2814  parenthesis followed by an asterisk. In Perl, they are generally of the form  .\" HTML <a href="#btrepeat">
2815  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  .\" </a>
2816  form is just (*VERB). Any number of these verbs may occur in a pattern. There  repeated groups,
2817  are two kinds:  .\"
2818    .\" HTML <a href="#btassert">
2819    .\" </a>
2820    assertions,
2821    .\"
2822    and in
2823    .\" HTML <a href="#btsub">
2824    .\" </a>
2825    subpatterns called as subroutines
2826    .\"
2827    (whether or not recursively) is documented below.
2828    .
2829    .
2830    .\" HTML <a name="nooptimize"></a>
2831    .SS "Optimizations that affect backtracking verbs"
2832    .rs
2833    .sp
2834    PCRE contains some optimizations that are used to speed up matching by running
2835    some checks at the start of each match attempt. For example, it may know the
2836    minimum length of matching subject, or that a particular character must be
2837    present. When one of these optimizations bypasses the running of a match, any
2838    included backtracking verbs will not, of course, be processed. You can suppress
2839    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2840    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2841    pattern with (*NO_START_OPT). There is more discussion of this option in the
2842    section entitled
2843    .\" HTML <a href="pcreapi.html#execoptions">
2844    .\" </a>
2845    "Option bits for \fBpcre_exec()\fP"
2846    .\"
2847    in the
2848    .\" HREF
2849    \fBpcreapi\fP
2850    .\"
2851    documentation.
2852    .P
2853    Experiments with Perl suggest that it too has similar optimizations, sometimes
2854    leading to anomalous results.
2855    .
2856  .  .
2857  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2858  .rs  .rs
2859  .sp  .sp
2860  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2861    followed by a name.
2862  .sp  .sp
2863     (*ACCEPT)     (*ACCEPT)
2864  .sp  .sp
2865  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
2866  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2867  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  subroutine, only that subpattern is ended successfully. Matching then continues
2868  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
2869  captured. For example:  assertion succeeds; in a negative assertion, the assertion fails.
2870    .P
2871    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2872    example:
2873  .sp  .sp
2874    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2875  .sp  .sp
2876  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
2877  captured.  the outer parentheses.
2878  .sp  .sp
2879    (*FAIL) or (*F)    (*FAIL) or (*F)
2880  .sp  .sp
2881  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
2882  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
2883  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2884  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 2889  callout feature, as for example in this
2889  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
2890  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2891  .  .
2892    .
2893    .SS "Recording which path was taken"
2894    .rs
2895    .sp
2896    There is one verb whose main purpose is to track how a match was arrived at,
2897    though it also has a secondary use in conjunction with advancing the match
2898    starting point (see (*SKIP) below).
2899    .sp
2900      (*MARK:NAME) or (*:NAME)
2901    .sp
2902    A name is always required with this verb. There may be as many instances of
2903    (*MARK) as you like in a pattern, and their names do not have to be unique.
2904    .P
2905    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2906    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2907    caller as described in the section entitled
2908    .\" HTML <a href="pcreapi.html#extradata">
2909    .\" </a>
2910    "Extra data for \fBpcre_exec()\fP"
2911    .\"
2912    in the
2913    .\" HREF
2914    \fBpcreapi\fP
2915    .\"
2916    documentation. Here is an example of \fBpcretest\fP output, where the /K
2917    modifier requests the retrieval and outputting of (*MARK) data:
2918    .sp
2919        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2920      data> XY
2921       0: XY
2922      MK: A
2923      XZ
2924       0: XZ
2925      MK: B
2926    .sp
2927    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2928    indicates which of the two alternatives matched. This is a more efficient way
2929    of obtaining this information than putting each alternative in its own
2930    capturing parentheses.
2931    .P
2932    If a verb with a name is encountered in a positive assertion that is true, the
2933    name is recorded and passed back if it is the last-encountered. This does not
2934    happen for negative assertions or failing positive assertions.
2935    .P
2936    After a partial match or a failed match, the last encountered name in the
2937    entire match process is returned. For example:
2938    .sp
2939        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2940      data> XP
2941      No match, mark = B
2942    .sp
2943    Note that in this unanchored example the mark is retained from the match
2944    attempt that started at the letter "X" in the subject. Subsequent match
2945    attempts starting at "P" and then with an empty string do not get as far as the
2946    (*MARK) item, but nevertheless do not reset it.
2947    .P
2948    If you are interested in (*MARK) values after failed matches, you should
2949    probably set the PCRE_NO_START_OPTIMIZE option
2950    .\" HTML <a href="#nooptimize">
2951    .\" </a>
2952    (see above)
2953    .\"
2954    to ensure that the match is always attempted.
2955    .
2956    .
2957  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2958  .rs  .rs
2959  .sp  .sp
2960  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2961  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
2962  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
2963    the verb. However, when one of these verbs appears inside an atomic group or an
2964    assertion that is true, its effect is confined to that group, because once the
2965    group has been matched, there is never any backtracking into it. In this
2966    situation, backtracking can "jump back" to the left of the entire atomic group
2967    or assertion. (Remember also, as stated above, that this localization also
2968    applies in subroutine calls.)
2969    .P
2970    These verbs differ in exactly what kind of failure occurs when backtracking
2971    reaches them. The behaviour described below is what happens when the verb is
2972    not in a subroutine or an assertion. Subsequent sections cover these special
2973    cases.
2974  .sp  .sp
2975    (*COMMIT)    (*COMMIT)
2976  .sp  .sp
2977  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
2978  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
2979  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
2980  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
2981  starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2982    committed to finding a match at the current starting point, or not at all. For
2983    example:
2984  .sp  .sp
2985    a+(*COMMIT)b    a+(*COMMIT)b
2986  .sp  .sp
2987  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
2988  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
2989  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2990    (*PRUNE)  match failure.
2991  .sp  .P
2992  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
2993  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2994  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.
2995  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .P
2996  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,
2997  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
2998  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot  \fBpcretest\fP example:
2999  be expressed in any other way.  .sp
3000        re> /(*COMMIT)abc/
3001      data> xyzabc
3002       0: abc
3003      xyzabc\eY
3004      No match
3005    .sp
3006    PCRE knows that any match must start with "a", so the optimization skips along
3007    the subject to "a" before running the first match attempt, which succeeds. When
3008    the optimization is disabled by the \eY escape in the second subject, the match
3009    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
3010    starting points.
3011    .sp
3012      (*PRUNE) or (*PRUNE:NAME)
3013    .sp
3014    This verb causes the match to fail at the current starting position in the
3015    subject if there is a later matching failure that causes backtracking to reach
3016    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3017    starting character then happens. Backtracking can occur as usual to the left of
3018    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3019    if there is no match to the right, backtracking cannot cross (*PRUNE). In
3020    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3021    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3022    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3023    as (*COMMIT).
3024    .P
3025    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3026    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3027    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3028  .sp  .sp
3029    (*SKIP)    (*SKIP)
3030  .sp  .sp
3031  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
3032  "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,
3033  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3034  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
3035    successful match. Consider:
3036  .sp  .sp
3037    a+(*SKIP)b    a+(*SKIP)b
3038  .sp  .sp
3039  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
3040  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
3041  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
3042  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
3043  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
3044  instead of skipping on to "c".  instead of skipping on to "c".
3045  .sp  .sp
3046    (*THEN)    (*SKIP:NAME)
3047  .sp  .sp
3048  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
3049  not match. That is, it cancels pending backtracking, but only within the  triggered, the previous path through the pattern is searched for the most
3050  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
3051  for a pattern-based if-then-else block:  is to the subject position that corresponds to that (*MARK) instead of to where
3052    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3053    (*SKIP) is ignored.
3054    .P
3055    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3056    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3057    .sp
3058      (*THEN) or (*THEN:NAME)
3059    .sp
3060    This verb causes a skip to the next innermost alternative when backtracking
3061    reaches it. That is, it cancels any further backtracking within the current
3062    alternative. Its name comes from the observation that it can be used for a
3063    pattern-based if-then-else block:
3064  .sp  .sp
3065    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3066  .sp  .sp
3067  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
3068  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
3069  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. If that
3070  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
3071    more alternatives, so there is a backtrack to whatever came before the entire
3072    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3073    .P
3074    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3075    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3076    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3077    .P
3078    A subpattern that does not contain a | character is just a part of the
3079    enclosing alternative; it is not a nested alternation with only one
3080    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3081    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3082    pattern fragments that do not contain any | characters at this level:
3083    .sp
3084      A (B(*THEN)C) | D
3085    .sp
3086    If A and B are matched, but there is a failure in C, matching does not
3087    backtrack into A; instead it moves to the next alternative, that is, D.
3088    However, if the subpattern containing (*THEN) is given an alternative, it
3089    behaves differently:
3090    .sp
3091      A (B(*THEN)C | (*FAIL)) | D
3092    .sp
3093    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3094    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3095    because there are no more alternatives to try. In this case, matching does now
3096    backtrack into A.
3097    .P
3098    Note that a conditional subpattern is not considered as having two
3099    alternatives, because only one is ever used. In other words, the | character in
3100    a conditional subpattern has a different meaning. Ignoring white space,
3101    consider:
3102    .sp
3103      ^.*? (?(?=a) a | b(*THEN)c )
3104    .sp
3105    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3106    it initially matches zero characters. The condition (?=a) then fails, the
3107    character "b" is matched, but "c" is not. At this point, matching does not
3108    backtrack to .*? as might perhaps be expected from the presence of the |
3109    character. The conditional subpattern is part of the single alternative that
3110    comprises the whole pattern, and so the match fails. (If there was a backtrack
3111    into .*?, allowing it to match "b", the match would succeed.)
3112    .P
3113    The verbs just described provide four different "strengths" of control when
3114    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3115    next alternative. (*PRUNE) comes next, failing the match at the current
3116    starting position, but allowing an advance to the next character (for an
3117    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3118    than one character. (*COMMIT) is the strongest, causing the entire match to
3119    fail.
3120    .
3121    .
3122    .SS "More than one backtracking verb"
3123    .rs
3124    .sp
3125    If more than one backtracking verb is present in a pattern, the one that is
3126    backtracked onto first acts. For example, consider this pattern, where A, B,
3127    etc. are complex pattern fragments:
3128    .sp
3129      (A(*COMMIT)B(*THEN)C|ABD)
3130    .sp
3131    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3132    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3133    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3134    not always the same as Perl's. It means that if two or more backtracking verbs
3135    appear in succession, all the the last of them has no effect. Consider this
3136    example:
3137    .sp
3138      ...(*COMMIT)(*PRUNE)...
3139    .sp
3140    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3141    it to be triggered, and its action is taken. There can never be a backtrack
3142    onto (*COMMIT).
3143    .
3144    .
3145    .\" HTML <a name="btrepeat"></a>
3146    .SS "Backtracking verbs in repeated groups"
3147    .rs
3148    .sp
3149    PCRE differs from Perl in its handling of backtracking verbs in repeated
3150    groups. For example, consider:
3151    .sp
3152      /(a(*COMMIT)b)+ac/
3153    .sp
3154    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3155    the second repeat of the group acts.
3156    .
3157    .
3158    .\" HTML <a name="btassert"></a>
3159    .SS "Backtracking verbs in assertions"
3160    .rs
3161    .sp
3162    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3163    .P
3164    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3165    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3166    fail without any further processing.
3167    .P
3168    The other backtracking verbs are not treated specially if they appear in a
3169    positive assertion. In particular, (*THEN) skips to the next alternative in the
3170    innermost enclosing group that has alternations, whether or not this is within
3171    the assertion.
3172    .P
3173    Negative assertions are, however, different, in order to ensure that changing a
3174    positive assertion into a negative assertion changes its result. Backtracking
3175    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3176    without considering any further alternative branches in the assertion.
3177    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3178    within the assertion (the normal behaviour), but if the assertion does not have
3179    such an alternative, (*THEN) behaves like (*PRUNE).
3180    .
3181    .
3182    .\" HTML <a name="btsub"></a>
3183    .SS "Backtracking verbs in subroutines"
3184    .rs
3185    .sp
3186    These behaviours occur whether or not the subpattern is called recursively.
3187    Perl's treatment of subroutines is different in some cases.
3188    .P
3189    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3190    an immediate backtrack.
3191    .P
3192    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3193    succeed without any further processing. Matching then continues after the
3194    subroutine call.
3195    .P
3196    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3197    the subroutine match to fail.
3198    .P
3199    (*THEN) skips to the next alternative in the innermost enclosing group within
3200    the subpattern that has alternatives. If there is no such group within the
3201    subpattern, (*THEN) causes the subroutine match to fail.
3202  .  .
3203  .  .
3204  .SH "SEE ALSO"  .SH "SEE ALSO"
3205  .rs  .rs
3206  .sp  .sp
3207  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3208    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3209  .  .
3210  .  .
3211  .SH AUTHOR  .SH AUTHOR
# Line 2245  Cambridge CB2 3QH, England. Line 3222  Cambridge CB2 3QH, England.
3222  .rs  .rs
3223  .sp  .sp
3224  .nf  .nf
3225  Last updated: 18 March 2009  Last updated: 09 November 2013
3226  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3227  .fi  .fi

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