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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "05 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"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
 regular expressions in great detail. This description of PCRE's regular  
 expressions is intended as reference material.  
 .P  
 The original operation of PCRE was on strings of one-byte characters. However,  
 there is now also support for UTF-8 character strings. To use this, you must  
 build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  
 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  
 places below. There is also a summary of UTF-8 features in the  
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
11  .\"  .\"
12  in the main  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22    .P
23    This document discusses the patterns that are supported by PCRE when one its
24    main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25    (16- or 32-bit), is used. PCRE also has alternative matching functions,
26    \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27    different algorithm that is not Perl-compatible. Some of the features discussed
28    below are not available when DFA matching is used. The advantages and
29    disadvantages of the alternative functions, and how they differ from the normal
30    functions, are discussed in the
31  .\" HREF  .\" HREF
32  \fBpcre\fP  \fBpcrematching\fP
33  .\"  .\"
34  page.  page.
35  .P  .
36  The remainder of this document discusses the patterns that are supported by  .
37  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  .SH "SPECIAL START-OF-PATTERN ITEMS"
38  From release 6.0, PCRE offers a second matching function,  .rs
39  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  .sp
40  Perl-compatible. The advantages and disadvantages of the alternative function,  A number of options that can be passed to \fBpcre_compile()\fP can also be set
41  and how it differs from the normal function, are discussed in the  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  \fBpcrematching\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73  .P  .P
74    Some applications that allow their users to supply patterns may wish to
75    restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76    option is set at compile time, (*UTF) etc. are not allowed, and their
77    appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82    .sp
83    Another special sequence that may appear at the start of a pattern is
84    .sp
85      (*UCP)
86    .sp
87    This has the same effect as setting the PCRE_UCP option: it causes sequences
88    such as \ed and \ew to use Unicode properties to determine character types,
89    instead of recognizing only characters with codes less than 128 via a lookup
90    table.
91    .
92    .
93    .SS "Disabling start-up optimizations"
94    .rs
95    .sp
96    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
97    PCRE_NO_START_OPTIMIZE option either at compile or matching time.
98    .
99    .
100    .\" HTML <a name="newlines"></a>
101    .SS "Newline conventions"
102    .rs
103    .sp
104    PCRE supports five different conventions for indicating line breaks in
105    strings: a single CR (carriage return) character, a single LF (linefeed)
106    character, the two-character sequence CRLF, any of the three preceding, or any
107    Unicode newline sequence. The
108    .\" HREF
109    \fBpcreapi\fP
110    .\"
111    page has
112    .\" HTML <a href="pcreapi.html#newlines">
113    .\" </a>
114    further discussion
115    .\"
116    about newlines, and shows how to set the newline convention in the
117    \fIoptions\fP arguments for the compiling and matching functions.
118    .P
119    It is also possible to specify a newline convention by starting a pattern
120    string with one of the following five sequences:
121    .sp
122      (*CR)        carriage return
123      (*LF)        linefeed
124      (*CRLF)      carriage return, followed by linefeed
125      (*ANYCRLF)   any of the three above
126      (*ANY)       all Unicode newline sequences
127    .sp
128    These override the default and the options given to the compiling function. For
129    example, on a Unix system where LF is the default newline sequence, the pattern
130    .sp
131      (*CR)a.b
132    .sp
133    changes the convention to CR. That pattern matches "a\enb" because LF is no
134    longer a newline. If more than one of these settings is present, the last one
135    is used.
136    .P
137    The newline convention affects where the circumflex and dollar assertions are
138    true. It also affects the interpretation of the dot metacharacter when
139    PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
140    what the \eR escape sequence matches. By default, this is any Unicode newline
141    sequence, for Perl compatibility. However, this can be changed; see the
142    description of \eR in the section entitled
143    .\" HTML <a href="#newlineseq">
144    .\" </a>
145    "Newline sequences"
146    .\"
147    below. A change of \eR setting can be combined with a change of newline
148    convention.
149    .
150    .
151    .SS "Setting match and recursion limits"
152    .rs
153    .sp
154    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
155    internal \fBmatch()\fP function is called and on the maximum depth of
156    recursive calls. These facilities are provided to catch runaway matches that
157    are provoked by patterns with huge matching trees (a typical example is a
158    pattern with nested unlimited repeats) and to avoid running out of system stack
159    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
160    gives an error return. The limits can also be set by items at the start of the
161    pattern of the form
162    .sp
163      (*LIMIT_MATCH=d)
164      (*LIMIT_RECURSION=d)
165    .sp
166    where d is any number of decimal digits. However, the value of the setting must
167    be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
168    for it to have any effect. In other words, the pattern writer can lower the
169    limits set by the programmer, but not raise them. If there is more than one
170    setting of one of these limits, the lower value is used.
171    .
172    .
173    .SH "EBCDIC CHARACTER CODES"
174    .rs
175    .sp
176    PCRE can be compiled to run in an environment that uses EBCDIC as its character
177    code rather than ASCII or Unicode (typically a mainframe system). In the
178    sections below, character code values are ASCII or Unicode; in an EBCDIC
179    environment these characters may have different code values, and there are no
180    code points greater than 255.
181    .
182    .
183    .SH "CHARACTERS AND METACHARACTERS"
184    .rs
185    .sp
186  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
187  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
188  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 45  corresponding characters in the subject. Line 191  corresponding characters in the subject.
191  .sp  .sp
192  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
193  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
194  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
195  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
196  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
197  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
198  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
199  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
200  UTF-8 support.  UTF support.
201  .P  .P
202  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
203  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 60  interpreted in some special way. Line 206  interpreted in some special way.
206  .P  .P
207  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
208  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
209  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
210  as follows:  are as follows:
211  .sp  .sp
212    \e      general escape character with several uses    \e      general escape character with several uses
213    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 238  a character class the only metacharacter
238  .sp  .sp
239  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
240  .  .
241    .
242  .SH BACKSLASH  .SH BACKSLASH
243  .rs  .rs
244  .sp  .sp
245  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
246  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
247  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
248  outside character classes.  both inside and outside character classes.
249  .P  .P
250  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.
251  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 106  otherwise be interpreted as a metacharac Line 253  otherwise be interpreted as a metacharac
253  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
254  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
255  .P  .P
256  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
257    backslash. All other characters (in particular, those whose codepoints are
258    greater than 127) are treated as literals.
259    .P
260    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
261  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
262  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
263  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
264  .P  .P
265  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
266  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 125  Perl, $ and @ cause variable interpolati Line 276  Perl, $ and @ cause variable interpolati
276    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
277  .sp  .sp
278  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
279    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
280    by \eE later in the pattern, the literal interpretation continues to the end of
281    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
282    a character class, this causes an error, because the character class is not
283    terminated.
284  .  .
285  .  .
286  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 134  The \eQ...\eE sequence is recognized bot Line 290  The \eQ...\eE sequence is recognized bot
290  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
291  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
292  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
293  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
294  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:  
295  .sp  .sp
296    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
297    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
298    \ee        escape (hex 1B)    \ee        escape (hex 1B)
299    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
300    \en        newline (hex 0A)    \en        linefeed (hex 0A)
301    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
302    \et        tab (hex 09)    \et        tab (hex 09)
303    \eddd      character with octal code ddd, or backreference    \e0dd      character with octal code 0dd
304      \eddd      character with octal code ddd, or back reference
305      \eo{ddd..} character with octal code ddd..
306    \exhh      character with hex code hh    \exhh      character with hex code hh
307    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
308      \euhhhh    character with hex code hhhh (JavaScript mode only)
309  .sp  .sp
310  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
311  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
312  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),
313  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
314  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
315  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.
316  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
317  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
318  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  extension to Unicode it is even less useful than it once was. It is, however,
319  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  recognized when PCRE is compiled in EBCDIC mode, where data items are always
320  and }, or if there is no terminating }, this form of escape is not recognized.  bytes. In this mode, all values are valid after \ec. If the next character is a
321  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
322  with no following digits, giving a character whose value is zero.  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
323  .P  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
324  Characters whose value is less than 256 can be defined by either of the two  characters also generate different values.
 syntaxes for \ex. There is no difference in the way they are handled. For  
 example, \exdc is exactly the same as \ex{dc}.  
325  .P  .P
326  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
327  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 173  specifies two binary zeros followed by a Line 329  specifies two binary zeros followed by a
329  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
330  follows is itself an octal digit.  follows is itself an octal digit.
331  .P  .P
332  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
333  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
334  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
335    numbers greater than 0777, and it also allows octal numbers and back references
336    to be unambiguously specified.
337    .P
338    For greater clarity and unambiguity, it is best to avoid following \e by a
339    digit greater than zero. Instead, use \eo{} or \ex{} to specify character
340    numbers, and \eg{} to specify back references. The following paragraphs
341    describe the old, ambiguous syntax.
342    .P
343    The handling of a backslash followed by a digit other than 0 is complicated,
344    and Perl has changed in recent releases, causing PCRE also to change. Outside a
345    character class, PCRE reads the digit and any following digits as a decimal
346    number. If the number is less than 8, or if there have been at least that many
347  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
348  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
349  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
# Line 188  following the discussion of Line 356  following the discussion of
356  parenthesized subpatterns.  parenthesized subpatterns.
357  .\"  .\"
358  .P  .P
359  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
360  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
361  digits following the backslash, ane uses them to generate a data character. Any  \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
362  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.
363  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:  
364  .sp  .sp
365    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
366  .\" JOIN  .\" JOIN
367    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
368              previous capturing subpatterns              previous capturing subpatterns
# Line 210  to \e777 are permitted. For example: Line 377  to \e777 are permitted. For example:
377              character with octal code 113              character with octal code 113
378  .\" JOIN  .\" JOIN
379    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
380              the byte consisting entirely of 1 bits              the value 255 (decimal)
381  .\" JOIN  .\" JOIN
382    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or the two
383              followed by the two characters "8" and "1"              characters "8" and "1"
384  .sp  .sp
385  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
386  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
387    digits are ever read.
388    .P
389    By default, after \ex that is not followed by {, from zero to two hexadecimal
390    digits are read (letters can be in upper or lower case). Any number of
391    hexadecimal digits may appear between \ex{ and }. If a character other than
392    a hexadecimal digit appears between \ex{ and }, or if there is no terminating
393    }, an error occurs.
394    .P
395    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
396    as just described only when it is followed by two hexadecimal digits.
397    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
398    code points greater than 256 is provided by \eu, which must be followed by
399    four hexadecimal digits; otherwise it matches a literal "u" character.
400  .P  .P
401    Characters whose value is less than 256 can be defined by either of the two
402    syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
403    way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
404    \eu00dc in JavaScript mode).
405    .
406    .
407    .SS "Constraints on character values"
408    .rs
409    .sp
410    Characters that are specified using octal or hexadecimal numbers are
411    limited to certain values, as follows:
412    .sp
413      8-bit non-UTF mode    less than 0x100
414      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
415      16-bit non-UTF mode   less than 0x10000
416      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
417      32-bit non-UTF mode   less than 0x100000000
418      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
419    .sp
420    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
421    "surrogate" codepoints), and 0xffef.
422    .
423    .
424    .SS "Escape sequences in character classes"
425    .rs
426    .sp
427  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
428  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
429  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
430  sequence \eX is interpreted as the character "X". Outside a character class,  .P
431  these sequences have different meanings  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
432  .\" HTML <a href="#uniextseq">  inside a character class. Like other unrecognized escape sequences, they are
433    treated as the literal characters "B", "R", and "X" by default, but cause an
434    error if the PCRE_EXTRA option is set. Outside a character class, these
435    sequences have different meanings.
436    .
437    .
438    .SS "Unsupported escape sequences"
439    .rs
440    .sp
441    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
442    handler and used to modify the case of following characters. By default, PCRE
443    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
444    option is set, \eU matches a "U" character, and \eu can be used to define a
445    character by code point, as described in the previous section.
446    .
447    .
448    .SS "Absolute and relative back references"
449    .rs
450    .sp
451    The sequence \eg followed by an unsigned or a negative number, optionally
452    enclosed in braces, is an absolute or relative back reference. A named back
453    reference can be coded as \eg{name}. Back references are discussed
454    .\" HTML <a href="#backreferences">
455  .\" </a>  .\" </a>
456  (see below).  later,
457    .\"
458    following the discussion of
459    .\" HTML <a href="#subpattern">
460    .\" </a>
461    parenthesized subpatterns.
462  .\"  .\"
463  .  .
464  .  .
465    .SS "Absolute and relative subroutine calls"
466    .rs
467    .sp
468    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
469    a number enclosed either in angle brackets or single quotes, is an alternative
470    syntax for referencing a subpattern as a "subroutine". Details are discussed
471    .\" HTML <a href="#onigurumasubroutines">
472    .\" </a>
473    later.
474    .\"
475    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
476    synonymous. The former is a back reference; the latter is a
477    .\" HTML <a href="#subpatternsassubroutines">
478    .\" </a>
479    subroutine
480    .\"
481    call.
482    .
483    .
484    .\" HTML <a name="genericchartypes"></a>
485  .SS "Generic character types"  .SS "Generic character types"
486  .rs  .rs
487  .sp  .sp
488  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
489  .sp  .sp
490    \ed     any decimal digit    \ed     any decimal digit
491    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
492    \es     any whitespace character    \eh     any horizontal white space character
493    \eS     any character that is not a whitespace character    \eH     any character that is not a horizontal white space character
494      \es     any white space character
495      \eS     any character that is not a white space character
496      \ev     any vertical white space character
497      \eV     any character that is not a vertical white space character
498    \ew     any "word" character    \ew     any "word" character
499    \eW     any "non-word" character    \eW     any "non-word" character
500  .sp  .sp
501  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.
502  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
503    .\" HTML <a href="#fullstopdot">
504    .\" </a>
505    the "." metacharacter
506    .\"
507    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
508    PCRE does not support this.
509  .P  .P
510  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
511    of characters into two disjoint sets. Any given character matches one, and only
512    one, of each pair. The sequences can appear both inside and outside character
513  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
514  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
515  there is no character to match.  there is no character to match.
516  .P  .P
517  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
518  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
519  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
520  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).
 does.)  
521  .P  .P
522  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
523  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
524  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
525  place (see  place (see
526  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 268  in the Line 531  in the
531  .\" HREF  .\" HREF
532  \fBpcreapi\fP  \fBpcreapi\fP
533  .\"  .\"
534  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
535  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
536  .P  accented letters, and these are then matched by \ew. The use of locales with
537  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
538  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
539  character property support is available. The use of locales with Unicode is  By default, in a UTF mode, characters with values greater than 128 never match
540  discouraged.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
541    their original meanings from before UTF support was available, mainly for
542    efficiency reasons. However, if PCRE is compiled with Unicode property support,
543    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
544    properties are used to determine character types, as follows:
545    .sp
546      \ed  any character that matches \ep{Nd} (decimal digit)
547      \es  any character that matches \ep{Z} or \eh or \ev
548      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
549    .sp
550    The upper case escapes match the inverse sets of characters. Note that \ed
551    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
552    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
553    \eB because they are defined in terms of \ew and \eW. Matching these sequences
554    is noticeably slower when PCRE_UCP is set.
555    .P
556    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
557    release 5.10. In contrast to the other sequences, which match only ASCII
558    characters by default, these always match certain high-valued codepoints,
559    whether or not PCRE_UCP is set. The horizontal space characters are:
560    .sp
561      U+0009     Horizontal tab (HT)
562      U+0020     Space
563      U+00A0     Non-break space
564      U+1680     Ogham space mark
565      U+180E     Mongolian vowel separator
566      U+2000     En quad
567      U+2001     Em quad
568      U+2002     En space
569      U+2003     Em space
570      U+2004     Three-per-em space
571      U+2005     Four-per-em space
572      U+2006     Six-per-em space
573      U+2007     Figure space
574      U+2008     Punctuation space
575      U+2009     Thin space
576      U+200A     Hair space
577      U+202F     Narrow no-break space
578      U+205F     Medium mathematical space
579      U+3000     Ideographic space
580    .sp
581    The vertical space characters are:
582    .sp
583      U+000A     Linefeed (LF)
584      U+000B     Vertical tab (VT)
585      U+000C     Form feed (FF)
586      U+000D     Carriage return (CR)
587      U+0085     Next line (NEL)
588      U+2028     Line separator
589      U+2029     Paragraph separator
590    .sp
591    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
592    relevant.
593    .
594    .
595    .\" HTML <a name="newlineseq"></a>
596    .SS "Newline sequences"
597    .rs
598    .sp
599    Outside a character class, by default, the escape sequence \eR matches any
600    Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
601    following:
602    .sp
603      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
604    .sp
605    This is an example of an "atomic group", details of which are given
606    .\" HTML <a href="#atomicgroup">
607    .\" </a>
608    below.
609    .\"
610    This particular group matches either the two-character sequence CR followed by
611    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
612    U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
613    line, U+0085). The two-character sequence is treated as a single unit that
614    cannot be split.
615    .P
616    In other modes, two additional characters whose codepoints are greater than 255
617    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
618    Unicode character property support is not needed for these characters to be
619    recognized.
620    .P
621    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
622    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
623    either at compile time or when the pattern is matched. (BSR is an abbrevation
624    for "backslash R".) This can be made the default when PCRE is built; if this is
625    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
626    It is also possible to specify these settings by starting a pattern string with
627    one of the following sequences:
628    .sp
629      (*BSR_ANYCRLF)   CR, LF, or CRLF only
630      (*BSR_UNICODE)   any Unicode newline sequence
631    .sp
632    These override the default and the options given to the compiling function, but
633    they can themselves be overridden by options given to a matching function. Note
634    that these special settings, which are not Perl-compatible, are recognized only
635    at the very start of a pattern, and that they must be in upper case. If more
636    than one of them is present, the last one is used. They can be combined with a
637    change of newline convention; for example, a pattern can start with:
638    .sp
639      (*ANY)(*BSR_ANYCRLF)
640    .sp
641    They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
642    (*UCP) special sequences. Inside a character class, \eR is treated as an
643    unrecognized escape sequence, and so matches the letter "R" by default, but
644    causes an error if PCRE_EXTRA is set.
645  .  .
646  .  .
647  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 649  discouraged.
649  .rs  .rs
650  .sp  .sp
651  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
652  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
653  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
654    characters whose codepoints are less than 256, but they do work in this mode.
655    The extra escape sequences are:
656  .sp  .sp
657    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
658    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
659    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
660  .sp  .sp
661  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
662  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
663  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
664  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
665  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
666    .\" </a>
667    next section).
668    .\"
669    Other Perl properties such as "InMusicalSymbols" are not currently supported by
670    PCRE. Note that \eP{Any} does not match any characters, so always causes a
671    match failure.
672  .P  .P
673  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
674  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 307  Those that are not part of an identified Line 682  Those that are not part of an identified
682  .P  .P
683  Arabic,  Arabic,
684  Armenian,  Armenian,
685    Avestan,
686    Balinese,
687    Bamum,
688    Batak,
689  Bengali,  Bengali,
690  Bopomofo,  Bopomofo,
691    Brahmi,
692  Braille,  Braille,
693  Buginese,  Buginese,
694  Buhid,  Buhid,
695  Canadian_Aboriginal,  Canadian_Aboriginal,
696    Carian,
697    Chakma,
698    Cham,
699  Cherokee,  Cherokee,
700  Common,  Common,
701  Coptic,  Coptic,
702    Cuneiform,
703  Cypriot,  Cypriot,
704  Cyrillic,  Cyrillic,
705  Deseret,  Deseret,
706  Devanagari,  Devanagari,
707    Egyptian_Hieroglyphs,
708  Ethiopic,  Ethiopic,
709  Georgian,  Georgian,
710  Glagolitic,  Glagolitic,
# Line 332  Hangul, Line 717  Hangul,
717  Hanunoo,  Hanunoo,
718  Hebrew,  Hebrew,
719  Hiragana,  Hiragana,
720    Imperial_Aramaic,
721  Inherited,  Inherited,
722    Inscriptional_Pahlavi,
723    Inscriptional_Parthian,
724    Javanese,
725    Kaithi,
726  Kannada,  Kannada,
727  Katakana,  Katakana,
728    Kayah_Li,
729  Kharoshthi,  Kharoshthi,
730  Khmer,  Khmer,
731  Lao,  Lao,
732  Latin,  Latin,
733    Lepcha,
734  Limbu,  Limbu,
735  Linear_B,  Linear_B,
736    Lisu,
737    Lycian,
738    Lydian,
739  Malayalam,  Malayalam,
740    Mandaic,
741    Meetei_Mayek,
742    Meroitic_Cursive,
743    Meroitic_Hieroglyphs,
744    Miao,
745  Mongolian,  Mongolian,
746  Myanmar,  Myanmar,
747  New_Tai_Lue,  New_Tai_Lue,
748    Nko,
749  Ogham,  Ogham,
750  Old_Italic,  Old_Italic,
751  Old_Persian,  Old_Persian,
752    Old_South_Arabian,
753    Old_Turkic,
754    Ol_Chiki,
755  Oriya,  Oriya,
756  Osmanya,  Osmanya,
757    Phags_Pa,
758    Phoenician,
759    Rejang,
760  Runic,  Runic,
761    Samaritan,
762    Saurashtra,
763    Sharada,
764  Shavian,  Shavian,
765  Sinhala,  Sinhala,
766    Sora_Sompeng,
767    Sundanese,
768  Syloti_Nagri,  Syloti_Nagri,
769  Syriac,  Syriac,
770  Tagalog,  Tagalog,
771  Tagbanwa,  Tagbanwa,
772  Tai_Le,  Tai_Le,
773    Tai_Tham,
774    Tai_Viet,
775    Takri,
776  Tamil,  Tamil,
777  Telugu,  Telugu,
778  Thaana,  Thaana,
# Line 365  Thai, Line 780  Thai,
780  Tibetan,  Tibetan,
781  Tifinagh,  Tifinagh,
782  Ugaritic,  Ugaritic,
783    Vai,
784  Yi.  Yi.
785  .P  .P
786  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
787  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
788  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
789  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
790  .P  .P
791  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
792  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 430  The special property L& is also supporte Line 846  The special property L& is also supporte
846  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
847  a modifier or "other".  a modifier or "other".
848  .P  .P
849  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
850    U+DFFF. Such characters are not valid in Unicode strings and so
851    cannot be tested by PCRE, unless UTF validity checking has been turned off
852    (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
853    PCRE_NO_UTF32_CHECK in the
854    .\" HREF
855    \fBpcreapi\fP
856    .\"
857    page). Perl does not support the Cs property.
858    .P
859    The long synonyms for property names that Perl supports (such as \ep{Letter})
860  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
861  properties with "Is".  properties with "Is".
862  .P  .P
# Line 439  Instead, this property is assumed for an Line 865  Instead, this property is assumed for an
865  Unicode table.  Unicode table.
866  .P  .P
867  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
868  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
869    the behaviour of current versions of Perl.
870  .P  .P
871  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
872  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
873  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
874    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
875    PCRE_UCP option or by starting the pattern with (*UCP).
876    .
877    .
878    .SS Extended grapheme clusters
879    .rs
880  .sp  .sp
881  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
882  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  
883  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
884  .\" </a>  .\" </a>
885  (see below).  (see below).
886  .\"  .\"
887  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
888  preceding character.  that was equivalent to
889    .sp
890      (?>\ePM\epM*)
891    .sp
892    That is, it matched a character without the "mark" property, followed by zero
893    or more characters with the "mark" property. Characters with the "mark"
894    property are typically non-spacing accents that affect the preceding character.
895    .P
896    This simple definition was extended in Unicode to include more complicated
897    kinds of composite character by giving each character a grapheme breaking
898    property, and creating rules that use these properties to define the boundaries
899    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
900    one of these clusters.
901    .P
902    \eX always matches at least one character. Then it decides whether to add
903    additional characters according to the following rules for ending a cluster:
904    .P
905    1. End at the end of the subject string.
906    .P
907    2. Do not end between CR and LF; otherwise end after any control character.
908    .P
909    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
910    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
911    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
912    character; an LVT or T character may be follwed only by a T character.
913  .P  .P
914  Matching characters by Unicode property is not fast, because PCRE has to search  4. Do not end before extending characters or spacing marks. Characters with
915  a structure that contains data for over fifteen thousand characters. That is  the "mark" property always have the "extend" grapheme breaking property.
916  why the traditional escape sequences such as \ed and \ew do not use Unicode  .P
917  properties in PCRE.  5. Do not end after prepend characters.
918    .P
919    6. Otherwise, end the cluster.
920    .
921    .
922    .\" HTML <a name="extraprops"></a>
923    .SS PCRE's additional properties
924    .rs
925    .sp
926    As well as the standard Unicode properties described above, PCRE supports four
927    more that make it possible to convert traditional escape sequences such as \ew
928    and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
929    properties internally when PCRE_UCP is set. However, they may also be used
930    explicitly. These properties are:
931    .sp
932      Xan   Any alphanumeric character
933      Xps   Any POSIX space character
934      Xsp   Any Perl space character
935      Xwd   Any Perl "word" character
936    .sp
937    Xan matches characters that have either the L (letter) or the N (number)
938    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
939    carriage return, and any other character that has the Z (separator) property.
940    Xsp is the same as Xps; it used to exclude vertical tab, for Perl
941    compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
942    matches the same characters as Xan, plus underscore.
943    .P
944    There is another non-standard property, Xuc, which matches any character that
945    can be represented by a Universal Character Name in C++ and other programming
946    languages. These are the characters $, @, ` (grave accent), and all characters
947    with Unicode code points greater than or equal to U+00A0, except for the
948    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
949    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
950    where H is a hexadecimal digit. Note that the Xuc property does not match these
951    sequences but the characters that they represent.)
952    .
953    .
954    .\" HTML <a name="resetmatchstart"></a>
955    .SS "Resetting the match start"
956    .rs
957    .sp
958    The escape sequence \eK causes any previously matched characters not to be
959    included in the final matched sequence. For example, the pattern:
960    .sp
961      foo\eKbar
962    .sp
963    matches "foobar", but reports that it has matched "bar". This feature is
964    similar to a lookbehind assertion
965    .\" HTML <a href="#lookbehind">
966    .\" </a>
967    (described below).
968    .\"
969    However, in this case, the part of the subject before the real match does not
970    have to be of fixed length, as lookbehind assertions do. The use of \eK does
971    not interfere with the setting of
972    .\" HTML <a href="#subpattern">
973    .\" </a>
974    captured substrings.
975    .\"
976    For example, when the pattern
977    .sp
978      (foo)\eKbar
979    .sp
980    matches "foobar", the first substring is still set to "foo".
981    .P
982    Perl documents that the use of \eK within assertions is "not well defined". In
983    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
984    ignored in negative assertions.
985  .  .
986  .  .
987  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
988  .SS "Simple assertions"  .SS "Simple assertions"
989  .rs  .rs
990  .sp  .sp
991  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
992  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
993  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
994  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 1000  The backslashed assertions are:
1000  .sp  .sp
1001    \eb     matches at a word boundary    \eb     matches at a word boundary
1002    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
1003    \eA     matches at start of subject    \eA     matches at the start of the subject
1004    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
1005    \ez     matches at end of subject            also matches before a newline at the end of the subject
1006    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
1007  .sp    \eG     matches at the first matching position in the subject
1008  These assertions may not appear in character classes (but note that \eb has a  .sp
1009  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
1010    character. If any other of these assertions appears in a character class, by
1011    default it matches the corresponding literal character (for example, \eB
1012    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
1013    escape sequence" error is generated instead.
1014  .P  .P
1015  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
1016  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
1017  \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
1018  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
1019    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1020    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1021    of word" or "end of word" metasequence. However, whatever follows \eb normally
1022    determines which it is. For example, the fragment \eba matches "a" at the start
1023    of a word.
1024  .P  .P
1025  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1026  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 523  regular expression. Line 1054  regular expression.
1054  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1055  .rs  .rs
1056  .sp  .sp
1057    The circumflex and dollar metacharacters are zero-width assertions. That is,
1058    they test for a particular condition being true without consuming any
1059    characters from the subject string.
1060    .P
1061  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1062  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
1063  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
1064  \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
1065  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1066  meaning  meaning
# Line 542  constrained to match only at the start o Line 1077  constrained to match only at the start o
1077  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1078  to be anchored.)  to be anchored.)
1079  .P  .P
1080  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
1081  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
1082  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
1083  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
1084  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
1085  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1086  .P  .P
1087  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
1088  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 573  end of the subject in both modes, and if Line 1108  end of the subject in both modes, and if
1108  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1109  .  .
1110  .  .
1111  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1112    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1113  .rs  .rs
1114  .sp  .sp
1115  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
1116  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
1117  line. In UTF-8 mode, the matched character may be more than one byte long. When  line.
1118  a line ending is defined as a single character (CR or LF), dot never matches  .P
1119  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
1120  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
1121  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
1122    (including isolated CRs and LFs). When any Unicode line endings are being
1123    recognized, dot does not match CR or LF or any of the other line ending
1124    characters.
1125  .P  .P
1126  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1127  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
1128  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
1129    to match it.
1130  .P  .P
1131  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
1132  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
1133  special meaning in a character class.  special meaning in a character class.
1134  .  .P
1135  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1136  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1137  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1138  .sp  name; PCRE does not support this.
1139  Outside a character class, the escape sequence \eC matches any one byte, both  .
1140  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  .
1141  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  .SH "MATCHING A SINGLE DATA UNIT"
1142  Because it breaks up UTF-8 characters into individual bytes, what remains in  .rs
1143  the string may be a malformed UTF-8 string. For this reason, the \eC escape  .sp
1144  sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1145    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1146    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1147    a 32-bit unit. Unlike a dot, \eC always
1148    matches line-ending characters. The feature is provided in Perl in order to
1149    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1150    used. Because \eC breaks up characters into individual data units, matching one
1151    unit with \eC in a UTF mode means that the rest of the string may start with a
1152    malformed UTF character. This has undefined results, because PCRE assumes that
1153    it is dealing with valid UTF strings (and by default it checks this at the
1154    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1155    PCRE_NO_UTF32_CHECK option is used).
1156  .P  .P
1157  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1158  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1159  .\" </a>  .\" </a>
1160  (described below),  (described below)
1161  .\"  .\"
1162  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
1163  the lookbehind.  the lookbehind.
1164    .P
1165    In general, the \eC escape sequence is best avoided. However, one
1166    way of using it that avoids the problem of malformed UTF characters is to use a
1167    lookahead to check the length of the next character, as in this pattern, which
1168    could be used with a UTF-8 string (ignore white space and line breaks):
1169    .sp
1170      (?| (?=[\ex00-\ex7f])(\eC) |
1171          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1172          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1173          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1174    .sp
1175    A group that starts with (?| resets the capturing parentheses numbers in each
1176    alternative (see
1177    .\" HTML <a href="#dupsubpatternnumber">
1178    .\" </a>
1179    "Duplicate Subpattern Numbers"
1180    .\"
1181    below). The assertions at the start of each branch check the next UTF-8
1182    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1183    character's individual bytes are then captured by the appropriate number of
1184    groups.
1185  .  .
1186  .  .
1187  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 617  the lookbehind. Line 1189  the lookbehind.
1189  .rs  .rs
1190  .sp  .sp
1191  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1192  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.
1193  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
1194  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
1195  escaped with a backslash.  a member of the class, it should be the first data character in the class
1196  .P  (after an initial circumflex, if present) or escaped with a backslash.
1197  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1198  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
1199  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
1200  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
1201  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
1202  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
1203    member of the class, ensure it is not the first character, or escape it with a
1204  backslash.  backslash.
1205  .P  .P
1206  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1207  [^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
1208  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1209  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
1210  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
1211  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
1212  string.  string.
1213  .P  .P
1214  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)
1215  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
1216    \ex{ escaping mechanism.
1217  .P  .P
1218  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
1219  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
1220  "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
1221  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
1222  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
1223  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1224  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1225  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
1226  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
1227  UTF-8 support.  well as with UTF support.
1228  .P  .P
1229  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
1230  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
1231  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1232  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
1233  .P  .P
1234  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
1235  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,
# Line 672  followed by two other characters. The oc Line 1246  followed by two other characters. The oc
1246  "]" can also be used to end a range.  "]" can also be used to end a range.
1247  .P  .P
1248  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
1249  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1250  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}].  
1251  .P  .P
1252  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
1253  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
1254  [][\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
1255  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1256  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
1257  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
1258  property support.  property support.
1259  .P  .P
1260  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,
1261  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
1262  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1263  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
1264  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
1265  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1266    .\" HTML <a href="#genericchartypes">
1267    .\" </a>
1268    "Generic character types"
1269    .\"
1270    above. The escape sequence \eb has a different meaning inside a character
1271    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1272    are not special inside a character class. Like any other unrecognized escape
1273    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1274    default, but cause an error if the PCRE_EXTRA option is set.
1275    .P
1276    A circumflex can conveniently be used with the upper case character types to
1277    specify a more restricted set of characters than the matching lower case type.
1278    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1279    whereas [\ew] includes underscore. A positive character class should be read as
1280    "something OR something OR ..." and a negative class as "NOT something AND NOT
1281    something AND NOT ...".
1282  .P  .P
1283  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1284  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 709  this notation. For example, Line 1298  this notation. For example,
1298    [01[:alpha:]%]    [01[:alpha:]%]
1299  .sp  .sp
1300  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1301  are  are:
1302  .sp  .sp
1303    alnum    letters and digits    alnum    letters and digits
1304    alpha    letters    alpha    letters
# Line 720  are Line 1309  are
1309    graph    printing characters, excluding space    graph    printing characters, excluding space
1310    lower    lower case letters    lower    lower case letters
1311    print    printing characters, including space    print    printing characters, including space
1312    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1313    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1314    upper    upper case letters    upper    upper case letters
1315    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1316    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1317  .sp  .sp
1318  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
1319  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
1320  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
1321  compatibility).  release 8.34. "Space" and \es now match the same set of characters.
1322  .P  .P
1323  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
1324  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 741  matches "1", "2", or any non-digit. PCRE Line 1330  matches "1", "2", or any non-digit. PCRE
1330  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
1331  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1332  .P  .P
1333  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
1334  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1335    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1336    character properties are used. This is achieved by replacing certain POSIX
1337    classes by other sequences, as follows:
1338    .sp
1339      [:alnum:]  becomes  \ep{Xan}
1340      [:alpha:]  becomes  \ep{L}
1341      [:blank:]  becomes  \eh
1342      [:digit:]  becomes  \ep{Nd}
1343      [:lower:]  becomes  \ep{Ll}
1344      [:space:]  becomes  \ep{Xps}
1345      [:upper:]  becomes  \ep{Lu}
1346      [:word:]   becomes  \ep{Xwd}
1347    .sp
1348    Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1349    classes are handled specially in UCP mode:
1350    .TP 10
1351    [:graph:]
1352    This matches characters that have glyphs that mark the page when printed. In
1353    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1354    properties, except for:
1355    .sp
1356      U+061C           Arabic Letter Mark
1357      U+180E           Mongolian Vowel Separator
1358      U+2066 - U+2069  Various "isolate"s
1359    .sp
1360    .TP 10
1361    [:print:]
1362    This matches the same characters as [:graph:] plus space characters that are
1363    not controls, that is, characters with the Zs property.
1364    .TP 10
1365    [:punct:]
1366    This matches all characters that have the Unicode P (punctuation) property,
1367    plus those characters whose code points are less than 128 that have the S
1368    (Symbol) property.
1369    .P
1370    The other POSIX classes are unchanged, and match only characters with code
1371    points less than 128.
1372  .  .
1373  .  .
1374  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 769  alternative in the subpattern. Line 1395  alternative in the subpattern.
1395  .rs  .rs
1396  .sp  .sp
1397  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1398  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1399  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1400    The option letters are
1401  .sp  .sp
1402    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1403    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1411  PCRE_MULTILINE while unsetting PCRE_DOTA
1411  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1412  unset.  unset.
1413  .P  .P
1414  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1415  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1416  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1417  the global options (and it will therefore show up in data extracted by the  .P
1418  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1419    subpattern parentheses), the change applies to the remainder of the pattern
1420    that follows. If the change is placed right at the start of a pattern, PCRE
1421    extracts it into the global options (and it will therefore show up in data
1422    extracted by the \fBpcre_fullinfo()\fP function).
1423  .P  .P
1424  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1425  pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1426  .sp  .sp
1427    (a(?i)b)c    (a(?i)b)c
1428  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1438  branch is abandoned before the option se
1438  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1439  behaviour otherwise.  behaviour otherwise.
1440  .P  .P
1441  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1442  changed in the same way as the Perl-compatible options by using the characters  application when the compiling or matching functions are called. In some cases
1443  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1444    what the application has set or what has been defaulted. Details are given in
1445    the section entitled
1446    .\" HTML <a href="#newlineseq">
1447    .\" </a>
1448    "Newline sequences"
1449    .\"
1450    above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1451    sequences that can be used to set UTF and Unicode property modes; they are
1452    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1453    options, respectively. The (*UTF) sequence is a generic version that can be
1454    used with any of the libraries. However, the application can set the
1455    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1456  .  .
1457  .  .
1458  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 823  Turning part of a pattern into a subpatt Line 1466  Turning part of a pattern into a subpatt
1466  .sp  .sp
1467    cat(aract|erpillar|)    cat(aract|erpillar|)
1468  .sp  .sp
1469  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1470  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1471  .sp  .sp
1472  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
1473  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
1474  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
1475  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1476  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1477  .P  .P
1478  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
1479    numbers for the capturing subpatterns. For example, if the string "the red
1480    king" is matched against the pattern
1481  .sp  .sp
1482    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1483  .sp  .sp
# Line 849  the string "the white queen" is matched Line 1494  the string "the white queen" is matched
1494    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1495  .sp  .sp
1496  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1497  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1498  .P  .P
1499  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1500  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 865  is reached, an option setting in one bra Line 1509  is reached, an option setting in one bra
1509  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1510  .  .
1511  .  .
1512    .\" HTML <a name="dupsubpatternnumber"></a>
1513    .SH "DUPLICATE SUBPATTERN NUMBERS"
1514    .rs
1515    .sp
1516    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1517    the same numbers for its capturing parentheses. Such a subpattern starts with
1518    (?| and is itself a non-capturing subpattern. For example, consider this
1519    pattern:
1520    .sp
1521      (?|(Sat)ur|(Sun))day
1522    .sp
1523    Because the two alternatives are inside a (?| group, both sets of capturing
1524    parentheses are numbered one. Thus, when the pattern matches, you can look
1525    at captured substring number one, whichever alternative matched. This construct
1526    is useful when you want to capture part, but not all, of one of a number of
1527    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1528    number is reset at the start of each branch. The numbers of any capturing
1529    parentheses that follow the subpattern start after the highest number used in
1530    any branch. The following example is taken from the Perl documentation. The
1531    numbers underneath show in which buffer the captured content will be stored.
1532    .sp
1533      # before  ---------------branch-reset----------- after
1534      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1535      # 1            2         2  3        2     3     4
1536    .sp
1537    A back reference to a numbered subpattern uses the most recent value that is
1538    set for that number by any subpattern. The following pattern matches "abcabc"
1539    or "defdef":
1540    .sp
1541      /(?|(abc)|(def))\e1/
1542    .sp
1543    In contrast, a subroutine call to a numbered subpattern always refers to the
1544    first one in the pattern with the given number. The following pattern matches
1545    "abcabc" or "defabc":
1546    .sp
1547      /(?|(abc)|(def))(?1)/
1548    .sp
1549    If a
1550    .\" HTML <a href="#conditions">
1551    .\" </a>
1552    condition test
1553    .\"
1554    for a subpattern's having matched refers to a non-unique number, the test is
1555    true if any of the subpatterns of that number have matched.
1556    .P
1557    An alternative approach to using this "branch reset" feature is to use
1558    duplicate named subpatterns, as described in the next section.
1559    .
1560    .
1561  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1562  .rs  .rs
1563  .sp  .sp
1564  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1565  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1566  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1567  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1568  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1569    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1570    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1571    have different names, but PCRE does not.
1572    .P
1573    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1574    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1575  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1576  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1577  .\" </a>  .\" </a>
1578  backreferences,  back references,
1579  .\"  .\"
1580  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1581  .\" </a>  .\" </a>
# Line 890  conditions, Line 1589  conditions,
1589  can be made by name as well as by number.  can be made by name as well as by number.
1590  .P  .P
1591  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1592  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1593  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1594  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1595  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1596  .P  .P
1597  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
1598  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
1599  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
1600  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
1601  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
1602  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
1603  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1604    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1605    (?P<DN>Tue)(?:sday)?|  .sp
1606    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1607    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1608    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1609      (?<DN>Thu)(?:rsday)?|
1610      (?<DN>Sat)(?:urday)?
1611  .sp  .sp
1612  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1613    (An alternative way of solving this problem is to use a "branch reset"
1614    subpattern, as described in the previous section.)
1615    .P
1616  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1617  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
1618  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1619  make a reference to a non-unique named subpattern from elsewhere in the  .P
1620  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
1621  details of the interfaces for handling named subpatterns, see the  the pattern, the subpatterns to which the name refers are checked in the order
1622    in which they appear in the overall pattern. The first one that is set is used
1623    for the reference. For example, this pattern matches both "foofoo" and
1624    "barbar" but not "foobar" or "barfoo":
1625    .sp
1626      (?:(?<n>foo)|(?<n>bar))\k<n>
1627    .sp
1628    .P
1629    If you make a subroutine call to a non-unique named subpattern, the one that
1630    corresponds to the first occurrence of the name is used. In the absence of
1631    duplicate numbers (see the previous section) this is the one with the lowest
1632    number.
1633    .P
1634    If you use a named reference in a condition
1635    test (see the
1636    .\"
1637    .\" HTML <a href="#conditions">
1638    .\" </a>
1639    section about conditions
1640    .\"
1641    below), either to check whether a subpattern has matched, or to check for
1642    recursion, all subpatterns with the same name are tested. If the condition is
1643    true for any one of them, the overall condition is true. This is the same
1644    behaviour as testing by number. For further details of the interfaces for
1645    handling named subpatterns, see the
1646  .\" HREF  .\" HREF
1647  \fBpcreapi\fP  \fBpcreapi\fP
1648  .\"  .\"
1649  documentation.  documentation.
1650    .P
1651    \fBWarning:\fP You cannot use different names to distinguish between two
1652    subpatterns with the same number because PCRE uses only the numbers when
1653    matching. For this reason, an error is given at compile time if different names
1654    are given to subpatterns with the same number. However, you can always give the
1655    same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1656    set.
1657  .  .
1658  .  .
1659  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1663  Repetition is specified by quantifiers,
1663  items:  items:
1664  .sp  .sp
1665    a literal data character    a literal data character
1666    the . metacharacter    the dot metacharacter
1667    the \eC escape sequence    the \eC escape sequence
1668    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1669    an escape such as \ed that matches a single character    the \eR escape sequence
1670      an escape such as \ed or \epL that matches a single character
1671    a character class    a character class
1672    a back reference (see next section)    a back reference (see next section)
1673    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1674      a subroutine call to a subpattern (recursive or otherwise)
1675  .sp  .sp
1676  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1677  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 959  where a quantifier is not allowed, or on Line 1696  where a quantifier is not allowed, or on
1696  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
1697  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1698  .P  .P
1699  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
1700  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
1701  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,
1702  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
1703  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1704  .P  .P
1705  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
1706  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1707    subpatterns that are referenced as
1708    .\" HTML <a href="#subpatternsassubroutines">
1709    .\" </a>
1710    subroutines
1711    .\"
1712    from elsewhere in the pattern (but see also the section entitled
1713    .\" HTML <a href="#subdefine">
1714    .\" </a>
1715    "Defining subpatterns for use by reference only"
1716    .\"
1717    below). Items other than subpatterns that have a {0} quantifier are omitted
1718    from the compiled pattern.
1719  .P  .P
1720  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1721  quantifiers have single-character abbreviations:  abbreviations:
1722  .sp  .sp
1723    *    is equivalent to {0,}    *    is equivalent to {0,}
1724    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1766  own right. Because it has two uses, it c
1766  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1767  way the rest of the pattern matches.  way the rest of the pattern matches.
1768  .P  .P
1769  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1770  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1771  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1772  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1776  is greater than 1 or with a limited maxi
1776  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1777  .P  .P
1778  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1779  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1780  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1781  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1782  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1037  In cases where it is known that the subj Line 1786  In cases where it is known that the subj
1786  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1787  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1788  .P  .P
1789  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1790  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1791  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1792  succeed. Consider, for example:  succeeds. Consider, for example:
1793  .sp  .sp
1794    (.*)abc\e1    (.*)abc\e1
1795  .sp  .sp
1796  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
1797  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1798  .P  .P
1799    Another case where implicit anchoring is not applied is when the leading .* is
1800    inside an atomic group. Once again, a match at the start may fail where a later
1801    one succeeds. Consider this pattern:
1802    .sp
1803      (?>.*?a)b
1804    .sp
1805    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1806    (*PRUNE) and (*SKIP) also disable this optimization.
1807    .P
1808  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1809  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1810  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1824  matches "aba" the value of the second ca
1824  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1825  .rs  .rs
1826  .sp  .sp
1827  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1828  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1829  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1830  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1831  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1832  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1833  .P  .P
1834  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1835  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1841  item, and then with 4, and so on, before
1841  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1842  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1843  .P  .P
1844  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1845  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1846  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1847  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1871  previous example can be rewritten as
1871  .sp  .sp
1872    \ed++foo    \ed++foo
1873  .sp  .sp
1874    Note that a possessive quantifier can be used with an entire group, for
1875    example:
1876    .sp
1877      (abc|xyz){2,3}+
1878    .sp
1879  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1880  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1881  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1882  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1883  .P  difference; possessive quantifiers should be slightly faster.
1884  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1885  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1886  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1887  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1888    package, and PCRE copied it from there. It ultimately found its way into Perl
1889    at release 5.10.
1890    .P
1891    PCRE has an optimization that automatically "possessifies" certain simple
1892    pattern constructs. For example, the sequence A+B is treated as A++B because
1893    there is no point in backtracking into a sequence of A's when B must follow.
1894  .P  .P
1895  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1896  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1167  numbers less than 10. A "forward back re Line 1936  numbers less than 10. A "forward back re
1936  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1937  in an earlier iteration.  in an earlier iteration.
1938  .P  .P
1939  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1940  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1941  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1942  "Non-printing characters"  "Non-printing characters"
1943  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1944  .\" </a>  .\" </a>
1945  above  above
1946  .\"  .\"
1947  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1948    no such problem when named parentheses are used. A back reference to any
1949    subpattern is possible using named parentheses (see below).
1950    .P
1951    Another way of avoiding the ambiguity inherent in the use of digits following a
1952    backslash is to use the \eg escape sequence. This escape must be followed by an
1953    unsigned number or a negative number, optionally enclosed in braces. These
1954    examples are all identical:
1955    .sp
1956      (ring), \e1
1957      (ring), \eg1
1958      (ring), \eg{1}
1959    .sp
1960    An unsigned number specifies an absolute reference without the ambiguity that
1961    is present in the older syntax. It is also useful when literal digits follow
1962    the reference. A negative number is a relative reference. Consider this
1963    example:
1964    .sp
1965      (abc(def)ghi)\eg{-1}
1966    .sp
1967    The sequence \eg{-1} is a reference to the most recently started capturing
1968    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1969    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1970    can be helpful in long patterns, and also in patterns that are created by
1971    joining together fragments that contain references within themselves.
1972  .P  .P
1973  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1974  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1990  back reference, the case of letters is r
1990  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1991  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1992  .P  .P
1993  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1994  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1995    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1996    back reference syntax, in which \eg can be used for both numeric and named
1997    references, is also supported. We could rewrite the above example in any of
1998    the following ways:
1999  .sp  .sp
2000      (?<p1>(?i)rah)\es+\ek<p1>
2001      (?'p1'(?i)rah)\es+\ek{p1}
2002    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
2003      (?<p1>(?i)rah)\es+\eg{p1}
2004  .sp  .sp
2005  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
2006  after the reference.  after the reference.
2007  .P  .P
2008  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
2009  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
2010  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
2011  .sp  .sp
2012    (a|(bc))\e2    (a|(bc))\e2
2013  .sp  .sp
2014  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
2015  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
2016  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
2017  with a digit character, some delimiter must be used to terminate the back  .P
2018  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
2019  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
2020    If the pattern continues with a digit character, some delimiter must be used to
2021    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
2022    white space. Otherwise, the \eg{ syntax or an empty comment (see
2023  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
2024  .\" </a>  .\" </a>
2025  "Comments"  "Comments"
2026  .\"  .\"
2027  below) can be used.  below) can be used.
2028  .P  .
2029    .SS "Recursive back references"
2030    .rs
2031    .sp
2032  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
2033  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.
2034  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1236  to the previous iteration. In order for Line 2042  to the previous iteration. In order for
2042  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
2043  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
2044  minimum of zero.  minimum of zero.
2045    .P
2046    Back references of this type cause the group that they reference to be treated
2047    as an
2048    .\" HTML <a href="#atomicgroup">
2049    .\" </a>
2050    atomic group.
2051    .\"
2052    Once the whole group has been matched, a subsequent matching failure cannot
2053    cause backtracking into the middle of the group.
2054  .  .
2055  .  .
2056  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1255  those that look ahead of the current pos Line 2070  those that look ahead of the current pos
2070  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,
2071  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.
2072  .P  .P
2073  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2074  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
2075  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2076  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2077  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2078  because it does not make sense for negative assertions.  .P
2079    For compatibility with Perl, assertion subpatterns may be repeated; though
2080    it makes no sense to assert the same thing several times, the side effect of
2081    capturing parentheses may occasionally be useful. In practice, there only three
2082    cases:
2083    .sp
2084    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2085    However, it may contain internal capturing parenthesized groups that are called
2086    from elsewhere via the
2087    .\" HTML <a href="#subpatternsassubroutines">
2088    .\" </a>
2089    subroutine mechanism.
2090    .\"
2091    .sp
2092    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2093    were {0,1}. At run time, the rest of the pattern match is tried with and
2094    without the assertion, the order depending on the greediness of the quantifier.
2095    .sp
2096    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2097    The assertion is obeyed just once when encountered during matching.
2098  .  .
2099  .  .
2100  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1289  lookbehind assertion is needed to achiev Line 2123  lookbehind assertion is needed to achiev
2123  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
2124  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
2125  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.
2126    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2127  .  .
2128  .  .
2129  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 2148  is permitted, but
2148  .sp  .sp
2149  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2150  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
2151  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
2152  match the same length of string. An assertion such as  length of string. An assertion such as
2153  .sp  .sp
2154    (?<=ab(c|de))    (?<=ab(c|de))
2155  .sp  .sp
2156  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
2157  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
2158    branches:
2159  .sp  .sp
2160    (?<=abc|abde)    (?<=abc|abde)
2161  .sp  .sp
2162    In some cases, the escape sequence \eK
2163    .\" HTML <a href="#resetmatchstart">
2164    .\" </a>
2165    (see above)
2166    .\"
2167    can be used instead of a lookbehind assertion to get round the fixed-length
2168    restriction.
2169    .P
2170  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2171  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
2172  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2173  match is deemed to fail.  assertion fails.
2174  .P  .P
2175  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
2176  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
2177  the length of the lookbehind. The \eX escape, which can match different numbers  it impossible to calculate the length of the lookbehind. The \eX and \eR
2178  of bytes, is also not permitted.  escapes, which can match different numbers of data units, are also not
2179  .P  permitted.
2180  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
2181  efficient matching at the end of the subject string. Consider a simple pattern  .\" HTML <a href="#subpatternsassubroutines">
2182  such as  .\" </a>
2183    "Subroutine"
2184    .\"
2185    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2186    as the subpattern matches a fixed-length string.
2187    .\" HTML <a href="#recursion">
2188    .\" </a>
2189    Recursion,
2190    .\"
2191    however, is not supported.
2192    .P
2193    Possessive quantifiers can be used in conjunction with lookbehind assertions to
2194    specify efficient matching of fixed-length strings at the end of subject
2195    strings. Consider a simple pattern such as
2196  .sp  .sp
2197    abcd$    abcd$
2198  .sp  .sp
# Line 1351  then all but the last two characters, an Line 2208  then all but the last two characters, an
2208  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
2209  if the pattern is written as  if the pattern is written as
2210  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
2211    ^.*+(?<=abcd)    ^.*+(?<=abcd)
2212  .sp  .sp
2213  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
2214  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
2215  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2216  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1403  characters that are not "999". Line 2256  characters that are not "999".
2256  .sp  .sp
2257  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2258  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2259  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2260  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2261  .sp  .sp
2262    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2263    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2264  .sp  .sp
2265  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2266  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
2267  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2268    itself contain nested subpatterns of any form, including conditional
2269    subpatterns; the restriction to two alternatives applies only at the level of
2270    the condition. This pattern fragment is an example where the alternatives are
2271    complex:
2272    .sp
2273      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2274    .sp
2275  .P  .P
2276  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
2277  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
2278  underscores, the condition is satisfied if the capturing subpattern of that  .
2279  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
2280  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
2281  named subpattern; if it cannot find one and the text consists entirely of  .sp
2282  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
2283  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
2284  recommended.  matched. If there is more than one capturing subpattern with the same number
2285    (see the earlier
2286    .\"
2287    .\" HTML <a href="#recursion">
2288    .\" </a>
2289    section about duplicate subpattern numbers),
2290    .\"
2291    the condition is true if any of them have matched. An alternative notation is
2292    to precede the digits with a plus or minus sign. In this case, the subpattern
2293    number is relative rather than absolute. The most recently opened parentheses
2294    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2295    loops it can also make sense to refer to subsequent groups. The next
2296    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2297    zero in any of these forms is not used; it provokes a compile-time error.)
2298  .P  .P
2299  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2300  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 1432  three parts for ease of discussion: Line 2305  three parts for ease of discussion:
2305  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2306  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
2307  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
2308  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2309  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,
2310  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
2311  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2312  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2313  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
2314  named subpattern gives this:  .P
2315    If you were embedding this pattern in a larger one, you could use a relative
2316    reference:
2317    .sp
2318      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2319  .sp  .sp
2320    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )  This makes the fragment independent of the parentheses in the larger pattern.
2321    .
2322    .SS "Checking for a used subpattern by name"
2323    .rs
2324    .sp
2325    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2326    subpattern by name. For compatibility with earlier versions of PCRE, which had
2327    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2328    there is a possible ambiguity with this syntax, because subpattern names may
2329    consist entirely of digits. PCRE looks first for a named subpattern; if it
2330    cannot find one and the name consists entirely of digits, PCRE looks for a
2331    subpattern of that number, which must be greater than zero. Using subpattern
2332    names that consist entirely of digits is not recommended.
2333    .P
2334    Rewriting the above example to use a named subpattern gives this:
2335    .sp
2336      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2337    .sp
2338    If the name used in a condition of this kind is a duplicate, the test is
2339    applied to all subpatterns of the same name, and is true if any one of them has
2340    matched.
2341    .
2342    .SS "Checking for pattern recursion"
2343    .rs
2344  .sp  .sp
2345  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
2346  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
2347  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
2348  Recursive patterns are described in the next section.  letter R, for example:
2349    .sp
2350      (?(R3)...) or (?(R&name)...)
2351    .sp
2352    the condition is true if the most recent recursion is into a subpattern whose
2353    number or name is given. This condition does not check the entire recursion
2354    stack. If the name used in a condition of this kind is a duplicate, the test is
2355    applied to all subpatterns of the same name, and is true if any one of them is
2356    the most recent recursion.
2357  .P  .P
2358  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
2359    .\" HTML <a href="#recursion">
2360    .\" </a>
2361    The syntax for recursive patterns
2362    .\"
2363    is described below.
2364    .
2365    .\" HTML <a name="subdefine"></a>
2366    .SS "Defining subpatterns for use by reference only"
2367    .rs
2368    .sp
2369    If the condition is the string (DEFINE), and there is no subpattern with the
2370    name DEFINE, the condition is always false. In this case, there may be only one
2371    alternative in the subpattern. It is always skipped if control reaches this
2372    point in the pattern; the idea of DEFINE is that it can be used to define
2373    subroutines that can be referenced from elsewhere. (The use of
2374    .\" HTML <a href="#subpatternsassubroutines">
2375    .\" </a>
2376    subroutines
2377    .\"
2378    is described below.) For example, a pattern to match an IPv4 address such as
2379    "192.168.23.245" could be written like this (ignore white space and line
2380    breaks):
2381    .sp
2382      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2383      \eb (?&byte) (\e.(?&byte)){3} \eb
2384    .sp
2385    The first part of the pattern is a DEFINE group inside which a another group
2386    named "byte" is defined. This matches an individual component of an IPv4
2387    address (a number less than 256). When matching takes place, this part of the
2388    pattern is skipped because DEFINE acts like a false condition. The rest of the
2389    pattern uses references to the named group to match the four dot-separated
2390    components of an IPv4 address, insisting on a word boundary at each end.
2391    .
2392    .SS "Assertion conditions"
2393    .rs
2394    .sp
2395    If the condition is not in any of the above formats, it must be an assertion.
2396  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2397  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2398  alternatives on the second line:  alternatives on the second line:
# Line 1467  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2412  dd-aaa-dd or dd-dd-dd, where aaa are let
2412  .SH COMMENTS  .SH COMMENTS
2413  .rs  .rs
2414  .sp  .sp
2415  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
2416  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,
2417  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
2418    subpattern name or number. The characters that make up a comment play no part
2419    in the pattern matching.
2420  .P  .P
2421  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
2422  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2423  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2424    this case continues to immediately after the next newline character or
2425    character sequence in the pattern. Which characters are interpreted as newlines
2426    is controlled by the options passed to a compiling function or by a special
2427    sequence at the start of the pattern, as described in the section entitled
2428    .\" HTML <a href="#newlines">
2429    .\" </a>
2430    "Newline conventions"
2431    .\"
2432    above. Note that the end of this type of comment is a literal newline sequence
2433    in the pattern; escape sequences that happen to represent a newline do not
2434    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2435    default newline convention is in force:
2436    .sp
2437      abc #comment \en still comment
2438    .sp
2439    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2440    a newline in the pattern. The sequence \en is still literal at this stage, so
2441    it does not terminate the comment. Only an actual character with the code value
2442    0x0a (the default newline) does so.
2443  .  .
2444  .  .
2445  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1483  next newline in the pattern. Line 2449  next newline in the pattern.
2449  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2450  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2451  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2452  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2453  that allows regular expressions to recurse (amongst other things). It does this  .P
2454  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2455  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2456  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2457    pattern using code interpolation to solve the parentheses problem can be
2458    created like this:
2459  .sp  .sp
2460    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2461  .sp  .sp
2462  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2463  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2464  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2465  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2466  .P  supports special syntax for recursion of the entire pattern, and also for
2467  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2468  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2469  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2470  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2471  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive subroutine call of the subpattern of the
2472  .P  given number, provided that it occurs inside that subpattern. (If not, it is a
2473  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2474  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2475  it contains untried alternatives and there is a subsequent matching failure.  non-recursive subroutine
2476    .\"
2477    call, which is described in the next section.) The special item (?R) or (?0) is
2478    a recursive call of the entire regular expression.
2479  .P  .P
2480  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2481  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2482  .sp  .sp
2483    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2484  .sp  .sp
2485  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2486  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
2487  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2488  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2489    to avoid backtracking into sequences of non-parentheses.
2490  .P  .P
2491  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
2492  pattern, so instead you could use this:  pattern, so instead you could use this:
2493  .sp  .sp
2494    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2495  .sp  .sp
2496  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
2497  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2498  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2499  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2500  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. Instead of (?1) in the
2501  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2502  .sp  parentheses preceding the recursion. In other words, a negative number counts
2503    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  capturing parentheses leftwards from the point at which it is encountered.
2504  .sp  .P
2505  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2506  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2507  when applying the pattern to strings that do not match. For example, when this  reference is not inside the parentheses that are referenced. They are always
2508  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2509    .\" </a>
2510    non-recursive subroutine
2511    .\"
2512    calls, as described in the next section.
2513    .P
2514    An alternative approach is to use named parentheses instead. The Perl syntax
2515    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2516    could rewrite the above example as follows:
2517    .sp
2518      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2519    .sp
2520    If there is more than one subpattern with the same name, the earliest one is
2521    used.
2522    .P
2523    This particular example pattern that we have been looking at contains nested
2524    unlimited repeats, and so the use of a possessive quantifier for matching
2525    strings of non-parentheses is important when applying the pattern to strings
2526    that do not match. For example, when this pattern is applied to
2527  .sp  .sp
2528    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2529  .sp  .sp
2530  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,
2531  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
2532  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
2533  before failure can be reported.  before failure can be reported.
2534  .P  .P
2535  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
2536  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
2537  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2538  .\" HREF  .\" HREF
2539  \fBpcrecallout\fP  \fBpcrecallout\fP
2540  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2542  documentation). If the pattern above is
2542  .sp  .sp
2543    (ab(cd)ef)    (ab(cd)ef)
2544  .sp  .sp
2545  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
2546  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
2547  .sp  matched at the top level, its final captured value is unset, even if it was
2548    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2549       ^                        ^  .P
2550       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2551  .sp  obtain extra memory to store data during a recursion, which it does by using
2552  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
2553  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.  
2554  .P  .P
2555  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.
2556  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1578  different alternatives for the recursive Line 2564  different alternatives for the recursive
2564  is the actual recursive call.  is the actual recursive call.
2565  .  .
2566  .  .
2567    .\" HTML <a name="recursiondifference"></a>
2568    .SS "Differences in recursion processing between PCRE and Perl"
2569    .rs
2570    .sp
2571    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2572    (like Python, but unlike Perl), a recursive subpattern call is always treated
2573    as an atomic group. That is, once it has matched some of the subject string, it
2574    is never re-entered, even if it contains untried alternatives and there is a
2575    subsequent matching failure. This can be illustrated by the following pattern,
2576    which purports to match a palindromic string that contains an odd number of
2577    characters (for example, "a", "aba", "abcba", "abcdcba"):
2578    .sp
2579      ^(.|(.)(?1)\e2)$
2580    .sp
2581    The idea is that it either matches a single character, or two identical
2582    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2583    it does not if the pattern is longer than three characters. Consider the
2584    subject string "abcba":
2585    .P
2586    At the top level, the first character is matched, but as it is not at the end
2587    of the string, the first alternative fails; the second alternative is taken
2588    and the recursion kicks in. The recursive call to subpattern 1 successfully
2589    matches the next character ("b"). (Note that the beginning and end of line
2590    tests are not part of the recursion).
2591    .P
2592    Back at the top level, the next character ("c") is compared with what
2593    subpattern 2 matched, which was "a". This fails. Because the recursion is
2594    treated as an atomic group, there are now no backtracking points, and so the
2595    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2596    try the second alternative.) However, if the pattern is written with the
2597    alternatives in the other order, things are different:
2598    .sp
2599      ^((.)(?1)\e2|.)$
2600    .sp
2601    This time, the recursing alternative is tried first, and continues to recurse
2602    until it runs out of characters, at which point the recursion fails. But this
2603    time we do have another alternative to try at the higher level. That is the big
2604    difference: in the previous case the remaining alternative is at a deeper
2605    recursion level, which PCRE cannot use.
2606    .P
2607    To change the pattern so that it matches all palindromic strings, not just
2608    those with an odd number of characters, it is tempting to change the pattern to
2609    this:
2610    .sp
2611      ^((.)(?1)\e2|.?)$
2612    .sp
2613    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2614    deeper recursion has matched a single character, it cannot be entered again in
2615    order to match an empty string. The solution is to separate the two cases, and
2616    write out the odd and even cases as alternatives at the higher level:
2617    .sp
2618      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2619    .sp
2620    If you want to match typical palindromic phrases, the pattern has to ignore all
2621    non-word characters, which can be done like this:
2622    .sp
2623      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2624    .sp
2625    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2626    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2627    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2628    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2629    more) to match typical phrases, and Perl takes so long that you think it has
2630    gone into a loop.
2631    .P
2632    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2633    string does not start with a palindrome that is shorter than the entire string.
2634    For example, although "abcba" is correctly matched, if the subject is "ababa",
2635    PCRE finds the palindrome "aba" at the start, then fails at top level because
2636    the end of the string does not follow. Once again, it cannot jump back into the
2637    recursion to try other alternatives, so the entire match fails.
2638    .P
2639    The second way in which PCRE and Perl differ in their recursion processing is
2640    in the handling of captured values. In Perl, when a subpattern is called
2641    recursively or as a subpattern (see the next section), it has no access to any
2642    values that were captured outside the recursion, whereas in PCRE these values
2643    can be referenced. Consider this pattern:
2644    .sp
2645      ^(.)(\e1|a(?2))
2646    .sp
2647    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2648    then in the second group, when the back reference \e1 fails to match "b", the
2649    second alternative matches "a" and then recurses. In the recursion, \e1 does
2650    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2651    match because inside the recursive call \e1 cannot access the externally set
2652    value.
2653    .
2654    .
2655  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2656  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2657  .rs  .rs
2658  .sp  .sp
2659  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
2660  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
2661  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2662  pattern  before or after the reference. A numbered reference can be absolute or
2663    relative, as in these examples:
2664    .sp
2665      (...(absolute)...)...(?2)...
2666      (...(relative)...)...(?-1)...
2667      (...(?+1)...(relative)...
2668    .sp
2669    An earlier example pointed out that the pattern
2670  .sp  .sp
2671    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2672  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2676  matches "sense and sensibility" and "res
2676    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2677  .sp  .sp
2678  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
2679  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
2680  which they refer. However, named references can refer to later subpatterns.  .P
2681    All subroutine calls, whether recursive or not, are always treated as atomic
2682    groups. That is, once a subroutine has matched some of the subject string, it
2683    is never re-entered, even if it contains untried alternatives and there is a
2684    subsequent matching failure. Any capturing parentheses that are set during the
2685    subroutine call revert to their previous values afterwards.
2686  .P  .P
2687  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Processing options such as case-independence are fixed when a subpattern is
2688  group. That is, once it has matched some of the subject string, it is never  defined, so if it is used as a subroutine, such options cannot be changed for
2689  re-entered, even if it contains untried alternatives and there is a subsequent  different calls. For example, consider this pattern:
2690  matching failure.  .sp
2691      (abc)(?i:(?-1))
2692    .sp
2693    It matches "abcabc". It does not match "abcABC" because the change of
2694    processing option does not affect the called subpattern.
2695    .
2696    .
2697    .\" HTML <a name="onigurumasubroutines"></a>
2698    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2699    .rs
2700    .sp
2701    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2702    a number enclosed either in angle brackets or single quotes, is an alternative
2703    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2704    are two of the examples used above, rewritten using this syntax:
2705    .sp
2706      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2707      (sens|respons)e and \eg'1'ibility
2708    .sp
2709    PCRE supports an extension to Oniguruma: if a number is preceded by a
2710    plus or a minus sign it is taken as a relative reference. For example:
2711    .sp
2712      (abc)(?i:\eg<-1>)
2713    .sp
2714    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2715    synonymous. The former is a back reference; the latter is a subroutine call.
2716  .  .
2717  .  .
2718  .SH CALLOUTS  .SH CALLOUTS
# Line 1614  same pair of parentheses when there is a Line 2725  same pair of parentheses when there is a
2725  .P  .P
2726  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
2727  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
2728  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
2729    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2730  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2731  .P  .P
2732  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 1622  function is to be called. If you want to Line 2734  function is to be called. If you want to
2734  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2735  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2736  .sp  .sp
2737    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2738  .sp  .sp
2739  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
2740  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2741  255.  255. If there is a conditional group in the pattern whose condition is an
2742  .P  assertion, an additional callout is inserted just before the condition. An
2743  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:
2744  set), the external function is called. It is provided with the number of the  .sp
2745  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2746  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2747  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
2748  description of the interface to the callout function is given in the  condition.
2749    .P
2750    During matching, when PCRE reaches a callout point, the external function is
2751    called. It is provided with the number of the callout, the position in the
2752    pattern, and, optionally, one item of data originally supplied by the caller of
2753    the matching function. The callout function may cause matching to proceed, to
2754    backtrack, or to fail altogether. A complete description of the interface to
2755    the callout function is given in the
2756  .\" HREF  .\" HREF
2757  \fBpcrecallout\fP  \fBpcrecallout\fP
2758  .\"  .\"
2759  documentation.  documentation.
2760    .
2761    .
2762    .\" HTML <a name="backtrackcontrol"></a>
2763    .SH "BACKTRACKING CONTROL"
2764    .rs
2765    .sp
2766    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2767    are still described in the Perl documentation as "experimental and subject to
2768    change or removal in a future version of Perl". It goes on to say: "Their usage
2769    in production code should be noted to avoid problems during upgrades." The same
2770    remarks apply to the PCRE features described in this section.
2771    .P
2772    The new verbs make use of what was previously invalid syntax: an opening
2773    parenthesis followed by an asterisk. They are generally of the form
2774    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2775    differently depending on whether or not a name is present. A name is any
2776    sequence of characters that does not include a closing parenthesis. The maximum
2777    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2778    libraries. If the name is empty, that is, if the closing parenthesis
2779    immediately follows the colon, the effect is as if the colon were not there.
2780    Any number of these verbs may occur in a pattern.
2781    .P
2782    Since these verbs are specifically related to backtracking, most of them can be
2783    used only when the pattern is to be matched using one of the traditional
2784    matching functions, because these use a backtracking algorithm. With the
2785    exception of (*FAIL), which behaves like a failing negative assertion, the
2786    backtracking control verbs cause an error if encountered by a DFA matching
2787    function.
2788    .P
2789    The behaviour of these verbs in
2790    .\" HTML <a href="#btrepeat">
2791    .\" </a>
2792    repeated groups,
2793    .\"
2794    .\" HTML <a href="#btassert">
2795    .\" </a>
2796    assertions,
2797    .\"
2798    and in
2799    .\" HTML <a href="#btsub">
2800    .\" </a>
2801    subpatterns called as subroutines
2802    .\"
2803    (whether or not recursively) is documented below.
2804    .
2805    .
2806    .\" HTML <a name="nooptimize"></a>
2807    .SS "Optimizations that affect backtracking verbs"
2808    .rs
2809    .sp
2810    PCRE contains some optimizations that are used to speed up matching by running
2811    some checks at the start of each match attempt. For example, it may know the
2812    minimum length of matching subject, or that a particular character must be
2813    present. When one of these optimizations bypasses the running of a match, any
2814    included backtracking verbs will not, of course, be processed. You can suppress
2815    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2816    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2817    pattern with (*NO_START_OPT). There is more discussion of this option in the
2818    section entitled
2819    .\" HTML <a href="pcreapi.html#execoptions">
2820    .\" </a>
2821    "Option bits for \fBpcre_exec()\fP"
2822    .\"
2823    in the
2824    .\" HREF
2825    \fBpcreapi\fP
2826    .\"
2827    documentation.
2828    .P
2829    Experiments with Perl suggest that it too has similar optimizations, sometimes
2830    leading to anomalous results.
2831    .
2832    .
2833    .SS "Verbs that act immediately"
2834    .rs
2835    .sp
2836    The following verbs act as soon as they are encountered. They may not be
2837    followed by a name.
2838    .sp
2839       (*ACCEPT)
2840    .sp
2841    This verb causes the match to end successfully, skipping the remainder of the
2842    pattern. However, when it is inside a subpattern that is called as a
2843    subroutine, only that subpattern is ended successfully. Matching then continues
2844    at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2845    assertion succeeds; in a negative assertion, the assertion fails.
2846    .P
2847    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2848    example:
2849    .sp
2850      A((?:A|B(*ACCEPT)|C)D)
2851    .sp
2852    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2853    the outer parentheses.
2854    .sp
2855      (*FAIL) or (*F)
2856    .sp
2857    This verb causes a matching failure, forcing backtracking to occur. It is
2858    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2859    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2860    Perl features that are not present in PCRE. The nearest equivalent is the
2861    callout feature, as for example in this pattern:
2862    .sp
2863      a+(?C)(*FAIL)
2864    .sp
2865    A match with the string "aaaa" always fails, but the callout is taken before
2866    each backtrack happens (in this example, 10 times).
2867    .
2868    .
2869    .SS "Recording which path was taken"
2870    .rs
2871    .sp
2872    There is one verb whose main purpose is to track how a match was arrived at,
2873    though it also has a secondary use in conjunction with advancing the match
2874    starting point (see (*SKIP) below).
2875    .sp
2876      (*MARK:NAME) or (*:NAME)
2877    .sp
2878    A name is always required with this verb. There may be as many instances of
2879    (*MARK) as you like in a pattern, and their names do not have to be unique.
2880    .P
2881    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2882    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2883    caller as described in the section entitled
2884    .\" HTML <a href="pcreapi.html#extradata">
2885    .\" </a>
2886    "Extra data for \fBpcre_exec()\fP"
2887    .\"
2888    in the
2889    .\" HREF
2890    \fBpcreapi\fP
2891    .\"
2892    documentation. Here is an example of \fBpcretest\fP output, where the /K
2893    modifier requests the retrieval and outputting of (*MARK) data:
2894    .sp
2895        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2896      data> XY
2897       0: XY
2898      MK: A
2899      XZ
2900       0: XZ
2901      MK: B
2902    .sp
2903    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2904    indicates which of the two alternatives matched. This is a more efficient way
2905    of obtaining this information than putting each alternative in its own
2906    capturing parentheses.
2907    .P
2908    If a verb with a name is encountered in a positive assertion that is true, the
2909    name is recorded and passed back if it is the last-encountered. This does not
2910    happen for negative assertions or failing positive assertions.
2911    .P
2912    After a partial match or a failed match, the last encountered name in the
2913    entire match process is returned. For example:
2914    .sp
2915        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2916      data> XP
2917      No match, mark = B
2918    .sp
2919    Note that in this unanchored example the mark is retained from the match
2920    attempt that started at the letter "X" in the subject. Subsequent match
2921    attempts starting at "P" and then with an empty string do not get as far as the
2922    (*MARK) item, but nevertheless do not reset it.
2923    .P
2924    If you are interested in (*MARK) values after failed matches, you should
2925    probably set the PCRE_NO_START_OPTIMIZE option
2926    .\" HTML <a href="#nooptimize">
2927    .\" </a>
2928    (see above)
2929    .\"
2930    to ensure that the match is always attempted.
2931    .
2932    .
2933    .SS "Verbs that act after backtracking"
2934    .rs
2935    .sp
2936    The following verbs do nothing when they are encountered. Matching continues
2937    with what follows, but if there is no subsequent match, causing a backtrack to
2938    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2939    the verb. However, when one of these verbs appears inside an atomic group or an
2940    assertion that is true, its effect is confined to that group, because once the
2941    group has been matched, there is never any backtracking into it. In this
2942    situation, backtracking can "jump back" to the left of the entire atomic group
2943    or assertion. (Remember also, as stated above, that this localization also
2944    applies in subroutine calls.)
2945  .P  .P
2946  .in 0  These verbs differ in exactly what kind of failure occurs when backtracking
2947  Last updated: 06 June 2006  reaches them. The behaviour described below is what happens when the verb is
2948  .br  not in a subroutine or an assertion. Subsequent sections cover these special
2949  Copyright (c) 1997-2006 University of Cambridge.  cases.
2950    .sp
2951      (*COMMIT)
2952    .sp
2953    This verb, which may not be followed by a name, causes the whole match to fail
2954    outright if there is a later matching failure that causes backtracking to reach
2955    it. Even if the pattern is unanchored, no further attempts to find a match by
2956    advancing the starting point take place. If (*COMMIT) is the only backtracking
2957    verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2958    committed to finding a match at the current starting point, or not at all. For
2959    example:
2960    .sp
2961      a+(*COMMIT)b
2962    .sp
2963    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2964    dynamic anchor, or "I've started, so I must finish." The name of the most
2965    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2966    match failure.
2967    .P
2968    If there is more than one backtracking verb in a pattern, a different one that
2969    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2970    match does not always guarantee that a match must be at this starting point.
2971    .P
2972    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2973    unless PCRE's start-of-match optimizations are turned off, as shown in this
2974    \fBpcretest\fP example:
2975    .sp
2976        re> /(*COMMIT)abc/
2977      data> xyzabc
2978       0: abc
2979      xyzabc\eY
2980      No match
2981    .sp
2982    PCRE knows that any match must start with "a", so the optimization skips along
2983    the subject to "a" before running the first match attempt, which succeeds. When
2984    the optimization is disabled by the \eY escape in the second subject, the match
2985    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2986    starting points.
2987    .sp
2988      (*PRUNE) or (*PRUNE:NAME)
2989    .sp
2990    This verb causes the match to fail at the current starting position in the
2991    subject if there is a later matching failure that causes backtracking to reach
2992    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2993    starting character then happens. Backtracking can occur as usual to the left of
2994    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2995    if there is no match to the right, backtracking cannot cross (*PRUNE). In
2996    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2997    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2998    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2999    as (*COMMIT).
3000    .P
3001    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3002    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3003    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3004    .sp
3005      (*SKIP)
3006    .sp
3007    This verb, when given without a name, is like (*PRUNE), except that if the
3008    pattern is unanchored, the "bumpalong" advance is not to the next character,
3009    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
3010    signifies that whatever text was matched leading up to it cannot be part of a
3011    successful match. Consider:
3012    .sp
3013      a+(*SKIP)b
3014    .sp
3015    If the subject is "aaaac...", after the first match attempt fails (starting at
3016    the first character in the string), the starting point skips on to start the
3017    next attempt at "c". Note that a possessive quantifer does not have the same
3018    effect as this example; although it would suppress backtracking during the
3019    first match attempt, the second attempt would start at the second character
3020    instead of skipping on to "c".
3021    .sp
3022      (*SKIP:NAME)
3023    .sp
3024    When (*SKIP) has an associated name, its behaviour is modified. When it is
3025    triggered, the previous path through the pattern is searched for the most
3026    recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3027    is to the subject position that corresponds to that (*MARK) instead of to where
3028    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3029    (*SKIP) is ignored.
3030    .P
3031    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3032    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3033    .sp
3034      (*THEN) or (*THEN:NAME)
3035    .sp
3036    This verb causes a skip to the next innermost alternative when backtracking
3037    reaches it. That is, it cancels any further backtracking within the current
3038    alternative. Its name comes from the observation that it can be used for a
3039    pattern-based if-then-else block:
3040    .sp
3041      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3042    .sp
3043    If the COND1 pattern matches, FOO is tried (and possibly further items after
3044    the end of the group if FOO succeeds); on failure, the matcher skips to the
3045    second alternative and tries COND2, without backtracking into COND1. If that
3046    succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3047    more alternatives, so there is a backtrack to whatever came before the entire
3048    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3049    .P
3050    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3051    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3052    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3053    .P
3054    A subpattern that does not contain a | character is just a part of the
3055    enclosing alternative; it is not a nested alternation with only one
3056    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3057    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3058    pattern fragments that do not contain any | characters at this level:
3059    .sp
3060      A (B(*THEN)C) | D
3061    .sp
3062    If A and B are matched, but there is a failure in C, matching does not
3063    backtrack into A; instead it moves to the next alternative, that is, D.
3064    However, if the subpattern containing (*THEN) is given an alternative, it
3065    behaves differently:
3066    .sp
3067      A (B(*THEN)C | (*FAIL)) | D
3068    .sp
3069    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3070    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3071    because there are no more alternatives to try. In this case, matching does now
3072    backtrack into A.
3073    .P
3074    Note that a conditional subpattern is not considered as having two
3075    alternatives, because only one is ever used. In other words, the | character in
3076    a conditional subpattern has a different meaning. Ignoring white space,
3077    consider:
3078    .sp
3079      ^.*? (?(?=a) a | b(*THEN)c )
3080    .sp
3081    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3082    it initially matches zero characters. The condition (?=a) then fails, the
3083    character "b" is matched, but "c" is not. At this point, matching does not
3084    backtrack to .*? as might perhaps be expected from the presence of the |
3085    character. The conditional subpattern is part of the single alternative that
3086    comprises the whole pattern, and so the match fails. (If there was a backtrack
3087    into .*?, allowing it to match "b", the match would succeed.)
3088    .P
3089    The verbs just described provide four different "strengths" of control when
3090    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3091    next alternative. (*PRUNE) comes next, failing the match at the current
3092    starting position, but allowing an advance to the next character (for an
3093    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3094    than one character. (*COMMIT) is the strongest, causing the entire match to
3095    fail.
3096    .
3097    .
3098    .SS "More than one backtracking verb"
3099    .rs
3100    .sp
3101    If more than one backtracking verb is present in a pattern, the one that is
3102    backtracked onto first acts. For example, consider this pattern, where A, B,
3103    etc. are complex pattern fragments:
3104    .sp
3105      (A(*COMMIT)B(*THEN)C|ABD)
3106    .sp
3107    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3108    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3109    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3110    not always the same as Perl's. It means that if two or more backtracking verbs
3111    appear in succession, all the the last of them has no effect. Consider this
3112    example:
3113    .sp
3114      ...(*COMMIT)(*PRUNE)...
3115    .sp
3116    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3117    it to be triggered, and its action is taken. There can never be a backtrack
3118    onto (*COMMIT).
3119    .
3120    .
3121    .\" HTML <a name="btrepeat"></a>
3122    .SS "Backtracking verbs in repeated groups"
3123    .rs
3124    .sp
3125    PCRE differs from Perl in its handling of backtracking verbs in repeated
3126    groups. For example, consider:
3127    .sp
3128      /(a(*COMMIT)b)+ac/
3129    .sp
3130    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3131    the second repeat of the group acts.
3132    .
3133    .
3134    .\" HTML <a name="btassert"></a>
3135    .SS "Backtracking verbs in assertions"
3136    .rs
3137    .sp
3138    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3139    .P
3140    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3141    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3142    fail without any further processing.
3143    .P
3144    The other backtracking verbs are not treated specially if they appear in a
3145    positive assertion. In particular, (*THEN) skips to the next alternative in the
3146    innermost enclosing group that has alternations, whether or not this is within
3147    the assertion.
3148    .P
3149    Negative assertions are, however, different, in order to ensure that changing a
3150    positive assertion into a negative assertion changes its result. Backtracking
3151    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3152    without considering any further alternative branches in the assertion.
3153    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3154    within the assertion (the normal behaviour), but if the assertion does not have
3155    such an alternative, (*THEN) behaves like (*PRUNE).
3156    .
3157    .
3158    .\" HTML <a name="btsub"></a>
3159    .SS "Backtracking verbs in subroutines"
3160    .rs
3161    .sp
3162    These behaviours occur whether or not the subpattern is called recursively.
3163    Perl's treatment of subroutines is different in some cases.
3164    .P
3165    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3166    an immediate backtrack.
3167    .P
3168    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3169    succeed without any further processing. Matching then continues after the
3170    subroutine call.
3171    .P
3172    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3173    the subroutine match to fail.
3174    .P
3175    (*THEN) skips to the next alternative in the innermost enclosing group within
3176    the subpattern that has alternatives. If there is no such group within the
3177    subpattern, (*THEN) causes the subroutine match to fail.
3178    .
3179    .
3180    .SH "SEE ALSO"
3181    .rs
3182    .sp
3183    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3184    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3185    .
3186    .
3187    .SH AUTHOR
3188    .rs
3189    .sp
3190    .nf
3191    Philip Hazel
3192    University Computing Service
3193    Cambridge CB2 3QH, England.
3194    .fi
3195    .
3196    .
3197    .SH REVISION
3198    .rs
3199    .sp
3200    .nf
3201    Last updated: 05 November 2013
3202    Copyright (c) 1997-2013 University of Cambridge.
3203    .fi

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