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1  .TH PCRE 3  .TH PCREPATTERN 3 "06 September 2013" "PCRE 8.34"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE REGULAR EXPRESSION DETAILS  .SH "PCRE REGULAR EXPRESSION DETAILS"
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 other books, some of which have copious  .\" HREF
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  \fBpcresyntax\fP
11  O'Reilly, covers them in great detail. The description here is intended as  .\"
12  reference documentation.  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  The basic operation of PCRE is on strings of bytes. However, there is also  conflict with the Perl syntax) in order to provide some compatibility with
15  support for UTF-8 character strings. To use this support you must build PCRE to  regular expressions in Python, .NET, and Oniguruma.
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  .P
17  option. How this affects the pattern matching is mentioned in several places  Perl's regular expressions are described in its own documentation, and
18  below. There is also a summary of UTF-8 features in the  regular expressions in general are covered in a number of books, some of which
19  .\" HTML <a href="pcre.html#utf8support">  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20  .\" </a>  published by O'Reilly, covers regular expressions in great detail. This
21  section on UTF-8 support  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
32    \fBpcrematching\fP
33  .\"  .\"
34  in the main  page.
35    .
36    .
37    .SH "SPECIAL START-OF-PATTERN ITEMS"
38    .rs
39    .sp
40    A number of options that can be passed to \fBpcre_compile()\fP can also be set
41    by special items at the start of a pattern. These are not Perl-compatible, but
42    are provided to make these options accessible to pattern writers who are not
43    able to change the program that processes the pattern. Any number of these
44    items may appear, but they must all be together right at the start of the
45    pattern string, and the letters must be in upper case.
46    .
47    .
48    .SS "UTF support"
49    .rs
50    .sp
51    The original operation of PCRE was on strings of one-byte characters. However,
52    there is now also support for UTF-8 strings in the original library, an
53    extra library that supports 16-bit and UTF-16 character strings, and a
54    third library that supports 32-bit and UTF-32 character strings. To use these
55    features, PCRE must be built to include appropriate support. When using UTF
56    strings you must either call the compiling function with the PCRE_UTF8,
57    PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58    these special sequences:
59    .sp
60      (*UTF8)
61      (*UTF16)
62      (*UTF32)
63      (*UTF)
64    .sp
65    (*UTF) is a generic sequence that can be used with any of the libraries.
66    Starting a pattern with such a sequence is equivalent to setting the relevant
67    option. How setting a UTF mode affects pattern matching is mentioned in several
68    places below. There is also a summary of features in the
69  .\" HREF  .\" HREF
70  \fBpcre\fR  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73    .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 by the caller of \fBpcre_exec()\fP for it to have
168    any effect. In other words, the pattern writer can lower the limit set by the
169    programmer, but not raise it. If there is more than one setting of one of these
170    limits, the lower value is used.
171    .
172    .
173    .SH "EBCDIC CHARACTER CODES"
174    .rs
175    .sp
176    PCRE can be compiled to run in an environment that uses EBCDIC as its character
177    code rather than ASCII or Unicode (typically a mainframe system). In the
178    sections below, character code values are ASCII or Unicode; in an EBCDIC
179    environment these characters may have different code values, and there are no
180    code points greater than 255.
181    .
182    .
183    .SH "CHARACTERS AND METACHARACTERS"
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
189    .sp
190    The quick brown fox    The quick brown fox
191    .sp
192  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
193  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
194  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In a UTF mode, PCRE always understands the concept of
195  \fImeta-characters\fR, which do not stand for themselves but instead are  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
197    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
199    ensure that PCRE is compiled with Unicode property support as well as with
200    UTF support.
201    .P
202    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
204    \fImetacharacters\fP, which do not stand for themselves but instead are
205  interpreted in some special way.  interpreted in some special way.
206    .P
207  There are two different sets of meta-characters: 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 meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
210  as follows:  are as follows:
211    .sp
212    \\      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)
214    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
215    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 224  as follows:
224    +      1 or more quantifier    +      1 or more quantifier
225           also "possessive quantifier"           also "possessive quantifier"
226    {      start min/max quantifier    {      start min/max quantifier
227    .sp
228  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
229  a character class the only meta-characters are:  a character class the only metacharacters are:
230    .sp
231    \\      general escape character    \e      general escape character
232    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
233    -      indicates character range    -      indicates character range
234    .\" JOIN
235    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
236             syntax)             syntax)
237    ]      terminates the character class    ]      terminates the character class
238    .sp
239  The following sections describe the use of each of the meta-characters.  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-alphameric character, it takes away any special meaning that character may  character that is not a number or a letter, it takes away any special meaning
247  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
250  For example, if you want to match a * character, you write \\* 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
252  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
253  non-alphameric 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 \\\\.  particular, if you want to match a backslash, you write \e\e.
255    .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 character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
263  backslash can be used to include a whitespace or # character as part of the  be used to include a white space or # character as part of the pattern.
264  pattern.  .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 \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
267  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
268  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
269    .sp
270    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
271    .sp
272    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
273      \eQabc$xyz\eE        abc$xyz        abc followed by the
274                                        contents of $xyz                                        contents of $xyz
275    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
276    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
277    .sp
278  The \\Q...\\E 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>
287    .SS "Non-printing characters"
288    .rs
289    .sp
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:
295  represents:  .sp
296      \ea        alarm, that is, the BEL character (hex 07)
297    \\a        alarm, that is, the BEL character (hex 07)    \ecx       "control-x", where x is any ASCII character
298    \\cx       "control-x", where x is any character    \ee        escape (hex 1B)
299    \\e        escape (hex 1B)    \ef        form feed (hex 0C)
300    \\f        formfeed (hex 0C)    \en        linefeed (hex 0A)
301    \\n        newline (hex 0A)    \er        carriage return (hex 0D)
302    \\r        carriage return (hex 0D)    \et        tab (hex 09)
303    \\t        tab (hex 09)    \eddd      character with octal code ddd, or back reference
304    \\ddd      character with octal code ddd, or backreference    \exhh      character with hex code hh
305    \\xhh      character with hex code hh    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
306    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \euhhhh    character with hex code hhhh (JavaScript mode only)
307    .sp
308  The precise effect of \\cx 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
309  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
310  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
311  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
312    data item (byte or 16-bit value) following \ec has a value greater than 127, a
313  After \\x, 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.
314  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  .P
315  appear between \\x{ and }, but the value of the character code must be less  The \ec facility was designed for use with ASCII characters, but with the
316  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  extension to Unicode it is even less useful than it once was. It is, however,
317  other than hexadecimal digits appear between \\x{ and }, or if there is no  recognized when PCRE is compiled in EBCDIC mode, where data items are always
318  terminating }, this form of escape is not recognized. Instead, the initial  bytes. In this mode, all values are valid after \ec. If the next character is a
319  \\x will be interpreted as a basic hexadecimal escape, with no following  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
320  digits, giving a byte whose value is zero.  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
321    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
322    characters also generate different values.
323    .P
324    By default, after \ex, from zero to two hexadecimal digits are read (letters
325    can be in upper or lower case). Any number of hexadecimal digits may appear
326    between \ex{ and }, but the character code is constrained as follows:
327    .sp
328      8-bit non-UTF mode    less than 0x100
329      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
330      16-bit non-UTF mode   less than 0x10000
331      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
332      32-bit non-UTF mode   less than 0x80000000
333      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
334    .sp
335    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
336    "surrogate" codepoints), and 0xffef.
337    .P
338    If characters other than hexadecimal digits appear between \ex{ and }, or if
339    there is no terminating }, this form of escape is not recognized. Instead, the
340    initial \ex will be interpreted as a basic hexadecimal escape, with no
341    following digits, giving a character whose value is zero.
342    .P
343    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
344    as just described only when it is followed by two hexadecimal digits.
345    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
346    code points greater than 256 is provided by \eu, which must be followed by
347    four hexadecimal digits; otherwise it matches a literal "u" character.
348    Character codes specified by \eu in JavaScript mode are constrained in the same
349    was as those specified by \ex in non-JavaScript mode.
350    .P
351  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
352  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
353  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
354    \eu00dc in JavaScript mode).
355  After \\0 up to two further octal digits are read. In both cases, if there  .P
356  are fewer than two digits, just those that are present are used. Thus the  After \e0 up to two further octal digits are read. If there are fewer than two
357  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  digits, just those that are present are used. Thus the sequence \e0\ex\e07
358  (code value 7). Make sure you supply two digits after the initial zero if the  specifies two binary zeros followed by a BEL character (code value 7). Make
359  character that follows is itself an octal digit.  sure you supply two digits after the initial zero if the pattern character that
360    follows is itself an octal digit.
361    .P
362  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
363  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
364  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
365  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
366  taken as a \fIback reference\fR. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
367  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
368    .\" </a>
369    later,
370    .\"
371    following the discussion of
372    .\" HTML <a href="#subpattern">
373    .\" </a>
374    parenthesized subpatterns.
375    .\"
376    .P
377  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
378  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
379  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
380  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. The value of the character is
381    constrained in the same way as characters specified in hexadecimal.
382  For example:  For example:
383    .sp
384    \\040   is another way of writing a space    \e040   is another way of writing an ASCII space
385    \\40    is the same, provided there are fewer than 40  .\" JOIN
386      \e40    is the same, provided there are fewer than 40
387              previous capturing subpatterns              previous capturing subpatterns
388    \\7     is always a back reference    \e7     is always a back reference
389    \\11    might be a back reference, or another way of  .\" JOIN
390      \e11    might be a back reference, or another way of
391              writing a tab              writing a tab
392    \\011   is always a tab    \e011   is always a tab
393    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
394    \\113   might be a back reference, otherwise the  .\" JOIN
395      \e113   might be a back reference, otherwise the
396              character with octal code 113              character with octal code 113
397    \\377   might be a back reference, otherwise  .\" JOIN
398              the byte consisting entirely of 1 bits    \e377   might be a back reference, otherwise
399    \\81    is either a back reference, or a binary zero              the value 255 (decimal)
400    .\" JOIN
401      \e81    is either a back reference, or a binary zero
402              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
403    .sp
404  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
405  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
406    .P
407  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
408  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, \eb is
409  addition, inside a character class, the sequence \\b is interpreted as the  interpreted as the backspace character (hex 08).
410  backspace character (hex 08). Outside a character class it has a different  .P
411  meaning (see below).  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
412    inside a character class. Like other unrecognized escape sequences, they are
413  The third use of backslash is for specifying generic character types:  treated as the literal characters "B", "R", and "X" by default, but cause an
414    error if the PCRE_EXTRA option is set. Outside a character class, these
415    \\d     any decimal digit  sequences have different meanings.
416    \\D     any character that is not a decimal digit  .
417    \\s     any whitespace character  .
418    \\S     any character that is not a whitespace character  .SS "Unsupported escape sequences"
419    \\w     any "word" character  .rs
420    \\W     any "non-word" character  .sp
421    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
422  Each pair of escape sequences partitions the complete set of characters into  handler and used to modify the case of following characters. By default, PCRE
423  two disjoint sets. Any given character matches one, and only one, of each pair.  does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
424    option is set, \eU matches a "U" character, and \eu can be used to define a
425  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  character by code point, as described in the previous section.
426  \\w, and always match \\D, \\S, and \\W.  .
427    .
428  For compatibility with Perl, \\s does not match the VT character (code 11).  .SS "Absolute and relative back references"
429  This makes it different from the the POSIX "space" class. The \\s characters  .rs
430  are HT (9), LF (10), FF (12), CR (13), and space (32).  .sp
431    The sequence \eg followed by an unsigned or a negative number, optionally
432  A "word" character is any letter or digit or the underscore character, that is,  enclosed in braces, is an absolute or relative back reference. A named back
433  any character which can be part of a Perl "word". The definition of letters and  reference can be coded as \eg{name}. Back references are discussed
434  digits is controlled by PCRE's character tables, and may vary if locale-  .\" HTML <a href="#backreferences">
435  specific matching is taking place (see  .\" </a>
436    later,
437    .\"
438    following the discussion of
439    .\" HTML <a href="#subpattern">
440    .\" </a>
441    parenthesized subpatterns.
442    .\"
443    .
444    .
445    .SS "Absolute and relative subroutine calls"
446    .rs
447    .sp
448    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
449    a number enclosed either in angle brackets or single quotes, is an alternative
450    syntax for referencing a subpattern as a "subroutine". Details are discussed
451    .\" HTML <a href="#onigurumasubroutines">
452    .\" </a>
453    later.
454    .\"
455    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
456    synonymous. The former is a back reference; the latter is a
457    .\" HTML <a href="#subpatternsassubroutines">
458    .\" </a>
459    subroutine
460    .\"
461    call.
462    .
463    .
464    .\" HTML <a name="genericchartypes"></a>
465    .SS "Generic character types"
466    .rs
467    .sp
468    Another use of backslash is for specifying generic character types:
469    .sp
470      \ed     any decimal digit
471      \eD     any character that is not a decimal digit
472      \eh     any horizontal white space character
473      \eH     any character that is not a horizontal white space character
474      \es     any white space character
475      \eS     any character that is not a white space character
476      \ev     any vertical white space character
477      \eV     any character that is not a vertical white space character
478      \ew     any "word" character
479      \eW     any "non-word" character
480    .sp
481    There is also the single sequence \eN, which matches a non-newline character.
482    This is the same as
483    .\" HTML <a href="#fullstopdot">
484    .\" </a>
485    the "." metacharacter
486    .\"
487    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
488    PCRE does not support this.
489    .P
490    Each pair of lower and upper case escape sequences partitions the complete set
491    of characters into two disjoint sets. Any given character matches one, and only
492    one, of each pair. The sequences can appear both inside and outside character
493    classes. They each match one character of the appropriate type. If the current
494    matching point is at the end of the subject string, all of them fail, because
495    there is no character to match.
496    .P
497    For compatibility with Perl, \es does not match the VT character (code 11).
498    This makes it different from the the POSIX "space" class. The \es characters
499    are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
500    included in a Perl script, \es may match the VT character. In PCRE, it never
501    does.
502    .P
503    A "word" character is an underscore or any character that is a letter or digit.
504    By default, the definition of letters and digits is controlled by PCRE's
505    low-valued character tables, and may vary if locale-specific matching is taking
506    place (see
507  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
508  .\" </a>  .\" </a>
509  "Locale support"  "Locale support"
510  .\"  .\"
511  in the  in the
512  .\" HREF  .\" HREF
513  \fBpcreapi\fR  \fBpcreapi\fP
514  .\"  .\"
515  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
516  than 128 are used for accented letters, and these are matched by \\w.  or "french" in Windows, some character codes greater than 128 are used for
517    accented letters, and these are then matched by \ew. The use of locales with
518  These character type sequences can appear both inside and outside character  Unicode is discouraged.
519  classes. They each match one character of the appropriate type. If the current  .P
520  matching point is at the end of the subject string, all of them fail, since  By default, in a UTF mode, characters with values greater than 128 never match
521  there is no character to match.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
522    their original meanings from before UTF support was available, mainly for
523  The fourth use of backslash is for certain simple assertions. An assertion  efficiency reasons. However, if PCRE is compiled with Unicode property support,
524    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
525    properties are used to determine character types, as follows:
526    .sp
527      \ed  any character that \ep{Nd} matches (decimal digit)
528      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
529      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
530    .sp
531    The upper case escapes match the inverse sets of characters. Note that \ed
532    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
533    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
534    \eB because they are defined in terms of \ew and \eW. Matching these sequences
535    is noticeably slower when PCRE_UCP is set.
536    .P
537    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
538    release 5.10. In contrast to the other sequences, which match only ASCII
539    characters by default, these always match certain high-valued codepoints,
540    whether or not PCRE_UCP is set. The horizontal space characters are:
541    .sp
542      U+0009     Horizontal tab (HT)
543      U+0020     Space
544      U+00A0     Non-break space
545      U+1680     Ogham space mark
546      U+180E     Mongolian vowel separator
547      U+2000     En quad
548      U+2001     Em quad
549      U+2002     En space
550      U+2003     Em space
551      U+2004     Three-per-em space
552      U+2005     Four-per-em space
553      U+2006     Six-per-em space
554      U+2007     Figure space
555      U+2008     Punctuation space
556      U+2009     Thin space
557      U+200A     Hair space
558      U+202F     Narrow no-break space
559      U+205F     Medium mathematical space
560      U+3000     Ideographic space
561    .sp
562    The vertical space characters are:
563    .sp
564      U+000A     Linefeed (LF)
565      U+000B     Vertical tab (VT)
566      U+000C     Form feed (FF)
567      U+000D     Carriage return (CR)
568      U+0085     Next line (NEL)
569      U+2028     Line separator
570      U+2029     Paragraph separator
571    .sp
572    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
573    relevant.
574    .
575    .
576    .\" HTML <a name="newlineseq"></a>
577    .SS "Newline sequences"
578    .rs
579    .sp
580    Outside a character class, by default, the escape sequence \eR matches any
581    Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
582    following:
583    .sp
584      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
585    .sp
586    This is an example of an "atomic group", details of which are given
587    .\" HTML <a href="#atomicgroup">
588    .\" </a>
589    below.
590    .\"
591    This particular group matches either the two-character sequence CR followed by
592    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
593    U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
594    line, U+0085). The two-character sequence is treated as a single unit that
595    cannot be split.
596    .P
597    In other modes, two additional characters whose codepoints are greater than 255
598    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
599    Unicode character property support is not needed for these characters to be
600    recognized.
601    .P
602    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
603    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
604    either at compile time or when the pattern is matched. (BSR is an abbrevation
605    for "backslash R".) This can be made the default when PCRE is built; if this is
606    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
607    It is also possible to specify these settings by starting a pattern string with
608    one of the following sequences:
609    .sp
610      (*BSR_ANYCRLF)   CR, LF, or CRLF only
611      (*BSR_UNICODE)   any Unicode newline sequence
612    .sp
613    These override the default and the options given to the compiling function, but
614    they can themselves be overridden by options given to a matching function. Note
615    that these special settings, which are not Perl-compatible, are recognized only
616    at the very start of a pattern, and that they must be in upper case. If more
617    than one of them is present, the last one is used. They can be combined with a
618    change of newline convention; for example, a pattern can start with:
619    .sp
620      (*ANY)(*BSR_ANYCRLF)
621    .sp
622    They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
623    (*UCP) special sequences. Inside a character class, \eR is treated as an
624    unrecognized escape sequence, and so matches the letter "R" by default, but
625    causes an error if PCRE_EXTRA is set.
626    .
627    .
628    .\" HTML <a name="uniextseq"></a>
629    .SS Unicode character properties
630    .rs
631    .sp
632    When PCRE is built with Unicode character property support, three additional
633    escape sequences that match characters with specific properties are available.
634    When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
635    characters whose codepoints are less than 256, but they do work in this mode.
636    The extra escape sequences are:
637    .sp
638      \ep{\fIxx\fP}   a character with the \fIxx\fP property
639      \eP{\fIxx\fP}   a character without the \fIxx\fP property
640      \eX       a Unicode extended grapheme cluster
641    .sp
642    The property names represented by \fIxx\fP above are limited to the Unicode
643    script names, the general category properties, "Any", which matches any
644    character (including newline), and some special PCRE properties (described
645    in the
646    .\" HTML <a href="#extraprops">
647    .\" </a>
648    next section).
649    .\"
650    Other Perl properties such as "InMusicalSymbols" are not currently supported by
651    PCRE. Note that \eP{Any} does not match any characters, so always causes a
652    match failure.
653    .P
654    Sets of Unicode characters are defined as belonging to certain scripts. A
655    character from one of these sets can be matched using a script name. For
656    example:
657    .sp
658      \ep{Greek}
659      \eP{Han}
660    .sp
661    Those that are not part of an identified script are lumped together as
662    "Common". The current list of scripts is:
663    .P
664    Arabic,
665    Armenian,
666    Avestan,
667    Balinese,
668    Bamum,
669    Batak,
670    Bengali,
671    Bopomofo,
672    Brahmi,
673    Braille,
674    Buginese,
675    Buhid,
676    Canadian_Aboriginal,
677    Carian,
678    Chakma,
679    Cham,
680    Cherokee,
681    Common,
682    Coptic,
683    Cuneiform,
684    Cypriot,
685    Cyrillic,
686    Deseret,
687    Devanagari,
688    Egyptian_Hieroglyphs,
689    Ethiopic,
690    Georgian,
691    Glagolitic,
692    Gothic,
693    Greek,
694    Gujarati,
695    Gurmukhi,
696    Han,
697    Hangul,
698    Hanunoo,
699    Hebrew,
700    Hiragana,
701    Imperial_Aramaic,
702    Inherited,
703    Inscriptional_Pahlavi,
704    Inscriptional_Parthian,
705    Javanese,
706    Kaithi,
707    Kannada,
708    Katakana,
709    Kayah_Li,
710    Kharoshthi,
711    Khmer,
712    Lao,
713    Latin,
714    Lepcha,
715    Limbu,
716    Linear_B,
717    Lisu,
718    Lycian,
719    Lydian,
720    Malayalam,
721    Mandaic,
722    Meetei_Mayek,
723    Meroitic_Cursive,
724    Meroitic_Hieroglyphs,
725    Miao,
726    Mongolian,
727    Myanmar,
728    New_Tai_Lue,
729    Nko,
730    Ogham,
731    Old_Italic,
732    Old_Persian,
733    Old_South_Arabian,
734    Old_Turkic,
735    Ol_Chiki,
736    Oriya,
737    Osmanya,
738    Phags_Pa,
739    Phoenician,
740    Rejang,
741    Runic,
742    Samaritan,
743    Saurashtra,
744    Sharada,
745    Shavian,
746    Sinhala,
747    Sora_Sompeng,
748    Sundanese,
749    Syloti_Nagri,
750    Syriac,
751    Tagalog,
752    Tagbanwa,
753    Tai_Le,
754    Tai_Tham,
755    Tai_Viet,
756    Takri,
757    Tamil,
758    Telugu,
759    Thaana,
760    Thai,
761    Tibetan,
762    Tifinagh,
763    Ugaritic,
764    Vai,
765    Yi.
766    .P
767    Each character has exactly one Unicode general category property, specified by
768    a two-letter abbreviation. For compatibility with Perl, negation can be
769    specified by including a circumflex between the opening brace and the property
770    name. For example, \ep{^Lu} is the same as \eP{Lu}.
771    .P
772    If only one letter is specified with \ep or \eP, it includes all the general
773    category properties that start with that letter. In this case, in the absence
774    of negation, the curly brackets in the escape sequence are optional; these two
775    examples have the same effect:
776    .sp
777      \ep{L}
778      \epL
779    .sp
780    The following general category property codes are supported:
781    .sp
782      C     Other
783      Cc    Control
784      Cf    Format
785      Cn    Unassigned
786      Co    Private use
787      Cs    Surrogate
788    .sp
789      L     Letter
790      Ll    Lower case letter
791      Lm    Modifier letter
792      Lo    Other letter
793      Lt    Title case letter
794      Lu    Upper case letter
795    .sp
796      M     Mark
797      Mc    Spacing mark
798      Me    Enclosing mark
799      Mn    Non-spacing mark
800    .sp
801      N     Number
802      Nd    Decimal number
803      Nl    Letter number
804      No    Other number
805    .sp
806      P     Punctuation
807      Pc    Connector punctuation
808      Pd    Dash punctuation
809      Pe    Close punctuation
810      Pf    Final punctuation
811      Pi    Initial punctuation
812      Po    Other punctuation
813      Ps    Open punctuation
814    .sp
815      S     Symbol
816      Sc    Currency symbol
817      Sk    Modifier symbol
818      Sm    Mathematical symbol
819      So    Other symbol
820    .sp
821      Z     Separator
822      Zl    Line separator
823      Zp    Paragraph separator
824      Zs    Space separator
825    .sp
826    The special property L& is also supported: it matches a character that has
827    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
828    a modifier or "other".
829    .P
830    The Cs (Surrogate) property applies only to characters in the range U+D800 to
831    U+DFFF. Such characters are not valid in Unicode strings and so
832    cannot be tested by PCRE, unless UTF validity checking has been turned off
833    (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
834    PCRE_NO_UTF32_CHECK in the
835    .\" HREF
836    \fBpcreapi\fP
837    .\"
838    page). Perl does not support the Cs property.
839    .P
840    The long synonyms for property names that Perl supports (such as \ep{Letter})
841    are not supported by PCRE, nor is it permitted to prefix any of these
842    properties with "Is".
843    .P
844    No character that is in the Unicode table has the Cn (unassigned) property.
845    Instead, this property is assumed for any code point that is not in the
846    Unicode table.
847    .P
848    Specifying caseless matching does not affect these escape sequences. For
849    example, \ep{Lu} always matches only upper case letters. This is different from
850    the behaviour of current versions of Perl.
851    .P
852    Matching characters by Unicode property is not fast, because PCRE has to do a
853    multistage table lookup in order to find a character's property. That is why
854    the traditional escape sequences such as \ed and \ew do not use Unicode
855    properties in PCRE by default, though you can make them do so by setting the
856    PCRE_UCP option or by starting the pattern with (*UCP).
857    .
858    .
859    .SS Extended grapheme clusters
860    .rs
861    .sp
862    The \eX escape matches any number of Unicode characters that form an "extended
863    grapheme cluster", and treats the sequence as an atomic group
864    .\" HTML <a href="#atomicgroup">
865    .\" </a>
866    (see below).
867    .\"
868    Up to and including release 8.31, PCRE matched an earlier, simpler definition
869    that was equivalent to
870    .sp
871      (?>\ePM\epM*)
872    .sp
873    That is, it matched a character without the "mark" property, followed by zero
874    or more characters with the "mark" property. Characters with the "mark"
875    property are typically non-spacing accents that affect the preceding character.
876    .P
877    This simple definition was extended in Unicode to include more complicated
878    kinds of composite character by giving each character a grapheme breaking
879    property, and creating rules that use these properties to define the boundaries
880    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
881    one of these clusters.
882    .P
883    \eX always matches at least one character. Then it decides whether to add
884    additional characters according to the following rules for ending a cluster:
885    .P
886    1. End at the end of the subject string.
887    .P
888    2. Do not end between CR and LF; otherwise end after any control character.
889    .P
890    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
891    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
892    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
893    character; an LVT or T character may be follwed only by a T character.
894    .P
895    4. Do not end before extending characters or spacing marks. Characters with
896    the "mark" property always have the "extend" grapheme breaking property.
897    .P
898    5. Do not end after prepend characters.
899    .P
900    6. Otherwise, end the cluster.
901    .
902    .
903    .\" HTML <a name="extraprops"></a>
904    .SS PCRE's additional properties
905    .rs
906    .sp
907    As well as the standard Unicode properties described above, PCRE supports four
908    more that make it possible to convert traditional escape sequences such as \ew
909    and \es and POSIX character classes to use Unicode properties. PCRE uses these
910    non-standard, non-Perl properties internally when PCRE_UCP is set. However,
911    they may also be used explicitly. These properties are:
912    .sp
913      Xan   Any alphanumeric character
914      Xps   Any POSIX space character
915      Xsp   Any Perl space character
916      Xwd   Any Perl "word" character
917    .sp
918    Xan matches characters that have either the L (letter) or the N (number)
919    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
920    carriage return, and any other character that has the Z (separator) property.
921    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
922    same characters as Xan, plus underscore.
923    .P
924    There is another non-standard property, Xuc, which matches any character that
925    can be represented by a Universal Character Name in C++ and other programming
926    languages. These are the characters $, @, ` (grave accent), and all characters
927    with Unicode code points greater than or equal to U+00A0, except for the
928    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
929    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
930    where H is a hexadecimal digit. Note that the Xuc property does not match these
931    sequences but the characters that they represent.)
932    .
933    .
934    .\" HTML <a name="resetmatchstart"></a>
935    .SS "Resetting the match start"
936    .rs
937    .sp
938    The escape sequence \eK causes any previously matched characters not to be
939    included in the final matched sequence. For example, the pattern:
940    .sp
941      foo\eKbar
942    .sp
943    matches "foobar", but reports that it has matched "bar". This feature is
944    similar to a lookbehind assertion
945    .\" HTML <a href="#lookbehind">
946    .\" </a>
947    (described below).
948    .\"
949    However, in this case, the part of the subject before the real match does not
950    have to be of fixed length, as lookbehind assertions do. The use of \eK does
951    not interfere with the setting of
952    .\" HTML <a href="#subpattern">
953    .\" </a>
954    captured substrings.
955    .\"
956    For example, when the pattern
957    .sp
958      (foo)\eKbar
959    .sp
960    matches "foobar", the first substring is still set to "foo".
961    .P
962    Perl documents that the use of \eK within assertions is "not well defined". In
963    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
964    ignored in negative assertions.
965    .
966    .
967    .\" HTML <a name="smallassertions"></a>
968    .SS "Simple assertions"
969    .rs
970    .sp
971    The final use of backslash is for certain simple assertions. An assertion
972  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,
973  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
974  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
975  assertions are  .\" HTML <a href="#bigassertions">
976    .\" </a>
977    \\b     matches at a word boundary  below.
978    \\B     matches when not at a word boundary  .\"
979    \\A     matches at start of subject  The backslashed assertions are:
980    \\Z     matches at end of subject or before newline at end  .sp
981    \\z     matches at end of subject    \eb     matches at a word boundary
982    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
983      \eA     matches at the start of the subject
984  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
985  different meaning, namely the backspace character, inside a character class).            also matches before a newline at the end of the subject
986      \ez     matches only at the end of the subject
987      \eG     matches at the first matching position in the subject
988    .sp
989    Inside a character class, \eb has a different meaning; it matches the backspace
990    character. If any other of these assertions appears in a character class, by
991    default it matches the corresponding literal character (for example, \eB
992    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
993    escape sequence" error is generated instead.
994    .P
995  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
996  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
997  \\w and the other matches \\W), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
998  first or last character matches \\w, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
999    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1000  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1001  dollar (described below) in that they only ever match at the very start and end  of word" or "end of word" metasequence. However, whatever follows \eb normally
1002  of the subject string, whatever options are set. Thus, they are independent of  determines which it is. For example, the fragment \eba matches "a" at the start
1003  multiline mode.  of a word.
1004    .P
1005  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1006  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  dollar (described in the next section) in that they only ever match at the very
1007  matching is to start at a point other than the beginning of the subject, \\A  start and end of the subject string, whatever options are set. Thus, they are
1008  can never match. The difference between \\Z and \\z is that \\Z matches before  independent of multiline mode. These three assertions are not affected by the
1009  a newline that is the last character of the string as well as at the end of the  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
1010  string, whereas \\z matches only at the end.  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
1011    argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
1012  The \\G assertion is true only when the current matching position is at the  at a point other than the beginning of the subject, \eA can never match. The
1013  start point of the match, as specified by the \fIstartoffset\fR argument of  difference between \eZ and \ez is that \eZ matches before a newline at the end
1014  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  of the string as well as at the very end, whereas \ez matches only at the end.
1015  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  .P
1016    The \eG assertion is true only when the current matching position is at the
1017    start point of the match, as specified by the \fIstartoffset\fP argument of
1018    \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
1019    non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
1020  arguments, you can mimic Perl's /g option, and it is in this kind of  arguments, you can mimic Perl's /g option, and it is in this kind of
1021  implementation where \\G can be useful.  implementation where \eG can be useful.
1022    .P
1023  Note, however, that PCRE's interpretation of \\G, as the start of the current  Note, however, that PCRE's interpretation of \eG, as the start of the current
1024  match, is subtly different from Perl's, which defines it as the end of the  match, is subtly different from Perl's, which defines it as the end of the
1025  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
1026  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
1027  reproduce this behaviour.  reproduce this behaviour.
1028    .P
1029  If all the alternatives of a pattern begin with \\G, the expression is anchored  If all the alternatives of a pattern begin with \eG, the expression is anchored
1030  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
1031  regular expression.  regular expression.
1032    .
1033  .SH CIRCUMFLEX AND DOLLAR  .
1034    .SH "CIRCUMFLEX AND DOLLAR"
1035  .rs  .rs
1036  .sp  .sp
1037    The circumflex and dollar metacharacters are zero-width assertions. That is,
1038    they test for a particular condition being true without consuming any
1039    characters from the subject string.
1040    .P
1041  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1042  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
1043  at the start of the subject string. If the \fIstartoffset\fR argument of  the start of the subject string. If the \fIstartoffset\fP argument of
1044  \fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1045  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1046  meaning (see below).  meaning
1047    .\" HTML <a href="#characterclass">
1048    .\" </a>
1049    (see below).
1050    .\"
1051    .P
1052  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
1053  alternatives are involved, but it should be the first thing in each alternative  alternatives are involved, but it should be the first thing in each alternative
1054  in which it appears if the pattern is ever to match that branch. If all  in which it appears if the pattern is ever to match that branch. If all
# Line 290  possible alternatives start with a circu Line 1056  possible alternatives start with a circu
1056  constrained to match only at the start of the subject, it is said to be an  constrained to match only at the start of the subject, it is said to be an
1057  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1058  to be anchored.)  to be anchored.)
1059    .P
1060  A dollar character is an assertion which is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1061  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1062  character that is the last character in the string (by default). Dollar need  the end of the string (by default). Note, however, that it does not actually
1063  not be the last character of the pattern if a number of alternatives are  match the newline. Dollar need not be the last character of the pattern if a
1064  involved, but it should be the last item in any branch in which it appears.  number of alternatives are involved, but it should be the last item in any
1065  Dollar has no special meaning in a character class.  branch in which it appears. Dollar has no special meaning in a character class.
1066    .P
1067  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1068  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
1069  does not affect the \\Z assertion.  does not affect the \eZ assertion.
1070    .P
1071  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
1072  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
1073  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
1074  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
1075  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
1076  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
1077  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
1078  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
1079  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
1080  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
1081    patterns that are anchored in single line mode because all branches start with
1082  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  ^ are not anchored in multiline mode, and a match for circumflex is possible
1083    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
1084    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
1085    .P
1086    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
1087  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
1088  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1089    .
1090  .SH FULL STOP (PERIOD, DOT)  .
1091    .\" HTML <a name="fullstopdot"></a>
1092    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1093  .rs  .rs
1094  .sp  .sp
1095  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
1096  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
1097  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line.
1098  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
1099  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
1100  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
1101  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
1102    (including isolated CRs and LFs). When any Unicode line endings are being
1103  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
1104  .rs  characters.
1105  .sp  .P
1106  Outside a character class, the escape sequence \\C matches any one byte, both  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
1107  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  option is set, a dot matches any one character, without exception. If the
1108  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  two-character sequence CRLF is present in the subject string, it takes two dots
1109  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
1110  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
1111    The handling of dot is entirely independent of the handling of circumflex and
1112  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  dollar, the only relationship being that they both involve newlines. Dot has no
1113  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
1114    .P
1115  .SH SQUARE BRACKETS  The escape sequence \eN behaves like a dot, except that it is not affected by
1116    the PCRE_DOTALL option. In other words, it matches any character except one
1117    that signifies the end of a line. Perl also uses \eN to match characters by
1118    name; PCRE does not support this.
1119    .
1120    .
1121    .SH "MATCHING A SINGLE DATA UNIT"
1122    .rs
1123    .sp
1124    Outside a character class, the escape sequence \eC matches any one data unit,
1125    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1126    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1127    a 32-bit unit. Unlike a dot, \eC always
1128    matches line-ending characters. The feature is provided in Perl in order to
1129    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1130    used. Because \eC breaks up characters into individual data units, matching one
1131    unit with \eC in a UTF mode means that the rest of the string may start with a
1132    malformed UTF character. This has undefined results, because PCRE assumes that
1133    it is dealing with valid UTF strings (and by default it checks this at the
1134    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1135    PCRE_NO_UTF32_CHECK option is used).
1136    .P
1137    PCRE does not allow \eC to appear in lookbehind assertions
1138    .\" HTML <a href="#lookbehind">
1139    .\" </a>
1140    (described below)
1141    .\"
1142    in a UTF mode, because this would make it impossible to calculate the length of
1143    the lookbehind.
1144    .P
1145    In general, the \eC escape sequence is best avoided. However, one
1146    way of using it that avoids the problem of malformed UTF characters is to use a
1147    lookahead to check the length of the next character, as in this pattern, which
1148    could be used with a UTF-8 string (ignore white space and line breaks):
1149    .sp
1150      (?| (?=[\ex00-\ex7f])(\eC) |
1151          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1152          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1153          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1154    .sp
1155    A group that starts with (?| resets the capturing parentheses numbers in each
1156    alternative (see
1157    .\" HTML <a href="#dupsubpatternnumber">
1158    .\" </a>
1159    "Duplicate Subpattern Numbers"
1160    .\"
1161    below). The assertions at the start of each branch check the next UTF-8
1162    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1163    character's individual bytes are then captured by the appropriate number of
1164    groups.
1165    .
1166    .
1167    .\" HTML <a name="characterclass"></a>
1168    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1169  .rs  .rs
1170  .sp  .sp
1171  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1172  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.
1173  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
1174  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
1175  escaped with a backslash.  a member of the class, it should be the first data character in the class
1176    (after an initial circumflex, if present) or escaped with a backslash.
1177  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1178  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
1179  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
1180  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
1181  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
1182  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
1183    member of the class, ensure it is not the first character, or escape it with a
1184  backslash.  backslash.
1185    .P
1186  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1187  [^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
1188  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
1189  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
1190  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion; it still consumes a character from the subject
1191  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
1192    string.
1193  In UTF-8 mode, characters with values greater than 255 can be included in a  .P
1194  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1195    can be included in a class as a literal string of data units, or by using the
1196    \ex{ escaping mechanism.
1197    .P
1198  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
1199  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
1200  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1201  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In a UTF mode, PCRE always understands the concept of
1202  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
1203    always possible. For characters with higher values, the concept of case is
1204  The newline character is never treated in any special way in character classes,  supported if PCRE is compiled with Unicode property support, but not otherwise.
1205  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching in a UTF mode for characters 128 and
1206  such as [^a] will always match a newline.  above, you must ensure that PCRE is compiled with Unicode property support as
1207    well as with UTF support.
1208    .P
1209    Characters that might indicate line breaks are never treated in any special way
1210    when matching character classes, whatever line-ending sequence is in use, and
1211    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1212    such as [^a] always matches one of these characters.
1213    .P
1214  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
1215  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,
1216  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
1217  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
1218  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class.
1219    .P
1220  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
1221  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
1222  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1223  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1224  the end of range, so [W-\\]46] is interpreted as a single class containing a  the end of range, so [W-\e]46] is interpreted as a class containing a range
1225  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
1226  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
1227    .P
1228  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
1229  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1230  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
1231  example [\\x{100}-\\x{2ff}].  .P
   
1232  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
1233  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
1234  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1235  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\exc8-\excb] matches accented E
1236    characters in both cases. In UTF modes, PCRE supports the concept of case for
1237  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a  characters with values greater than 128 only when it is compiled with Unicode
1238  character class, and add the characters that they match to the class. For  property support.
1239  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
1240  conveniently be used with the upper case character types to specify a more  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1241  restricted set of characters than the matching lower case type. For example,  \eV, \ew, and \eW may appear in a character class, and add the characters that
1242  the class [^\\W_] matches any letter or digit, but not underscore.  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1243    digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1244  All non-alphameric characters other than \\, -, ^ (at the start) and the  and their upper case partners, just as it does when they appear outside a
1245  terminating ] are non-special in character classes, but it does no harm if they  character class, as described in the section entitled
1246  are escaped.  .\" HTML <a href="#genericchartypes">
1247    .\" </a>
1248  .SH POSIX CHARACTER CLASSES  "Generic character types"
1249    .\"
1250    above. The escape sequence \eb has a different meaning inside a character
1251    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1252    are not special inside a character class. Like any other unrecognized escape
1253    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1254    default, but cause an error if the PCRE_EXTRA option is set.
1255    .P
1256    A circumflex can conveniently be used with the upper case character types to
1257    specify a more restricted set of characters than the matching lower case type.
1258    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1259    whereas [\ew] includes underscore. A positive character class should be read as
1260    "something OR something OR ..." and a negative class as "NOT something AND NOT
1261    something AND NOT ...".
1262    .P
1263    The only metacharacters that are recognized in character classes are backslash,
1264    hyphen (only where it can be interpreted as specifying a range), circumflex
1265    (only at the start), opening square bracket (only when it can be interpreted as
1266    introducing a POSIX class name - see the next section), and the terminating
1267    closing square bracket. However, escaping other non-alphanumeric characters
1268    does no harm.
1269    .
1270    .
1271    .SH "POSIX CHARACTER CLASSES"
1272  .rs  .rs
1273  .sp  .sp
1274  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
1275  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1276  this notation. For example,  this notation. For example,
1277    .sp
1278    [01[:alpha:]%]    [01[:alpha:]%]
1279    .sp
1280  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1281  are  are:
1282    .sp
1283    alnum    letters and digits    alnum    letters and digits
1284    alpha    letters    alpha    letters
1285    ascii    character codes 0 - 127    ascii    character codes 0 - 127
1286    blank    space or tab only    blank    space or tab only
1287    cntrl    control characters    cntrl    control characters
1288    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
1289    graph    printing characters, excluding space    graph    printing characters, excluding space
1290    lower    lower case letters    lower    lower case letters
1291    print    printing characters, including space    print    printing characters, including space
1292    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1293    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
1294    upper    upper case letters    upper    upper case letters
1295    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
1296    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1297    .sp
1298  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
1299  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
1300  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
1301  compatibility).  compatibility).
1302    .P
1303  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
1304  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
1305  after the colon. For example,  after the colon. For example,
1306    .sp
1307    [12[:^digit:]]    [12[:^digit:]]
1308    .sp
1309  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1310  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1311  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1312    .P
1313  In UTF-8 mode, characters with values greater than 255 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1314  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1315    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1316  .SH VERTICAL BAR  character properties are used. This is achieved by replacing the POSIX classes
1317    by other sequences, as follows:
1318    .sp
1319      [:alnum:]  becomes  \ep{Xan}
1320      [:alpha:]  becomes  \ep{L}
1321      [:blank:]  becomes  \eh
1322      [:digit:]  becomes  \ep{Nd}
1323      [:lower:]  becomes  \ep{Ll}
1324      [:space:]  becomes  \ep{Xps}
1325      [:upper:]  becomes  \ep{Lu}
1326      [:word:]   becomes  \ep{Xwd}
1327    .sp
1328    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1329    classes are unchanged, and match only characters with code points less than
1330    128.
1331    .
1332    .
1333    .SH "VERTICAL BAR"
1334  .rs  .rs
1335  .sp  .sp
1336  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1337  the pattern  the pattern
1338    .sp
1339    gilbert|sullivan    gilbert|sullivan
1340    .sp
1341  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1342  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1343  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1344  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
1345  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
1346  pattern as well as the alternative in the subpattern.  .\" </a>
1347    (defined below),
1348  .SH INTERNAL OPTION SETTING  .\"
1349    "succeeds" means matching the rest of the main pattern as well as the
1350    alternative in the subpattern.
1351    .
1352    .
1353    .SH "INTERNAL OPTION SETTING"
1354  .rs  .rs
1355  .sp  .sp
1356  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1357  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
1358  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1359    The option letters are
1360    .sp
1361    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1362    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
1363    s  for PCRE_DOTALL    s  for PCRE_DOTALL
1364    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
1365    .sp
1366  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
1367  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
1368  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1369  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1370  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
1371  unset.  unset.
1372    .P
1373  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
1374  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
1375  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1376  the global options (and it will therefore show up in data extracted by the  .P
1377  \fBpcre_fullinfo()\fR function).  When one of these option changes occurs at top level (that is, not inside
1378    subpattern parentheses), the change applies to the remainder of the pattern
1379  An option change within a subpattern affects only that part of the current  that follows. If the change is placed right at the start of a pattern, PCRE
1380  pattern that follows it, so  extracts it into the global options (and it will therefore show up in data
1381    extracted by the \fBpcre_fullinfo()\fP function).
1382    .P
1383    An option change within a subpattern (see below for a description of
1384    subpatterns) affects only that part of the subpattern that follows it, so
1385    .sp
1386    (a(?i)b)c    (a(?i)b)c
1387    .sp
1388  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1389  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
1390  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
1391  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
1392    .sp
1393    (a(?i)b|c)    (a(?i)b|c)
1394    .sp
1395  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
1396  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1397  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1398  behaviour otherwise.  behaviour otherwise.
1399    .P
1400  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1401  same way as the Perl-compatible options by using the characters U and X  application when the compiling or matching functions are called. In some cases
1402  respectively. The (?X) flag setting is special in that it must always occur  the pattern can contain special leading sequences such as (*CRLF) to override
1403  earlier in the pattern than any of the additional features it turns on, even  what the application has set or what has been defaulted. Details are given in
1404  when it is at top level. It is best put at the start.  the section entitled
1405    .\" HTML <a href="#newlineseq">
1406    .\" </a>
1407    "Newline sequences"
1408    .\"
1409    above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1410    sequences that can be used to set UTF and Unicode property modes; they are
1411    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1412    options, respectively. The (*UTF) sequence is a generic version that can be
1413    used with any of the libraries. However, the application can set the
1414    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1415    .
1416    .
1417    .\" HTML <a name="subpattern"></a>
1418  .SH SUBPATTERNS  .SH SUBPATTERNS
1419  .rs  .rs
1420  .sp  .sp
1421  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1422  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1423    .sp
1424  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
1425    .sp
1426    cat(aract|erpillar|)    cat(aract|erpillar|)
1427    .sp
1428  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1429  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1430    .sp
1431  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
1432  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
1433  the subpattern is passed back to the caller via the \fIovector\fR argument of  subpattern is passed back to the caller via the \fIovector\fP argument of the
1434  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1435  from 1) to obtain the numbers of the capturing subpatterns.  the DFA matching functions do not support capturing.)
1436    .P
1437  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
1438    numbers for the capturing subpatterns. For example, if the string "the red
1439    king" is matched against the pattern
1440    .sp
1441    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1442    .sp
1443  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
1444  2, and 3, respectively.  2, and 3, respectively.
1445    .P
1446  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
1447  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
1448  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
1449  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
1450  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1451  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1452    .sp
1453    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1454    .sp
1455  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
1456  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1457  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1458  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
1459  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1460  the ":". Thus the two patterns  the ":". Thus the two patterns
1461    .sp
1462    (?i:saturday|sunday)    (?i:saturday|sunday)
1463    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1464    .sp
1465  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1466  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1467  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1468  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1469    .
1470  .SH NAMED SUBPATTERNS  .
1471    .\" HTML <a name="dupsubpatternnumber"></a>
1472    .SH "DUPLICATE SUBPATTERN NUMBERS"
1473    .rs
1474    .sp
1475    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1476    the same numbers for its capturing parentheses. Such a subpattern starts with
1477    (?| and is itself a non-capturing subpattern. For example, consider this
1478    pattern:
1479    .sp
1480      (?|(Sat)ur|(Sun))day
1481    .sp
1482    Because the two alternatives are inside a (?| group, both sets of capturing
1483    parentheses are numbered one. Thus, when the pattern matches, you can look
1484    at captured substring number one, whichever alternative matched. This construct
1485    is useful when you want to capture part, but not all, of one of a number of
1486    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1487    number is reset at the start of each branch. The numbers of any capturing
1488    parentheses that follow the subpattern start after the highest number used in
1489    any branch. The following example is taken from the Perl documentation. The
1490    numbers underneath show in which buffer the captured content will be stored.
1491    .sp
1492      # before  ---------------branch-reset----------- after
1493      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1494      # 1            2         2  3        2     3     4
1495    .sp
1496    A back reference to a numbered subpattern uses the most recent value that is
1497    set for that number by any subpattern. The following pattern matches "abcabc"
1498    or "defdef":
1499    .sp
1500      /(?|(abc)|(def))\e1/
1501    .sp
1502    In contrast, a subroutine call to a numbered subpattern always refers to the
1503    first one in the pattern with the given number. The following pattern matches
1504    "abcabc" or "defabc":
1505    .sp
1506      /(?|(abc)|(def))(?1)/
1507    .sp
1508    If a
1509    .\" HTML <a href="#conditions">
1510    .\" </a>
1511    condition test
1512    .\"
1513    for a subpattern's having matched refers to a non-unique number, the test is
1514    true if any of the subpatterns of that number have matched.
1515    .P
1516    An alternative approach to using this "branch reset" feature is to use
1517    duplicate named subpatterns, as described in the next section.
1518    .
1519    .
1520    .SH "NAMED SUBPATTERNS"
1521  .rs  .rs
1522  .sp  .sp
1523  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
1524  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1525  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
1526  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1527  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1528  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1529    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1530  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1531  PCRE API provides function calls for extracting the name-to-number translation  .P
1532  table from a compiled pattern. For further details see the  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1533    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1534    parentheses from other parts of the pattern, such as
1535    .\" HTML <a href="#backreferences">
1536    .\" </a>
1537    back references,
1538    .\"
1539    .\" HTML <a href="#recursion">
1540    .\" </a>
1541    recursion,
1542    .\"
1543    and
1544    .\" HTML <a href="#conditions">
1545    .\" </a>
1546    conditions,
1547    .\"
1548    can be made by name as well as by number.
1549    .P
1550    Names consist of up to 32 alphanumeric characters and underscores. Named
1551    capturing parentheses are still allocated numbers as well as names, exactly as
1552    if the names were not present. The PCRE API provides function calls for
1553    extracting the name-to-number translation table from a compiled pattern. There
1554    is also a convenience function for extracting a captured substring by name.
1555    .P
1556    By default, a name must be unique within a pattern, but it is possible to relax
1557    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1558    names are also always permitted for subpatterns with the same number, set up as
1559    described in the previous section.) Duplicate names can be useful for patterns
1560    where only one instance of the named parentheses can match. Suppose you want to
1561    match the name of a weekday, either as a 3-letter abbreviation or as the full
1562    name, and in both cases you want to extract the abbreviation. This pattern
1563    (ignoring the line breaks) does the job:
1564    .sp
1565      (?<DN>Mon|Fri|Sun)(?:day)?|
1566      (?<DN>Tue)(?:sday)?|
1567      (?<DN>Wed)(?:nesday)?|
1568      (?<DN>Thu)(?:rsday)?|
1569      (?<DN>Sat)(?:urday)?
1570    .sp
1571    There are five capturing substrings, but only one is ever set after a match.
1572    (An alternative way of solving this problem is to use a "branch reset"
1573    subpattern, as described in the previous section.)
1574    .P
1575    The convenience function for extracting the data by name returns the substring
1576    for the first (and in this example, the only) subpattern of that name that
1577    matched. This saves searching to find which numbered subpattern it was.
1578    .P
1579    If you make a back reference to a non-unique named subpattern from elsewhere in
1580    the pattern, the subpatterns to which the name refers are checked in the order
1581    in which they appear in the overall pattern. The first one that is set is used
1582    for the reference. For example, this pattern matches both "foofoo" and
1583    "barbar" but not "foobar" or "barfoo":
1584    .sp
1585      (?:(?<n>foo)|(?<n>bar))\k<n>
1586    .sp
1587    .P
1588    If you make a subroutine call to a non-unique named subpattern, the one that
1589    corresponds to the first occurrence of the name is used. In the absence of
1590    duplicate numbers (see the previous section) this is the one with the lowest
1591    number.
1592    .P
1593    If you use a named reference in a condition
1594    test (see the
1595    .\"
1596    .\" HTML <a href="#conditions">
1597    .\" </a>
1598    section about conditions
1599    .\"
1600    below), either to check whether a subpattern has matched, or to check for
1601    recursion, all subpatterns with the same name are tested. If the condition is
1602    true for any one of them, the overall condition is true. This is the same
1603    behaviour as testing by number. For further details of the interfaces for
1604    handling named subpatterns, see the
1605  .\" HREF  .\" HREF
1606  \fBpcreapi\fR  \fBpcreapi\fP
1607  .\"  .\"
1608  documentation.  documentation.
1609    .P
1610    \fBWarning:\fP You cannot use different names to distinguish between two
1611    subpatterns with the same number because PCRE uses only the numbers when
1612    matching. For this reason, an error is given at compile time if different names
1613    are given to subpatterns with the same number. However, you can always give the
1614    same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1615    set.
1616    .
1617    .
1618  .SH REPETITION  .SH REPETITION
1619  .rs  .rs
1620  .sp  .sp
1621  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1622  items:  items:
1623    .sp
1624    a literal data character    a literal data character
1625    the . metacharacter    the dot metacharacter
1626    the \\C escape sequence    the \eC escape sequence
1627    escapes such as \\d that match single characters    the \eX escape sequence
1628      the \eR escape sequence
1629      an escape such as \ed or \epL that matches a single character
1630    a character class    a character class
1631    a back reference (see next section)    a back reference (see next section)
1632    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1633      a subroutine call to a subpattern (recursive or otherwise)
1634    .sp
1635  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1636  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1637  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1638  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1639    .sp
1640    z{2,4}    z{2,4}
1641    .sp
1642  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1643  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
1644  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
1645  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1646    .sp
1647    [aeiou]{3,}    [aeiou]{3,}
1648    .sp
1649  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1650    .sp
1651    \\d{8}    \ed{8}
1652    .sp
1653  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
1654  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
1655  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1656  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1657    .P
1658  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1659  bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1660  which is represented by a two-byte sequence.  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1661    \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1662    several data units long (and they may be of different lengths).
1663    .P
1664  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1665  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1666    subpatterns that are referenced as
1667  For convenience (and historical compatibility) the three most common  .\" HTML <a href="#subpatternsassubroutines">
1668  quantifiers have single-character abbreviations:  .\" </a>
1669    subroutines
1670    .\"
1671    from elsewhere in the pattern (but see also the section entitled
1672    .\" HTML <a href="#subdefine">
1673    .\" </a>
1674    "Defining subpatterns for use by reference only"
1675    .\"
1676    below). Items other than subpatterns that have a {0} quantifier are omitted
1677    from the compiled pattern.
1678    .P
1679    For convenience, the three most common quantifiers have single-character
1680    abbreviations:
1681    .sp
1682    *    is equivalent to {0,}    *    is equivalent to {0,}
1683    +    is equivalent to {1,}    +    is equivalent to {1,}
1684    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1685    .sp
1686  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
1687  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
1688    .sp
1689    (a?)*    (a?)*
1690    .sp
1691  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
1692  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
1693  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
1694  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1695    .P
1696  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
1697  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1698  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1699  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
1700  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1701  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1702    .sp
1703    /\\*.*\\*/    /\e*.*\e*/
1704    .sp
1705  to the string  to the string
1706    .sp
1707    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1708    .sp
1709  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
1710  item.  item.
1711    .P
1712  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
1713  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1714  pattern  pattern
1715    .sp
1716    /\\*.*?\\*/    /\e*.*?\e*/
1717    .sp
1718  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
1719  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1720  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1721  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1722    .sp
1723    \\d??\\d    \ed??\ed
1724    .sp
1725  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
1726  way the rest of the pattern matches.  way the rest of the pattern matches.
1727    .P
1728  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),
1729  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
1730  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
1731  default behaviour.  default behaviour.
1732    .P
1733  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1734  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
1735  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1736    .P
1737  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
1738  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
1739  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1740  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
1741  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
1742  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1743    .P
1744  In cases where it is known that the subject string contains no newlines, it is  In cases where it is known that the subject string contains no newlines, it is
1745  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1746  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1747    .P
1748  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1749  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1750  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
1751  succeed. Consider, for example:  succeeds. Consider, for example:
1752    .sp
1753    (.*)abc\\1    (.*)abc\e1
1754    .sp
1755  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1756  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1757    .P
1758    Another case where implicit anchoring is not applied is when the leading .* is
1759    inside an atomic group. Once again, a match at the start may fail where a later
1760    one succeeds. Consider this pattern:
1761    .sp
1762      (?>.*?a)b
1763    .sp
1764    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1765    (*PRUNE) and (*SKIP) also disable this optimization.
1766    .P
1767  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1768  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1769    .sp
1770    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1771    .sp
1772  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1773  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1774  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1775  example, after  example, after
1776    .sp
1777    /(a|(b))+/    /(a|(b))+/
1778    .sp
1779  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1780    .
1781  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1782    .\" HTML <a name="atomicgroup"></a>
1783    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1784  .rs  .rs
1785  .sp  .sp
1786  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1787  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
1788  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
1789  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
1790  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
1791  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1792    .P
1793  Consider, for example, the pattern \\d+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1794    .sp
1795    123456bar    123456bar
1796    .sp
1797  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1798  action of the matcher is to try again with only 5 digits matching the \\d+  action of the matcher is to try again with only 5 digits matching the \ed+
1799  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1800  (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
1801  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.
1802    .P
1803  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
1804  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
1805  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1806    .sp
1807    (?>\\d+)bar    (?>\ed+)foo
1808    .sp
1809  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1810  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1811  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1812  normal.  normal.
1813    .P
1814  An alternative description is that a subpattern of this type matches the string  An alternative description is that a subpattern of this type matches the string
1815  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1816  the current point in the subject string.  the current point in the subject string.
1817    .P
1818  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1819  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1820  everything it can. So, while both \\d+ and \\d+? are prepared to adjust the  everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1821  number of digits they match in order to make the rest of the pattern match,  number of digits they match in order to make the rest of the pattern match,
1822  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1823    .P
1824  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1825  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1826  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1827  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1828  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1829  previous example can be rewritten as  previous example can be rewritten as
1830    .sp
1831    \\d++bar    \ed++foo
1832    .sp
1833    Note that a possessive quantifier can be used with an entire group, for
1834    example:
1835    .sp
1836      (abc|xyz){2,3}+
1837    .sp
1838  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1839  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
1840  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
1841  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1842    difference; possessive quantifiers should be slightly faster.
1843  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1844  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1845    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1846    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1847    package, and PCRE copied it from there. It ultimately found its way into Perl
1848    at release 5.10.
1849    .P
1850    PCRE has an optimization that automatically "possessifies" certain simple
1851    pattern constructs. For example, the sequence A+B is treated as A++B because
1852    there is no point in backtracking into a sequence of A's when B must follow.
1853    .P
1854  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
1855  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
1856  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1857  pattern  pattern
1858    .sp
1859    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1860    .sp
1861  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1862  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1863  quickly. However, if it is applied to  quickly. However, if it is applied to
1864    .sp
1865    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1866    .sp
1867  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1868  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \eD+ repeat and the external * repeat in a
1869  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1870  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1871  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1872  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1873  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1874    an atomic group, like this:
1875    ((?>\\D+)|<\\d+>)*[!?]  .sp
1876      ((?>\eD+)|<\ed+>)*[!?]
1877    .sp
1878  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1879    .
1880  .SH BACK REFERENCES  .
1881    .\" HTML <a name="backreferences"></a>
1882    .SH "BACK REFERENCES"
1883  .rs  .rs
1884  .sp  .sp
1885  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1886  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1887  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1888  previous capturing left parentheses.  previous capturing left parentheses.
1889    .P
1890  However, if the decimal number following the backslash is less than 10, it is  However, if the decimal number following the backslash is less than 10, it is
1891  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1892  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1893  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1894  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1895  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1896    in an earlier iteration.
1897    .P
1898    It is not possible to have a numerical "forward back reference" to a subpattern
1899    whose number is 10 or more using this syntax because a sequence such as \e50 is
1900    interpreted as a character defined in octal. See the subsection entitled
1901    "Non-printing characters"
1902    .\" HTML <a href="#digitsafterbackslash">
1903    .\" </a>
1904    above
1905    .\"
1906    for further details of the handling of digits following a backslash. There is
1907    no such problem when named parentheses are used. A back reference to any
1908    subpattern is possible using named parentheses (see below).
1909    .P
1910    Another way of avoiding the ambiguity inherent in the use of digits following a
1911    backslash is to use the \eg escape sequence. This escape must be followed by an
1912    unsigned number or a negative number, optionally enclosed in braces. These
1913    examples are all identical:
1914    .sp
1915      (ring), \e1
1916      (ring), \eg1
1917      (ring), \eg{1}
1918    .sp
1919    An unsigned number specifies an absolute reference without the ambiguity that
1920    is present in the older syntax. It is also useful when literal digits follow
1921    the reference. A negative number is a relative reference. Consider this
1922    example:
1923    .sp
1924      (abc(def)ghi)\eg{-1}
1925    .sp
1926    The sequence \eg{-1} is a reference to the most recently started capturing
1927    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1928    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1929    can be helpful in long patterns, and also in patterns that are created by
1930    joining together fragments that contain references within themselves.
1931    .P
1932  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1933  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1934  itself (see  itself (see
# Line 832  itself (see Line 1937  itself (see
1937  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1938  .\"  .\"
1939  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1940    .sp
1941    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1942    .sp
1943  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1944  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1945  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1946    .sp
1947    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1948    .sp
1949  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
1950  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1951    .P
1952  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1953  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1954    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1955    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1956    references, is also supported. We could rewrite the above example in any of
1957    the following ways:
1958    .sp
1959      (?<p1>(?i)rah)\es+\ek<p1>
1960      (?'p1'(?i)rah)\es+\ek{p1}
1961      (?P<p1>(?i)rah)\es+(?P=p1)
1962      (?<p1>(?i)rah)\es+\eg{p1}
1963    .sp
1964    A subpattern that is referenced by name may appear in the pattern before or
1965    after the reference.
1966    .P
1967  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
1968  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1969  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1970    .sp
1971    (a|(bc))\\2    (a|(bc))\e2
1972    .sp
1973  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
1974  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
1975  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1976  with a digit character, some delimiter must be used to terminate the back  .P
1977  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1978  Otherwise an empty comment can be used.  following a backslash are taken as part of a potential back reference number.
1979    If the pattern continues with a digit character, some delimiter must be used to
1980    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1981    white space. Otherwise, the \eg{ syntax or an empty comment (see
1982    .\" HTML <a href="#comments">
1983    .\" </a>
1984    "Comments"
1985    .\"
1986    below) can be used.
1987    .
1988    .SS "Recursive back references"
1989    .rs
1990    .sp
1991  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
1992  when the subpattern is first used, so, for example, (a\\1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1993  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1994  example, the pattern  example, the pattern
1995    .sp
1996    (a|b\\1)+    (a|b\e1)+
1997    .sp
1998  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1999  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
2000  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
2001  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
2002  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
2003  minimum of zero.  minimum of zero.
2004    .P
2005    Back references of this type cause the group that they reference to be treated
2006    as an
2007    .\" HTML <a href="#atomicgroup">
2008    .\" </a>
2009    atomic group.
2010    .\"
2011    Once the whole group has been matched, a subsequent matching failure cannot
2012    cause backtracking into the middle of the group.
2013    .
2014    .
2015    .\" HTML <a name="bigassertions"></a>
2016  .SH ASSERTIONS  .SH ASSERTIONS
2017  .rs  .rs
2018  .sp  .sp
2019  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
2020  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
2021  assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.  assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
2022    .\" HTML <a href="#smallassertions">
2023    .\" </a>
2024    above.
2025    .\"
2026    .P
2027  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
2028  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
2029  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
2030    except that it does not cause the current matching position to be changed.
2031  An assertion subpattern is matched in the normal way, except that it does not  .P
2032  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns. If such an assertion
2033  with (?= for positive assertions and (?! for negative assertions. For example,  contains capturing subpatterns within it, these are counted for the purposes of
2034    numbering the capturing subpatterns in the whole pattern. However, substring
2035    \\w+(?=;)  capturing is carried out only for positive assertions. (Perl sometimes, but not
2036    always, does do capturing in negative assertions.)
2037    .P
2038    For compatibility with Perl, assertion subpatterns may be repeated; though
2039    it makes no sense to assert the same thing several times, the side effect of
2040    capturing parentheses may occasionally be useful. In practice, there only three
2041    cases:
2042    .sp
2043    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2044    However, it may contain internal capturing parenthesized groups that are called
2045    from elsewhere via the
2046    .\" HTML <a href="#subpatternsassubroutines">
2047    .\" </a>
2048    subroutine mechanism.
2049    .\"
2050    .sp
2051    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2052    were {0,1}. At run time, the rest of the pattern match is tried with and
2053    without the assertion, the order depending on the greediness of the quantifier.
2054    .sp
2055    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2056    The assertion is obeyed just once when encountered during matching.
2057    .
2058    .
2059    .SS "Lookahead assertions"
2060    .rs
2061    .sp
2062    Lookahead assertions start with (?= for positive assertions and (?! for
2063    negative assertions. For example,
2064    .sp
2065      \ew+(?=;)
2066    .sp
2067  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
2068  the match, and  the match, and
2069    .sp
2070    foo(?!bar)    foo(?!bar)
2071    .sp
2072  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
2073  apparently similar pattern  apparently similar pattern
2074    .sp
2075    (?!foo)bar    (?!foo)bar
2076    .sp
2077  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
2078  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
2079  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
2080  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
2081    .P
2082  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
2083  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
2084  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.
2085    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2086    .
2087    .
2088    .\" HTML <a name="lookbehind"></a>
2089    .SS "Lookbehind assertions"
2090    .rs
2091    .sp
2092  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
2093  negative assertions. For example,  negative assertions. For example,
2094    .sp
2095    (?<!foo)bar    (?<!foo)bar
2096    .sp
2097  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
2098  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
2099  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
2100  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
2101    .sp
2102    (?<=bullock|donkey)    (?<=bullock|donkey)
2103    .sp
2104  is permitted, but  is permitted, but
2105    .sp
2106    (?<!dogs?|cats?)    (?<!dogs?|cats?)
2107    .sp
2108  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2109  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
2110  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
2111  match the same length of string. An assertion such as  length of string. An assertion such as
2112    .sp
2113    (?<=ab(c|de))    (?<=ab(c|de))
2114    .sp
2115  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
2116  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
2117    branches:
2118    .sp
2119    (?<=abc|abde)    (?<=abc|abde)
2120    .sp
2121    In some cases, the escape sequence \eK
2122    .\" HTML <a href="#resetmatchstart">
2123    .\" </a>
2124    (see above)
2125    .\"
2126    can be used instead of a lookbehind assertion to get round the fixed-length
2127    restriction.
2128    .P
2129  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2130  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
2131  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2132  match is deemed to fail.  assertion fails.
2133    .P
2134  PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
2135  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
2136  the length of the lookbehind.  it impossible to calculate the length of the lookbehind. The \eX and \eR
2137    escapes, which can match different numbers of data units, are also not
2138  Atomic groups can be used in conjunction with lookbehind assertions to specify  permitted.
2139  efficient matching at the end of the subject string. Consider a simple pattern  .P
2140  such as  .\" HTML <a href="#subpatternsassubroutines">
2141    .\" </a>
2142    "Subroutine"
2143    .\"
2144    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2145    as the subpattern matches a fixed-length string.
2146    .\" HTML <a href="#recursion">
2147    .\" </a>
2148    Recursion,
2149    .\"
2150    however, is not supported.
2151    .P
2152    Possessive quantifiers can be used in conjunction with lookbehind assertions to
2153    specify efficient matching of fixed-length strings at the end of subject
2154    strings. Consider a simple pattern such as
2155    .sp
2156    abcd$    abcd$
2157    .sp
2158  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
2159  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
2160  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
2161    .sp
2162    ^.*abcd$    ^.*abcd$
2163    .sp
2164  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
2165  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
2166  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
2167  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,
2168  if the pattern is written as  if the pattern is written as
2169    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
2170    ^.*+(?<=abcd)    ^.*+(?<=abcd)
2171    .sp
2172  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
2173  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
2174  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2175  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
2176    .
2177    .
2178    .SS "Using multiple assertions"
2179    .rs
2180    .sp
2181  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
2182    .sp
2183    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
2184    .sp
2185  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
2186  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
2187  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
2188  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
2189  This pattern does \fInot\fR match "foo" preceded by six characters, the first  This pattern does \fInot\fP match "foo" preceded by six characters, the first
2190  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
2191  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
2192    .sp
2193    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
2194    .sp
2195  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
2196  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
2197  preceding three characters are not "999".  preceding three characters are not "999".
2198    .P
2199  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
2200    .sp
2201    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
2202    .sp
2203  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2204  preceded by "foo", while  preceded by "foo", while
2205    .sp
2206    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
2207    .sp
2208  is another pattern which matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
2209  characters that are not "999".  characters that are not "999".
2210    .
2211  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
2212  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
2213  of assertion contains capturing subpatterns within it, these are counted for  .SH "CONDITIONAL SUBPATTERNS"
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
   
 .SH CONDITIONAL SUBPATTERNS  
2214  .rs  .rs
2215  .sp  .sp
2216  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2217  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2218  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2219  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2220    .sp
2221    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2222    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2223    .sp
2224  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2225  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
2226  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2227    itself contain nested subpatterns of any form, including conditional
2228  There are three kinds of condition. If the text between the parentheses  subpatterns; the restriction to two alternatives applies only at the level of
2229  consists of a sequence of digits, the condition is satisfied if the capturing  the condition. This pattern fragment is an example where the alternatives are
2230  subpattern of that number has previously matched. The number must be greater  complex:
2231  than zero. Consider the following pattern, which contains non-significant white  .sp
2232  space to make it more readable (assume the PCRE_EXTENDED option) and to divide    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2233  it into three parts for ease of discussion:  .sp
2234    .P
2235    ( \\( )?    [^()]+    (?(1) \\) )  There are four kinds of condition: references to subpatterns, references to
2236    recursion, a pseudo-condition called DEFINE, and assertions.
2237    .
2238    .SS "Checking for a used subpattern by number"
2239    .rs
2240    .sp
2241    If the text between the parentheses consists of a sequence of digits, the
2242    condition is true if a capturing subpattern of that number has previously
2243    matched. If there is more than one capturing subpattern with the same number
2244    (see the earlier
2245    .\"
2246    .\" HTML <a href="#recursion">
2247    .\" </a>
2248    section about duplicate subpattern numbers),
2249    .\"
2250    the condition is true if any of them have matched. An alternative notation is
2251    to precede the digits with a plus or minus sign. In this case, the subpattern
2252    number is relative rather than absolute. The most recently opened parentheses
2253    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2254    loops it can also make sense to refer to subsequent groups. The next
2255    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2256    zero in any of these forms is not used; it provokes a compile-time error.)
2257    .P
2258    Consider the following pattern, which contains non-significant white space to
2259    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2260    three parts for ease of discussion:
2261    .sp
2262      ( \e( )?    [^()]+    (?(1) \e) )
2263    .sp
2264  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2265  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
2266  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
2267  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2268  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,
2269  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
2270  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2271  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2272  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2273    .P
2274  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2275  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2276  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2277      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2278  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
2279    This makes the fragment independent of the parentheses in the larger pattern.
2280    .
2281    .SS "Checking for a used subpattern by name"
2282    .rs
2283    .sp
2284    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2285    subpattern by name. For compatibility with earlier versions of PCRE, which had
2286    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2287    there is a possible ambiguity with this syntax, because subpattern names may
2288    consist entirely of digits. PCRE looks first for a named subpattern; if it
2289    cannot find one and the name consists entirely of digits, PCRE looks for a
2290    subpattern of that number, which must be greater than zero. Using subpattern
2291    names that consist entirely of digits is not recommended.
2292    .P
2293    Rewriting the above example to use a named subpattern gives this:
2294    .sp
2295      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2296    .sp
2297    If the name used in a condition of this kind is a duplicate, the test is
2298    applied to all subpatterns of the same name, and is true if any one of them has
2299    matched.
2300    .
2301    .SS "Checking for pattern recursion"
2302    .rs
2303    .sp
2304    If the condition is the string (R), and there is no subpattern with the name R,
2305    the condition is true if a recursive call to the whole pattern or any
2306    subpattern has been made. If digits or a name preceded by ampersand follow the
2307    letter R, for example:
2308    .sp
2309      (?(R3)...) or (?(R&name)...)
2310    .sp
2311    the condition is true if the most recent recursion is into a subpattern whose
2312    number or name is given. This condition does not check the entire recursion
2313    stack. If the name used in a condition of this kind is a duplicate, the test is
2314    applied to all subpatterns of the same name, and is true if any one of them is
2315    the most recent recursion.
2316    .P
2317    At "top level", all these recursion test conditions are false.
2318    .\" HTML <a href="#recursion">
2319    .\" </a>
2320    The syntax for recursive patterns
2321    .\"
2322    is described below.
2323    .
2324    .\" HTML <a name="subdefine"></a>
2325    .SS "Defining subpatterns for use by reference only"
2326    .rs
2327    .sp
2328    If the condition is the string (DEFINE), and there is no subpattern with the
2329    name DEFINE, the condition is always false. In this case, there may be only one
2330    alternative in the subpattern. It is always skipped if control reaches this
2331    point in the pattern; the idea of DEFINE is that it can be used to define
2332    subroutines that can be referenced from elsewhere. (The use of
2333    .\" HTML <a href="#subpatternsassubroutines">
2334    .\" </a>
2335    subroutines
2336    .\"
2337    is described below.) For example, a pattern to match an IPv4 address such as
2338    "192.168.23.245" could be written like this (ignore white space and line
2339    breaks):
2340    .sp
2341      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2342      \eb (?&byte) (\e.(?&byte)){3} \eb
2343    .sp
2344    The first part of the pattern is a DEFINE group inside which a another group
2345    named "byte" is defined. This matches an individual component of an IPv4
2346    address (a number less than 256). When matching takes place, this part of the
2347    pattern is skipped because DEFINE acts like a false condition. The rest of the
2348    pattern uses references to the named group to match the four dot-separated
2349    components of an IPv4 address, insisting on a word boundary at each end.
2350    .
2351    .SS "Assertion conditions"
2352    .rs
2353    .sp
2354    If the condition is not in any of the above formats, it must be an assertion.
2355  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2356  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2357  alternatives on the second line:  alternatives on the second line:
2358    .sp
2359    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
2360    \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )    \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
2361    .sp
2362  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
2363  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
2364  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
2365  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
2366  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
2367  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2368    .
2369    .
2370    .\" HTML <a name="comments"></a>
2371  .SH COMMENTS  .SH COMMENTS
2372  .rs  .rs
2373  .sp  .sp
2374  The sequence (?# marks the start of a comment which continues up to the next  There are two ways of including comments in patterns that are processed by
2375  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,
2376  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
2377    subpattern name or number. The characters that make up a comment play no part
2378  If the PCRE_EXTENDED option is set, an unescaped # character outside a  in the pattern matching.
2379  character class introduces a comment that continues up to the next newline  .P
2380  character in the pattern.  The sequence (?# marks the start of a comment that continues up to the next
2381    closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2382  .SH RECURSIVE PATTERNS  option is set, an unescaped # character also introduces a comment, which in
2383    this case continues to immediately after the next newline character or
2384    character sequence in the pattern. Which characters are interpreted as newlines
2385    is controlled by the options passed to a compiling function or by a special
2386    sequence at the start of the pattern, as described in the section entitled
2387    .\" HTML <a href="#newlines">
2388    .\" </a>
2389    "Newline conventions"
2390    .\"
2391    above. Note that the end of this type of comment is a literal newline sequence
2392    in the pattern; escape sequences that happen to represent a newline do not
2393    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2394    default newline convention is in force:
2395    .sp
2396      abc #comment \en still comment
2397    .sp
2398    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2399    a newline in the pattern. The sequence \en is still literal at this stage, so
2400    it does not terminate the comment. Only an actual character with the code value
2401    0x0a (the default newline) does so.
2402    .
2403    .
2404    .\" HTML <a name="recursion"></a>
2405    .SH "RECURSIVE PATTERNS"
2406  .rs  .rs
2407  .sp  .sp
2408  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2409  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2410  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
2411  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
2412  experimental facility that allows regular expressions to recurse (amongst other  .P
2413  things). It does this by interpolating Perl code in the expression at run time,  For some time, Perl has provided a facility that allows regular expressions to
2414  and the code can refer to the expression itself. A Perl pattern to solve the  recurse (amongst other things). It does this by interpolating Perl code in the
2415  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2416    pattern using code interpolation to solve the parentheses problem can be
2417    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
2418    .sp
2419      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2420    .sp
2421  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
2422  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2423  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2424  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2425    supports special syntax for recursion of the entire pattern, and also for
2426  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2427  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.
2428  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2429  "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
2430  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive subroutine call of the subpattern of the
2431    given number, provided that it occurs inside that subpattern. (If not, it is a
2432  For example, this PCRE pattern solves the nested parentheses problem (assume  .\" HTML <a href="#subpatternsassubroutines">
2433  the PCRE_EXTENDED option is set so that white space is ignored):  .\" </a>
2434    non-recursive subroutine
2435    \\( ( (?>[^()]+) | (?R) )* \\)  .\"
2436    call, which is described in the next section.) The special item (?R) or (?0) is
2437    a recursive call of the entire regular expression.
2438    .P
2439    This PCRE pattern solves the nested parentheses problem (assume the
2440    PCRE_EXTENDED option is set so that white space is ignored):
2441    .sp
2442      \e( ( [^()]++ | (?R) )* \e)
2443    .sp
2444  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2445  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
2446  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2447  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2448    to avoid backtracking into sequences of non-parentheses.
2449    .P
2450  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
2451  pattern, so instead you could use this:  pattern, so instead you could use this:
2452    .sp
2453    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( [^()]++ | (?1) )* \e) )
2454    .sp
2455  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
2456  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2457  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2458  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
2459  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
2460  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2461    parentheses preceding the recursion. In other words, a negative number counts
2462    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  capturing parentheses leftwards from the point at which it is encountered.
2463    .P
2464  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2465  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2466  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
2467  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2468    .\" </a>
2469    non-recursive subroutine
2470    .\"
2471    calls, as described in the next section.
2472    .P
2473    An alternative approach is to use named parentheses instead. The Perl syntax
2474    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2475    could rewrite the above example as follows:
2476    .sp
2477      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2478    .sp
2479    If there is more than one subpattern with the same name, the earliest one is
2480    used.
2481    .P
2482    This particular example pattern that we have been looking at contains nested
2483    unlimited repeats, and so the use of a possessive quantifier for matching
2484    strings of non-parentheses is important when applying the pattern to strings
2485    that do not match. For example, when this pattern is applied to
2486    .sp
2487    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2488    .sp
2489  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,
2490  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
2491  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
2492  before failure can be reported.  before failure can be reported.
2493    .P
2494  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
2495  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
2496  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2497  .\" HREF  .\" HREF
2498  \fBpcrecallout\fR  \fBpcrecallout\fP
2499  .\"  .\"
2500  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2501    .sp
2502    (ab(cd)ef)    (ab(cd)ef)
2503    .sp
2504  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
2505  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
2506    matched at the top level, its final captured value is unset, even if it was
2507    \\( ( ( (?>[^()]+) | (?R) )* ) \\)  (temporarily) set at a deeper level during the matching process.
2508       ^                        ^  .P
2509       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2510    obtain extra memory to store data during a recursion, which it does by using
2511  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
2512  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2513  has to obtain extra memory to store data during a recursion, which it does by  .P
 using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
   
2514  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.
2515  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
2516  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2517  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
2518    .sp
2519    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
2520    .sp
2521  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
2522  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2523  is the actual recursive call.  is the actual recursive call.
2524    .
2525    .
2526    .\" HTML <a name="recursiondifference"></a>
2527    .SS "Differences in recursion processing between PCRE and Perl"
2528    .rs
2529    .sp
2530    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2531    (like Python, but unlike Perl), a recursive subpattern call is always treated
2532    as an atomic group. That is, once it has matched some of the subject string, it
2533    is never re-entered, even if it contains untried alternatives and there is a
2534    subsequent matching failure. This can be illustrated by the following pattern,
2535    which purports to match a palindromic string that contains an odd number of
2536    characters (for example, "a", "aba", "abcba", "abcdcba"):
2537    .sp
2538      ^(.|(.)(?1)\e2)$
2539    .sp
2540    The idea is that it either matches a single character, or two identical
2541    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2542    it does not if the pattern is longer than three characters. Consider the
2543    subject string "abcba":
2544    .P
2545    At the top level, the first character is matched, but as it is not at the end
2546    of the string, the first alternative fails; the second alternative is taken
2547    and the recursion kicks in. The recursive call to subpattern 1 successfully
2548    matches the next character ("b"). (Note that the beginning and end of line
2549    tests are not part of the recursion).
2550    .P
2551    Back at the top level, the next character ("c") is compared with what
2552    subpattern 2 matched, which was "a". This fails. Because the recursion is
2553    treated as an atomic group, there are now no backtracking points, and so the
2554    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2555    try the second alternative.) However, if the pattern is written with the
2556    alternatives in the other order, things are different:
2557    .sp
2558      ^((.)(?1)\e2|.)$
2559    .sp
2560    This time, the recursing alternative is tried first, and continues to recurse
2561    until it runs out of characters, at which point the recursion fails. But this
2562    time we do have another alternative to try at the higher level. That is the big
2563    difference: in the previous case the remaining alternative is at a deeper
2564    recursion level, which PCRE cannot use.
2565    .P
2566    To change the pattern so that it matches all palindromic strings, not just
2567    those with an odd number of characters, it is tempting to change the pattern to
2568    this:
2569    .sp
2570      ^((.)(?1)\e2|.?)$
2571    .sp
2572    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2573    deeper recursion has matched a single character, it cannot be entered again in
2574    order to match an empty string. The solution is to separate the two cases, and
2575    write out the odd and even cases as alternatives at the higher level:
2576    .sp
2577      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2578    .sp
2579    If you want to match typical palindromic phrases, the pattern has to ignore all
2580    non-word characters, which can be done like this:
2581    .sp
2582      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2583    .sp
2584    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2585    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2586    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2587    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2588    more) to match typical phrases, and Perl takes so long that you think it has
2589    gone into a loop.
2590    .P
2591    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2592    string does not start with a palindrome that is shorter than the entire string.
2593    For example, although "abcba" is correctly matched, if the subject is "ababa",
2594    PCRE finds the palindrome "aba" at the start, then fails at top level because
2595    the end of the string does not follow. Once again, it cannot jump back into the
2596    recursion to try other alternatives, so the entire match fails.
2597    .P
2598    The second way in which PCRE and Perl differ in their recursion processing is
2599    in the handling of captured values. In Perl, when a subpattern is called
2600    recursively or as a subpattern (see the next section), it has no access to any
2601    values that were captured outside the recursion, whereas in PCRE these values
2602    can be referenced. Consider this pattern:
2603    .sp
2604      ^(.)(\e1|a(?2))
2605    .sp
2606    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2607    then in the second group, when the back reference \e1 fails to match "b", the
2608    second alternative matches "a" and then recurses. In the recursion, \e1 does
2609    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2610    match because inside the recursive call \e1 cannot access the externally set
2611    value.
2612    .
2613    .
2614  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2615  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
2616  .rs  .rs
2617  .sp  .sp
2618  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2619  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2620  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2621  pattern  before or after the reference. A numbered reference can be absolute or
2622    relative, as in these examples:
2623    (sens|respons)e and \\1ibility  .sp
2624      (...(absolute)...)...(?2)...
2625      (...(relative)...)...(?-1)...
2626      (...(?+1)...(relative)...
2627    .sp
2628    An earlier example pointed out that the pattern
2629    .sp
2630      (sens|respons)e and \e1ibility
2631    .sp
2632  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
2633  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
2634    .sp
2635    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2636    .sp
2637  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2638  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2639  refer.  .P
2640    All subroutine calls, whether recursive or not, are always treated as atomic
2641    groups. That is, once a subroutine has matched some of the subject string, it
2642    is never re-entered, even if it contains untried alternatives and there is a
2643    subsequent matching failure. Any capturing parentheses that are set during the
2644    subroutine call revert to their previous values afterwards.
2645    .P
2646    Processing options such as case-independence are fixed when a subpattern is
2647    defined, so if it is used as a subroutine, such options cannot be changed for
2648    different calls. For example, consider this pattern:
2649    .sp
2650      (abc)(?i:(?-1))
2651    .sp
2652    It matches "abcabc". It does not match "abcABC" because the change of
2653    processing option does not affect the called subpattern.
2654    .
2655    .
2656    .\" HTML <a name="onigurumasubroutines"></a>
2657    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2658    .rs
2659    .sp
2660    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2661    a number enclosed either in angle brackets or single quotes, is an alternative
2662    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2663    are two of the examples used above, rewritten using this syntax:
2664    .sp
2665      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2666      (sens|respons)e and \eg'1'ibility
2667    .sp
2668    PCRE supports an extension to Oniguruma: if a number is preceded by a
2669    plus or a minus sign it is taken as a relative reference. For example:
2670    .sp
2671      (abc)(?i:\eg<-1>)
2672    .sp
2673    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2674    synonymous. The former is a back reference; the latter is a subroutine call.
2675    .
2676    .
2677  .SH CALLOUTS  .SH CALLOUTS
2678  .rs  .rs
2679  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 2681  Perl has a feature whereby using the seq
2681  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
2682  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
2683  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
2684    .P
2685  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2686  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
2687  function by putting its entry point in the global variable \fIpcre_callout\fR.  function by putting its entry point in the global variable \fIpcre_callout\fP
2688    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2689  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2690    .P
2691  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2692  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
2693  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.
2694  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2695    .sp
2696    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2697    .sp
2698  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2699  set), the external function is called. It is provided with the number of the  automatically installed before each item in the pattern. They are all numbered
2700  callout, and, optionally, one item of data originally supplied by the caller of  255. If there is a conditional group in the pattern whose condition is an
2701  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  assertion, an additional callout is inserted just before the condition. An
2702  fail altogether. A complete description of the interface to the callout  explicit callout may also be set at this position, as in this example:
2703  function is given in the  .sp
2704      (?(?C9)(?=a)abc|def)
2705    .sp
2706    Note that this applies only to assertion conditions, not to other types of
2707    condition.
2708    .P
2709    During matching, when PCRE reaches a callout point, the external function is
2710    called. It is provided with the number of the callout, the position in the
2711    pattern, and, optionally, one item of data originally supplied by the caller of
2712    the matching function. The callout function may cause matching to proceed, to
2713    backtrack, or to fail altogether. A complete description of the interface to
2714    the callout function is given in the
2715    .\" HREF
2716    \fBpcrecallout\fP
2717    .\"
2718    documentation.
2719    .
2720    .
2721    .\" HTML <a name="backtrackcontrol"></a>
2722    .SH "BACKTRACKING CONTROL"
2723    .rs
2724    .sp
2725    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2726    are still described in the Perl documentation as "experimental and subject to
2727    change or removal in a future version of Perl". It goes on to say: "Their usage
2728    in production code should be noted to avoid problems during upgrades." The same
2729    remarks apply to the PCRE features described in this section.
2730    .P
2731    The new verbs make use of what was previously invalid syntax: an opening
2732    parenthesis followed by an asterisk. They are generally of the form
2733    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2734    differently depending on whether or not a name is present. A name is any
2735    sequence of characters that does not include a closing parenthesis. The maximum
2736    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2737    libraries. If the name is empty, that is, if the closing parenthesis
2738    immediately follows the colon, the effect is as if the colon were not there.
2739    Any number of these verbs may occur in a pattern.
2740    .P
2741    Since these verbs are specifically related to backtracking, most of them can be
2742    used only when the pattern is to be matched using one of the traditional
2743    matching functions, because these use a backtracking algorithm. With the
2744    exception of (*FAIL), which behaves like a failing negative assertion, the
2745    backtracking control verbs cause an error if encountered by a DFA matching
2746    function.
2747    .P
2748    The behaviour of these verbs in
2749    .\" HTML <a href="#btrepeat">
2750    .\" </a>
2751    repeated groups,
2752    .\"
2753    .\" HTML <a href="#btassert">
2754    .\" </a>
2755    assertions,
2756    .\"
2757    and in
2758    .\" HTML <a href="#btsub">
2759    .\" </a>
2760    subpatterns called as subroutines
2761    .\"
2762    (whether or not recursively) is documented below.
2763    .
2764    .
2765    .\" HTML <a name="nooptimize"></a>
2766    .SS "Optimizations that affect backtracking verbs"
2767    .rs
2768    .sp
2769    PCRE contains some optimizations that are used to speed up matching by running
2770    some checks at the start of each match attempt. For example, it may know the
2771    minimum length of matching subject, or that a particular character must be
2772    present. When one of these optimizations bypasses the running of a match, any
2773    included backtracking verbs will not, of course, be processed. You can suppress
2774    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2775    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2776    pattern with (*NO_START_OPT). There is more discussion of this option in the
2777    section entitled
2778    .\" HTML <a href="pcreapi.html#execoptions">
2779    .\" </a>
2780    "Option bits for \fBpcre_exec()\fP"
2781    .\"
2782    in the
2783  .\" HREF  .\" HREF
2784  \fBpcrecallout\fR  \fBpcreapi\fP
2785  .\"  .\"
2786  documentation.  documentation.
2787    .P
2788  .in 0  Experiments with Perl suggest that it too has similar optimizations, sometimes
2789  Last updated: 03 February 2003  leading to anomalous results.
2790  .br  .
2791  Copyright (c) 1997-2003 University of Cambridge.  .
2792    .SS "Verbs that act immediately"
2793    .rs
2794    .sp
2795    The following verbs act as soon as they are encountered. They may not be
2796    followed by a name.
2797    .sp
2798       (*ACCEPT)
2799    .sp
2800    This verb causes the match to end successfully, skipping the remainder of the
2801    pattern. However, when it is inside a subpattern that is called as a
2802    subroutine, only that subpattern is ended successfully. Matching then continues
2803    at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2804    assertion succeeds; in a negative assertion, the assertion fails.
2805    .P
2806    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2807    example:
2808    .sp
2809      A((?:A|B(*ACCEPT)|C)D)
2810    .sp
2811    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2812    the outer parentheses.
2813    .sp
2814      (*FAIL) or (*F)
2815    .sp
2816    This verb causes a matching failure, forcing backtracking to occur. It is
2817    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2818    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2819    Perl features that are not present in PCRE. The nearest equivalent is the
2820    callout feature, as for example in this pattern:
2821    .sp
2822      a+(?C)(*FAIL)
2823    .sp
2824    A match with the string "aaaa" always fails, but the callout is taken before
2825    each backtrack happens (in this example, 10 times).
2826    .
2827    .
2828    .SS "Recording which path was taken"
2829    .rs
2830    .sp
2831    There is one verb whose main purpose is to track how a match was arrived at,
2832    though it also has a secondary use in conjunction with advancing the match
2833    starting point (see (*SKIP) below).
2834    .sp
2835      (*MARK:NAME) or (*:NAME)
2836    .sp
2837    A name is always required with this verb. There may be as many instances of
2838    (*MARK) as you like in a pattern, and their names do not have to be unique.
2839    .P
2840    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2841    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2842    caller as described in the section entitled
2843    .\" HTML <a href="pcreapi.html#extradata">
2844    .\" </a>
2845    "Extra data for \fBpcre_exec()\fP"
2846    .\"
2847    in the
2848    .\" HREF
2849    \fBpcreapi\fP
2850    .\"
2851    documentation. Here is an example of \fBpcretest\fP output, where the /K
2852    modifier requests the retrieval and outputting of (*MARK) data:
2853    .sp
2854        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2855      data> XY
2856       0: XY
2857      MK: A
2858      XZ
2859       0: XZ
2860      MK: B
2861    .sp
2862    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2863    indicates which of the two alternatives matched. This is a more efficient way
2864    of obtaining this information than putting each alternative in its own
2865    capturing parentheses.
2866    .P
2867    If a verb with a name is encountered in a positive assertion that is true, the
2868    name is recorded and passed back if it is the last-encountered. This does not
2869    happen for negative assertions or failing positive assertions.
2870    .P
2871    After a partial match or a failed match, the last encountered name in the
2872    entire match process is returned. For example:
2873    .sp
2874        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2875      data> XP
2876      No match, mark = B
2877    .sp
2878    Note that in this unanchored example the mark is retained from the match
2879    attempt that started at the letter "X" in the subject. Subsequent match
2880    attempts starting at "P" and then with an empty string do not get as far as the
2881    (*MARK) item, but nevertheless do not reset it.
2882    .P
2883    If you are interested in (*MARK) values after failed matches, you should
2884    probably set the PCRE_NO_START_OPTIMIZE option
2885    .\" HTML <a href="#nooptimize">
2886    .\" </a>
2887    (see above)
2888    .\"
2889    to ensure that the match is always attempted.
2890    .
2891    .
2892    .SS "Verbs that act after backtracking"
2893    .rs
2894    .sp
2895    The following verbs do nothing when they are encountered. Matching continues
2896    with what follows, but if there is no subsequent match, causing a backtrack to
2897    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2898    the verb. However, when one of these verbs appears inside an atomic group or an
2899    assertion that is true, its effect is confined to that group, because once the
2900    group has been matched, there is never any backtracking into it. In this
2901    situation, backtracking can "jump back" to the left of the entire atomic group
2902    or assertion. (Remember also, as stated above, that this localization also
2903    applies in subroutine calls.)
2904    .P
2905    These verbs differ in exactly what kind of failure occurs when backtracking
2906    reaches them. The behaviour described below is what happens when the verb is
2907    not in a subroutine or an assertion. Subsequent sections cover these special
2908    cases.
2909    .sp
2910      (*COMMIT)
2911    .sp
2912    This verb, which may not be followed by a name, causes the whole match to fail
2913    outright if there is a later matching failure that causes backtracking to reach
2914    it. Even if the pattern is unanchored, no further attempts to find a match by
2915    advancing the starting point take place. If (*COMMIT) is the only backtracking
2916    verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2917    committed to finding a match at the current starting point, or not at all. For
2918    example:
2919    .sp
2920      a+(*COMMIT)b
2921    .sp
2922    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2923    dynamic anchor, or "I've started, so I must finish." The name of the most
2924    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2925    match failure.
2926    .P
2927    If there is more than one backtracking verb in a pattern, a different one that
2928    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2929    match does not always guarantee that a match must be at this starting point.
2930    .P
2931    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2932    unless PCRE's start-of-match optimizations are turned off, as shown in this
2933    \fBpcretest\fP example:
2934    .sp
2935        re> /(*COMMIT)abc/
2936      data> xyzabc
2937       0: abc
2938      xyzabc\eY
2939      No match
2940    .sp
2941    PCRE knows that any match must start with "a", so the optimization skips along
2942    the subject to "a" before running the first match attempt, which succeeds. When
2943    the optimization is disabled by the \eY escape in the second subject, the match
2944    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2945    starting points.
2946    .sp
2947      (*PRUNE) or (*PRUNE:NAME)
2948    .sp
2949    This verb causes the match to fail at the current starting position in the
2950    subject if there is a later matching failure that causes backtracking to reach
2951    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2952    starting character then happens. Backtracking can occur as usual to the left of
2953    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2954    if there is no match to the right, backtracking cannot cross (*PRUNE). In
2955    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2956    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2957    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2958    as (*COMMIT).
2959    .P
2960    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2961    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2962    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2963    .sp
2964      (*SKIP)
2965    .sp
2966    This verb, when given without a name, is like (*PRUNE), except that if the
2967    pattern is unanchored, the "bumpalong" advance is not to the next character,
2968    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2969    signifies that whatever text was matched leading up to it cannot be part of a
2970    successful match. Consider:
2971    .sp
2972      a+(*SKIP)b
2973    .sp
2974    If the subject is "aaaac...", after the first match attempt fails (starting at
2975    the first character in the string), the starting point skips on to start the
2976    next attempt at "c". Note that a possessive quantifer does not have the same
2977    effect as this example; although it would suppress backtracking during the
2978    first match attempt, the second attempt would start at the second character
2979    instead of skipping on to "c".
2980    .sp
2981      (*SKIP:NAME)
2982    .sp
2983    When (*SKIP) has an associated name, its behaviour is modified. When it is
2984    triggered, the previous path through the pattern is searched for the most
2985    recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
2986    is to the subject position that corresponds to that (*MARK) instead of to where
2987    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
2988    (*SKIP) is ignored.
2989    .P
2990    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2991    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2992    .sp
2993      (*THEN) or (*THEN:NAME)
2994    .sp
2995    This verb causes a skip to the next innermost alternative when backtracking
2996    reaches it. That is, it cancels any further backtracking within the current
2997    alternative. Its name comes from the observation that it can be used for a
2998    pattern-based if-then-else block:
2999    .sp
3000      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3001    .sp
3002    If the COND1 pattern matches, FOO is tried (and possibly further items after
3003    the end of the group if FOO succeeds); on failure, the matcher skips to the
3004    second alternative and tries COND2, without backtracking into COND1. If that
3005    succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3006    more alternatives, so there is a backtrack to whatever came before the entire
3007    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3008    .P
3009    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3010    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3011    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3012    .P
3013    A subpattern that does not contain a | character is just a part of the
3014    enclosing alternative; it is not a nested alternation with only one
3015    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3016    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3017    pattern fragments that do not contain any | characters at this level:
3018    .sp
3019      A (B(*THEN)C) | D
3020    .sp
3021    If A and B are matched, but there is a failure in C, matching does not
3022    backtrack into A; instead it moves to the next alternative, that is, D.
3023    However, if the subpattern containing (*THEN) is given an alternative, it
3024    behaves differently:
3025    .sp
3026      A (B(*THEN)C | (*FAIL)) | D
3027    .sp
3028    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3029    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3030    because there are no more alternatives to try. In this case, matching does now
3031    backtrack into A.
3032    .P
3033    Note that a conditional subpattern is not considered as having two
3034    alternatives, because only one is ever used. In other words, the | character in
3035    a conditional subpattern has a different meaning. Ignoring white space,
3036    consider:
3037    .sp
3038      ^.*? (?(?=a) a | b(*THEN)c )
3039    .sp
3040    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3041    it initially matches zero characters. The condition (?=a) then fails, the
3042    character "b" is matched, but "c" is not. At this point, matching does not
3043    backtrack to .*? as might perhaps be expected from the presence of the |
3044    character. The conditional subpattern is part of the single alternative that
3045    comprises the whole pattern, and so the match fails. (If there was a backtrack
3046    into .*?, allowing it to match "b", the match would succeed.)
3047    .P
3048    The verbs just described provide four different "strengths" of control when
3049    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3050    next alternative. (*PRUNE) comes next, failing the match at the current
3051    starting position, but allowing an advance to the next character (for an
3052    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3053    than one character. (*COMMIT) is the strongest, causing the entire match to
3054    fail.
3055    .
3056    .
3057    .SS "More than one backtracking verb"
3058    .rs
3059    .sp
3060    If more than one backtracking verb is present in a pattern, the one that is
3061    backtracked onto first acts. For example, consider this pattern, where A, B,
3062    etc. are complex pattern fragments:
3063    .sp
3064      (A(*COMMIT)B(*THEN)C|ABD)
3065    .sp
3066    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3067    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3068    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3069    not always the same as Perl's. It means that if two or more backtracking verbs
3070    appear in succession, all the the last of them has no effect. Consider this
3071    example:
3072    .sp
3073      ...(*COMMIT)(*PRUNE)...
3074    .sp
3075    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3076    it to be triggered, and its action is taken. There can never be a backtrack
3077    onto (*COMMIT).
3078    .
3079    .
3080    .\" HTML <a name="btrepeat"></a>
3081    .SS "Backtracking verbs in repeated groups"
3082    .rs
3083    .sp
3084    PCRE differs from Perl in its handling of backtracking verbs in repeated
3085    groups. For example, consider:
3086    .sp
3087      /(a(*COMMIT)b)+ac/
3088    .sp
3089    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3090    the second repeat of the group acts.
3091    .
3092    .
3093    .\" HTML <a name="btassert"></a>
3094    .SS "Backtracking verbs in assertions"
3095    .rs
3096    .sp
3097    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3098    .P
3099    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3100    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3101    fail without any further processing.
3102    .P
3103    The other backtracking verbs are not treated specially if they appear in a
3104    positive assertion. In particular, (*THEN) skips to the next alternative in the
3105    innermost enclosing group that has alternations, whether or not this is within
3106    the assertion.
3107    .P
3108    Negative assertions are, however, different, in order to ensure that changing a
3109    positive assertion into a negative assertion changes its result. Backtracking
3110    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3111    without considering any further alternative branches in the assertion.
3112    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3113    within the assertion (the normal behaviour), but if the assertion does not have
3114    such an alternative, (*THEN) behaves like (*PRUNE).
3115    .
3116    .
3117    .\" HTML <a name="btsub"></a>
3118    .SS "Backtracking verbs in subroutines"
3119    .rs
3120    .sp
3121    These behaviours occur whether or not the subpattern is called recursively.
3122    Perl's treatment of subroutines is different in some cases.
3123    .P
3124    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3125    an immediate backtrack.
3126    .P
3127    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3128    succeed without any further processing. Matching then continues after the
3129    subroutine call.
3130    .P
3131    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3132    the subroutine match to fail.
3133    .P
3134    (*THEN) skips to the next alternative in the innermost enclosing group within
3135    the subpattern that has alternatives. If there is no such group within the
3136    subpattern, (*THEN) causes the subroutine match to fail.
3137    .
3138    .
3139    .SH "SEE ALSO"
3140    .rs
3141    .sp
3142    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3143    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3144    .
3145    .
3146    .SH AUTHOR
3147    .rs
3148    .sp
3149    .nf
3150    Philip Hazel
3151    University Computing Service
3152    Cambridge CB2 3QH, England.
3153    .fi
3154    .
3155    .
3156    .SH REVISION
3157    .rs
3158    .sp
3159    .nf
3160    Last updated: 06 September 2013
3161    Copyright (c) 1997-2013 University of Cambridge.
3162    .fi

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