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1  .TH PCRE 3  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
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
 O'Reilly, covers them in great detail. The description here is intended as  
 reference documentation.  
   
 The basic operation of PCRE is on strings of bytes. However, there is also  
 support for UTF-8 character strings. To use this support you must build PCRE to  
 include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  
 option. How this affects the pattern matching is mentioned in several places  
 below. There is also a summary of UTF-8 features in the  
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
11  .\"  .\"
12  in the main  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22    .P
23    The original operation of PCRE was on strings of one-byte characters. However,
24    there is now also support for UTF-8 strings in the original library, and a
25    second library that supports 16-bit and UTF-16 character strings. To use these
26    features, PCRE must be built to include appropriate support. When using UTF
27    strings you must either call the compiling function with the PCRE_UTF8 or
28    PCRE_UTF16 option, or the pattern must start with one of these special
29    sequences:
30    .sp
31      (*UTF8)
32      (*UTF16)
33    .sp
34    Starting a pattern with such a sequence is equivalent to setting the relevant
35    option. This feature is not Perl-compatible. How setting a UTF mode affects
36    pattern matching is mentioned in several places below. There is also a summary
37    of features in the
38  .\" HREF  .\" HREF
39  \fBpcre\fR  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42    .P
43    Another special sequence that may appear at the start of a pattern or in
44    combination with (*UTF8) or (*UTF16) is:
45    .sp
46      (*UCP)
47    .sp
48    This has the same effect as setting the PCRE_UCP option: it causes sequences
49    such as \ed and \ew to use Unicode properties to determine character types,
50    instead of recognizing only characters with codes less than 128 via a lookup
51    table.
52    .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .P
58    The remainder of this document discusses the patterns that are supported by
59    PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60    \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61    functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62    a different algorithm that is not Perl-compatible. Some of the features
63    discussed below are not available when DFA matching is used. The advantages and
64    disadvantages of the alternative functions, and how they differ from the normal
65    functions, are discussed in the
66    .\" HREF
67    \fBpcrematching\fP
68    .\"
69    page.
70    .
71    .
72    .\" HTML <a name="newlines"></a>
73    .SH "NEWLINE CONVENTIONS"
74    .rs
75    .sp
76    PCRE supports five different conventions for indicating line breaks in
77    strings: a single CR (carriage return) character, a single LF (linefeed)
78    character, the two-character sequence CRLF, any of the three preceding, or any
79    Unicode newline sequence. The
80    .\" HREF
81    \fBpcreapi\fP
82    .\"
83    page has
84    .\" HTML <a href="pcreapi.html#newlines">
85    .\" </a>
86    further discussion
87    .\"
88    about newlines, and shows how to set the newline convention in the
89    \fIoptions\fP arguments for the compiling and matching functions.
90    .P
91    It is also possible to specify a newline convention by starting a pattern
92    string with one of the following five sequences:
93    .sp
94      (*CR)        carriage return
95      (*LF)        linefeed
96      (*CRLF)      carriage return, followed by linefeed
97      (*ANYCRLF)   any of the three above
98      (*ANY)       all Unicode newline sequences
99    .sp
100    These override the default and the options given to the compiling function. For
101    example, on a Unix system where LF is the default newline sequence, the pattern
102    .sp
103      (*CR)a.b
104    .sp
105    changes the convention to CR. That pattern matches "a\enb" because LF is no
106    longer a newline. Note that these special settings, which are not
107    Perl-compatible, are recognized only at the very start of a pattern, and that
108    they must be in upper case. If more than one of them is present, the last one
109    is used.
110    .P
111    The newline convention affects the interpretation of the dot metacharacter when
112    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113    affect what the \eR escape sequence matches. By default, this is any Unicode
114    newline sequence, for Perl compatibility. However, this can be changed; see the
115    description of \eR in the section entitled
116    .\" HTML <a href="#newlineseq">
117    .\" </a>
118    "Newline sequences"
119    .\"
120    below. A change of \eR setting can be combined with a change of newline
121    convention.
122    .
123    .
124    .SH "CHARACTERS AND METACHARACTERS"
125    .rs
126    .sp
127  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
128  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
129  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
130    .sp
131    The quick brown fox    The quick brown fox
132    .sp
133  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
134  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
135  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
136  \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
137    always possible. For characters with higher values, the concept of case is
138    supported if PCRE is compiled with Unicode property support, but not otherwise.
139    If you want to use caseless matching for characters 128 and above, you must
140    ensure that PCRE is compiled with Unicode property support as well as with
141    UTF support.
142    .P
143    The power of regular expressions comes from the ability to include alternatives
144    and repetitions in the pattern. These are encoded in the pattern by the use of
145    \fImetacharacters\fP, which do not stand for themselves but instead are
146  interpreted in some special way.  interpreted in some special way.
147    .P
148  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
149  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
150  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
151  as follows:  are as follows:
152    .sp
153    \\      general escape character with several uses    \e      general escape character with several uses
154    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
155    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
156    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 165  as follows:
165    +      1 or more quantifier    +      1 or more quantifier
166           also "possessive quantifier"           also "possessive quantifier"
167    {      start min/max quantifier    {      start min/max quantifier
168    .sp
169  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
170  a character class the only meta-characters are:  a character class the only metacharacters are:
171    .sp
172    \\      general escape character    \e      general escape character
173    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
174    -      indicates character range    -      indicates character range
175    .\" JOIN
176    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
177             syntax)             syntax)
178    ]      terminates the character class    ]      terminates the character class
179    .sp
180  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
181    .
182    .
183  .SH BACKSLASH  .SH BACKSLASH
184  .rs  .rs
185  .sp  .sp
186  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
187  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
188  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
189  outside character classes.  both inside and outside character classes.
190    .P
191  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.
192  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
193  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
194  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
195  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
196    .P
197  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
198    backslash. All other characters (in particular, those whose codepoints are
199    greater than 127) are treated as literals.
200    .P
201    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
202  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
203  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
204  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.
205  pattern.  .P
   
206  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
207  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
208  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
209  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
210    .sp
211    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
212    .sp
213    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
214      \eQabc$xyz\eE        abc$xyz        abc followed by the
215                                        contents of $xyz                                        contents of $xyz
216    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
217    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
218    .sp
219  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
220    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221    by \eE later in the pattern, the literal interpretation continues to the end of
222    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223    a character class, this causes an error, because the character class is not
224    terminated.
225    .
226    .
227    .\" HTML <a name="digitsafterbackslash"></a>
228    .SS "Non-printing characters"
229    .rs
230    .sp
231  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
232  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
233  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
234  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
235  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
236  represents:  .sp
237      \ea        alarm, that is, the BEL character (hex 07)
238    \\a        alarm, that is, the BEL character (hex 07)    \ecx       "control-x", where x is any ASCII character
239    \\cx       "control-x", where x is any character    \ee        escape (hex 1B)
240    \\e        escape (hex 1B)    \ef        form feed (hex 0C)
241    \\f        formfeed (hex 0C)    \en        linefeed (hex 0A)
242    \\n        newline (hex 0A)    \er        carriage return (hex 0D)
243    \\r        carriage return (hex 0D)    \et        tab (hex 09)
244    \\t        tab (hex 09)    \eddd      character with octal code ddd, or back reference
245    \\ddd      character with octal code ddd, or backreference    \exhh      character with hex code hh
246    \\xhh      character with hex code hh    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \euhhhh    character with hex code hhhh (JavaScript mode only)
248    .sp
249  The precise effect of \\cx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
250  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
251  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
252  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253    than 127, a compile-time error occurs. This locks out non-ASCII characters in
254  After \\x, from zero to two hexadecimal digits are read (letters can be in  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
255  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  lower case letter is converted to upper case, and then the 0xc0 bits are
256  appear between \\x{ and }, but the value of the character code must be less  flipped.)
257  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  .P
258  other than hexadecimal digits appear between \\x{ and }, or if there is no  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  terminating }, this form of escape is not recognized. Instead, the initial  can be in upper or lower case). Any number of hexadecimal digits may appear
260  \\x will be interpreted as a basic hexadecimal escape, with no following  between \ex{ and }, but the character code is constrained as follows:
261  digits, giving a byte whose value is zero.  .sp
262      8-bit non-UTF mode    less than 0x100
263      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
264      16-bit non-UTF mode   less than 0x10000
265      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
266    .sp
267    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
268    "surrogate" codepoints).
269    .P
270    If characters other than hexadecimal digits appear between \ex{ and }, or if
271    there is no terminating }, this form of escape is not recognized. Instead, the
272    initial \ex will be interpreted as a basic hexadecimal escape, with no
273    following digits, giving a character whose value is zero.
274    .P
275    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
276    as just described only when it is followed by two hexadecimal digits.
277    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
278    code points greater than 256 is provided by \eu, which must be followed by
279    four hexadecimal digits; otherwise it matches a literal "u" character.
280    .P
281  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
282  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
283  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
284    \eu00dc in JavaScript mode).
285  After \\0 up to two further octal digits are read. In both cases, if there  .P
286  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
287  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
288  (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
289  character that follows is itself an octal digit.  sure you supply two digits after the initial zero if the pattern character that
290    follows is itself an octal digit.
291    .P
292  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.
293  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
294  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
295  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
296  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
297  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
298    .\" </a>
299    later,
300    .\"
301    following the discussion of
302    .\" HTML <a href="#subpattern">
303    .\" </a>
304    parenthesized subpatterns.
305    .\"
306    .P
307  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
308  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
309  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
310  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. The value of the character is
311    constrained in the same way as characters specified in hexadecimal.
312  For example:  For example:
313    .sp
314    \\040   is another way of writing a space    \e040   is another way of writing a space
315    \\40    is the same, provided there are fewer than 40  .\" JOIN
316      \e40    is the same, provided there are fewer than 40
317              previous capturing subpatterns              previous capturing subpatterns
318    \\7     is always a back reference    \e7     is always a back reference
319    \\11    might be a back reference, or another way of  .\" JOIN
320      \e11    might be a back reference, or another way of
321              writing a tab              writing a tab
322    \\011   is always a tab    \e011   is always a tab
323    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
324    \\113   might be a back reference, otherwise the  .\" JOIN
325      \e113   might be a back reference, otherwise the
326              character with octal code 113              character with octal code 113
327    \\377   might be a back reference, otherwise  .\" JOIN
328              the byte consisting entirely of 1 bits    \e377   might be a back reference, otherwise
329    \\81    is either a back reference, or a binary zero              the value 255 (decimal)
330    .\" JOIN
331      \e81    is either a back reference, or a binary zero
332              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
333    .sp
334  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
335  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
336    .P
337  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
338  (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
339  addition, inside a character class, the sequence \\b is interpreted as the  interpreted as the backspace character (hex 08).
340  backspace character (hex 08). Outside a character class it has a different  .P
341  meaning (see below).  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
342    inside a character class. Like other unrecognized escape sequences, they are
343  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
344    error if the PCRE_EXTRA option is set. Outside a character class, these
345    \\d     any decimal digit  sequences have different meanings.
346    \\D     any character that is not a decimal digit  .
347    \\s     any whitespace character  .
348    \\S     any character that is not a whitespace character  .SS "Unsupported escape sequences"
349    \\w     any "word" character  .rs
350    \\W     any "non-word" character  .sp
351    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
352  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
353  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
354    option is set, \eU matches a "U" character, and \eu can be used to define a
355  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.
356  \\w, and always match \\D, \\S, and \\W.  .
357    .
358  For compatibility with Perl, \\s does not match the VT character (code 11).  .SS "Absolute and relative back references"
359  This makes it different from the the POSIX "space" class. The \\s characters  .rs
360  are HT (9), LF (10), FF (12), CR (13), and space (32).  .sp
361    The sequence \eg followed by an unsigned or a negative number, optionally
362  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
363  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
364  digits is controlled by PCRE's character tables, and may vary if locale-  .\" HTML <a href="#backreferences">
365  specific matching is taking place (see  .\" </a>
366    later,
367    .\"
368    following the discussion of
369    .\" HTML <a href="#subpattern">
370    .\" </a>
371    parenthesized subpatterns.
372    .\"
373    .
374    .
375    .SS "Absolute and relative subroutine calls"
376    .rs
377    .sp
378    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
379    a number enclosed either in angle brackets or single quotes, is an alternative
380    syntax for referencing a subpattern as a "subroutine". Details are discussed
381    .\" HTML <a href="#onigurumasubroutines">
382    .\" </a>
383    later.
384    .\"
385    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
386    synonymous. The former is a back reference; the latter is a
387    .\" HTML <a href="#subpatternsassubroutines">
388    .\" </a>
389    subroutine
390    .\"
391    call.
392    .
393    .
394    .\" HTML <a name="genericchartypes"></a>
395    .SS "Generic character types"
396    .rs
397    .sp
398    Another use of backslash is for specifying generic character types:
399    .sp
400      \ed     any decimal digit
401      \eD     any character that is not a decimal digit
402      \eh     any horizontal white space character
403      \eH     any character that is not a horizontal white space character
404      \es     any white space character
405      \eS     any character that is not a white space character
406      \ev     any vertical white space character
407      \eV     any character that is not a vertical white space character
408      \ew     any "word" character
409      \eW     any "non-word" character
410    .sp
411    There is also the single sequence \eN, which matches a non-newline character.
412    This is the same as
413    .\" HTML <a href="#fullstopdot">
414    .\" </a>
415    the "." metacharacter
416    .\"
417    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
418    PCRE does not support this.
419    .P
420    Each pair of lower and upper case escape sequences partitions the complete set
421    of characters into two disjoint sets. Any given character matches one, and only
422    one, of each pair. The sequences can appear both inside and outside character
423    classes. They each match one character of the appropriate type. If the current
424    matching point is at the end of the subject string, all of them fail, because
425    there is no character to match.
426    .P
427    For compatibility with Perl, \es does not match the VT character (code 11).
428    This makes it different from the the POSIX "space" class. The \es characters
429    are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
430    included in a Perl script, \es may match the VT character. In PCRE, it never
431    does.
432    .P
433    A "word" character is an underscore or any character that is a letter or digit.
434    By default, the definition of letters and digits is controlled by PCRE's
435    low-valued character tables, and may vary if locale-specific matching is taking
436    place (see
437  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
438  .\" </a>  .\" </a>
439  "Locale support"  "Locale support"
440  .\"  .\"
441  in the  in the
442  .\" HREF  .\" HREF
443  \fBpcreapi\fR  \fBpcreapi\fP
444  .\"  .\"
445  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,
446  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
447    accented letters, and these are then matched by \ew. The use of locales with
448  These character type sequences can appear both inside and outside character  Unicode is discouraged.
449  classes. They each match one character of the appropriate type. If the current  .P
450  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
451  there is no character to match.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
452    their original meanings from before UTF support was available, mainly for
453  The fourth use of backslash is for certain simple assertions. An assertion  efficiency reasons. However, if PCRE is compiled with Unicode property support,
454    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
455    properties are used to determine character types, as follows:
456    .sp
457      \ed  any character that \ep{Nd} matches (decimal digit)
458      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
459      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
460    .sp
461    The upper case escapes match the inverse sets of characters. Note that \ed
462    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
463    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
464    \eB because they are defined in terms of \ew and \eW. Matching these sequences
465    is noticeably slower when PCRE_UCP is set.
466    .P
467    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
468    release 5.10. In contrast to the other sequences, which match only ASCII
469    characters by default, these always match certain high-valued codepoints,
470    whether or not PCRE_UCP is set. The horizontal space characters are:
471    .sp
472      U+0009     Horizontal tab
473      U+0020     Space
474      U+00A0     Non-break space
475      U+1680     Ogham space mark
476      U+180E     Mongolian vowel separator
477      U+2000     En quad
478      U+2001     Em quad
479      U+2002     En space
480      U+2003     Em space
481      U+2004     Three-per-em space
482      U+2005     Four-per-em space
483      U+2006     Six-per-em space
484      U+2007     Figure space
485      U+2008     Punctuation space
486      U+2009     Thin space
487      U+200A     Hair space
488      U+202F     Narrow no-break space
489      U+205F     Medium mathematical space
490      U+3000     Ideographic space
491    .sp
492    The vertical space characters are:
493    .sp
494      U+000A     Linefeed
495      U+000B     Vertical tab
496      U+000C     Form feed
497      U+000D     Carriage return
498      U+0085     Next line
499      U+2028     Line separator
500      U+2029     Paragraph separator
501    .sp
502    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
503    relevant.
504    .
505    .
506    .\" HTML <a name="newlineseq"></a>
507    .SS "Newline sequences"
508    .rs
509    .sp
510    Outside a character class, by default, the escape sequence \eR matches any
511    Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
512    following:
513    .sp
514      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
515    .sp
516    This is an example of an "atomic group", details of which are given
517    .\" HTML <a href="#atomicgroup">
518    .\" </a>
519    below.
520    .\"
521    This particular group matches either the two-character sequence CR followed by
522    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
523    U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
524    line, U+0085). The two-character sequence is treated as a single unit that
525    cannot be split.
526    .P
527    In other modes, two additional characters whose codepoints are greater than 255
528    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
529    Unicode character property support is not needed for these characters to be
530    recognized.
531    .P
532    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
533    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
534    either at compile time or when the pattern is matched. (BSR is an abbrevation
535    for "backslash R".) This can be made the default when PCRE is built; if this is
536    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
537    It is also possible to specify these settings by starting a pattern string with
538    one of the following sequences:
539    .sp
540      (*BSR_ANYCRLF)   CR, LF, or CRLF only
541      (*BSR_UNICODE)   any Unicode newline sequence
542    .sp
543    These override the default and the options given to the compiling function, but
544    they can themselves be overridden by options given to a matching function. Note
545    that these special settings, which are not Perl-compatible, are recognized only
546    at the very start of a pattern, and that they must be in upper case. If more
547    than one of them is present, the last one is used. They can be combined with a
548    change of newline convention; for example, a pattern can start with:
549    .sp
550      (*ANY)(*BSR_ANYCRLF)
551    .sp
552    They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
553    sequences. Inside a character class, \eR is treated as an unrecognized escape
554    sequence, and so matches the letter "R" by default, but causes an error if
555    PCRE_EXTRA is set.
556    .
557    .
558    .\" HTML <a name="uniextseq"></a>
559    .SS Unicode character properties
560    .rs
561    .sp
562    When PCRE is built with Unicode character property support, three additional
563    escape sequences that match characters with specific properties are available.
564    When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
565    characters whose codepoints are less than 256, but they do work in this mode.
566    The extra escape sequences are:
567    .sp
568      \ep{\fIxx\fP}   a character with the \fIxx\fP property
569      \eP{\fIxx\fP}   a character without the \fIxx\fP property
570      \eX       an extended Unicode sequence
571    .sp
572    The property names represented by \fIxx\fP above are limited to the Unicode
573    script names, the general category properties, "Any", which matches any
574    character (including newline), and some special PCRE properties (described
575    in the
576    .\" HTML <a href="#extraprops">
577    .\" </a>
578    next section).
579    .\"
580    Other Perl properties such as "InMusicalSymbols" are not currently supported by
581    PCRE. Note that \eP{Any} does not match any characters, so always causes a
582    match failure.
583    .P
584    Sets of Unicode characters are defined as belonging to certain scripts. A
585    character from one of these sets can be matched using a script name. For
586    example:
587    .sp
588      \ep{Greek}
589      \eP{Han}
590    .sp
591    Those that are not part of an identified script are lumped together as
592    "Common". The current list of scripts is:
593    .P
594    Arabic,
595    Armenian,
596    Avestan,
597    Balinese,
598    Bamum,
599    Batak,
600    Bengali,
601    Bopomofo,
602    Brahmi,
603    Braille,
604    Buginese,
605    Buhid,
606    Canadian_Aboriginal,
607    Carian,
608    Chakma,
609    Cham,
610    Cherokee,
611    Common,
612    Coptic,
613    Cuneiform,
614    Cypriot,
615    Cyrillic,
616    Deseret,
617    Devanagari,
618    Egyptian_Hieroglyphs,
619    Ethiopic,
620    Georgian,
621    Glagolitic,
622    Gothic,
623    Greek,
624    Gujarati,
625    Gurmukhi,
626    Han,
627    Hangul,
628    Hanunoo,
629    Hebrew,
630    Hiragana,
631    Imperial_Aramaic,
632    Inherited,
633    Inscriptional_Pahlavi,
634    Inscriptional_Parthian,
635    Javanese,
636    Kaithi,
637    Kannada,
638    Katakana,
639    Kayah_Li,
640    Kharoshthi,
641    Khmer,
642    Lao,
643    Latin,
644    Lepcha,
645    Limbu,
646    Linear_B,
647    Lisu,
648    Lycian,
649    Lydian,
650    Malayalam,
651    Mandaic,
652    Meetei_Mayek,
653    Meroitic_Cursive,
654    Meroitic_Hieroglyphs,
655    Miao,
656    Mongolian,
657    Myanmar,
658    New_Tai_Lue,
659    Nko,
660    Ogham,
661    Old_Italic,
662    Old_Persian,
663    Old_South_Arabian,
664    Old_Turkic,
665    Ol_Chiki,
666    Oriya,
667    Osmanya,
668    Phags_Pa,
669    Phoenician,
670    Rejang,
671    Runic,
672    Samaritan,
673    Saurashtra,
674    Sharada,
675    Shavian,
676    Sinhala,
677    Sora_Sompeng,
678    Sundanese,
679    Syloti_Nagri,
680    Syriac,
681    Tagalog,
682    Tagbanwa,
683    Tai_Le,
684    Tai_Tham,
685    Tai_Viet,
686    Takri,
687    Tamil,
688    Telugu,
689    Thaana,
690    Thai,
691    Tibetan,
692    Tifinagh,
693    Ugaritic,
694    Vai,
695    Yi.
696    .P
697    Each character has exactly one Unicode general category property, specified by
698    a two-letter abbreviation. For compatibility with Perl, negation can be
699    specified by including a circumflex between the opening brace and the property
700    name. For example, \ep{^Lu} is the same as \eP{Lu}.
701    .P
702    If only one letter is specified with \ep or \eP, it includes all the general
703    category properties that start with that letter. In this case, in the absence
704    of negation, the curly brackets in the escape sequence are optional; these two
705    examples have the same effect:
706    .sp
707      \ep{L}
708      \epL
709    .sp
710    The following general category property codes are supported:
711    .sp
712      C     Other
713      Cc    Control
714      Cf    Format
715      Cn    Unassigned
716      Co    Private use
717      Cs    Surrogate
718    .sp
719      L     Letter
720      Ll    Lower case letter
721      Lm    Modifier letter
722      Lo    Other letter
723      Lt    Title case letter
724      Lu    Upper case letter
725    .sp
726      M     Mark
727      Mc    Spacing mark
728      Me    Enclosing mark
729      Mn    Non-spacing mark
730    .sp
731      N     Number
732      Nd    Decimal number
733      Nl    Letter number
734      No    Other number
735    .sp
736      P     Punctuation
737      Pc    Connector punctuation
738      Pd    Dash punctuation
739      Pe    Close punctuation
740      Pf    Final punctuation
741      Pi    Initial punctuation
742      Po    Other punctuation
743      Ps    Open punctuation
744    .sp
745      S     Symbol
746      Sc    Currency symbol
747      Sk    Modifier symbol
748      Sm    Mathematical symbol
749      So    Other symbol
750    .sp
751      Z     Separator
752      Zl    Line separator
753      Zp    Paragraph separator
754      Zs    Space separator
755    .sp
756    The special property L& is also supported: it matches a character that has
757    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
758    a modifier or "other".
759    .P
760    The Cs (Surrogate) property applies only to characters in the range U+D800 to
761    U+DFFF. Such characters are not valid in Unicode strings and so
762    cannot be tested by PCRE, unless UTF validity checking has been turned off
763    (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
764    .\" HREF
765    \fBpcreapi\fP
766    .\"
767    page). Perl does not support the Cs property.
768    .P
769    The long synonyms for property names that Perl supports (such as \ep{Letter})
770    are not supported by PCRE, nor is it permitted to prefix any of these
771    properties with "Is".
772    .P
773    No character that is in the Unicode table has the Cn (unassigned) property.
774    Instead, this property is assumed for any code point that is not in the
775    Unicode table.
776    .P
777    Specifying caseless matching does not affect these escape sequences. For
778    example, \ep{Lu} always matches only upper case letters.
779    .P
780    The \eX escape matches any number of Unicode characters that form an extended
781    Unicode sequence. \eX is equivalent to
782    .sp
783      (?>\ePM\epM*)
784    .sp
785    That is, it matches a character without the "mark" property, followed by zero
786    or more characters with the "mark" property, and treats the sequence as an
787    atomic group
788    .\" HTML <a href="#atomicgroup">
789    .\" </a>
790    (see below).
791    .\"
792    Characters with the "mark" property are typically accents that affect the
793    preceding character. None of them have codepoints less than 256, so in
794    8-bit non-UTF-8 mode \eX matches any one character.
795    .P
796    Note that recent versions of Perl have changed \eX to match what Unicode calls
797    an "extended grapheme cluster", which has a more complicated definition.
798    .P
799    Matching characters by Unicode property is not fast, because PCRE has to search
800    a structure that contains data for over fifteen thousand characters. That is
801    why the traditional escape sequences such as \ed and \ew do not use Unicode
802    properties in PCRE by default, though you can make them do so by setting the
803    PCRE_UCP option or by starting the pattern with (*UCP).
804    .
805    .
806    .\" HTML <a name="extraprops"></a>
807    .SS PCRE's additional properties
808    .rs
809    .sp
810    As well as the standard Unicode properties described in the previous
811    section, PCRE supports four more that make it possible to convert traditional
812    escape sequences such as \ew and \es and POSIX character classes to use Unicode
813    properties. PCRE uses these non-standard, non-Perl properties internally when
814    PCRE_UCP is set. They are:
815    .sp
816      Xan   Any alphanumeric character
817      Xps   Any POSIX space character
818      Xsp   Any Perl space character
819      Xwd   Any Perl "word" character
820    .sp
821    Xan matches characters that have either the L (letter) or the N (number)
822    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
823    carriage return, and any other character that has the Z (separator) property.
824    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
825    same characters as Xan, plus underscore.
826    .
827    .
828    .\" HTML <a name="resetmatchstart"></a>
829    .SS "Resetting the match start"
830    .rs
831    .sp
832    The escape sequence \eK causes any previously matched characters not to be
833    included in the final matched sequence. For example, the pattern:
834    .sp
835      foo\eKbar
836    .sp
837    matches "foobar", but reports that it has matched "bar". This feature is
838    similar to a lookbehind assertion
839    .\" HTML <a href="#lookbehind">
840    .\" </a>
841    (described below).
842    .\"
843    However, in this case, the part of the subject before the real match does not
844    have to be of fixed length, as lookbehind assertions do. The use of \eK does
845    not interfere with the setting of
846    .\" HTML <a href="#subpattern">
847    .\" </a>
848    captured substrings.
849    .\"
850    For example, when the pattern
851    .sp
852      (foo)\eKbar
853    .sp
854    matches "foobar", the first substring is still set to "foo".
855    .P
856    Perl documents that the use of \eK within assertions is "not well defined". In
857    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
858    ignored in negative assertions.
859    .
860    .
861    .\" HTML <a name="smallassertions"></a>
862    .SS "Simple assertions"
863    .rs
864    .sp
865    The final use of backslash is for certain simple assertions. An assertion
866  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,
867  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
868  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
869  assertions are  .\" HTML <a href="#bigassertions">
870    .\" </a>
871    \\b     matches at a word boundary  below.
872    \\B     matches when not at a word boundary  .\"
873    \\A     matches at start of subject  The backslashed assertions are:
874    \\Z     matches at end of subject or before newline at end  .sp
875    \\z     matches at end of subject    \eb     matches at a word boundary
876    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
877      \eA     matches at the start of the subject
878  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
879  different meaning, namely the backspace character, inside a character class).            also matches before a newline at the end of the subject
880      \ez     matches only at the end of the subject
881      \eG     matches at the first matching position in the subject
882    .sp
883    Inside a character class, \eb has a different meaning; it matches the backspace
884    character. If any other of these assertions appears in a character class, by
885    default it matches the corresponding literal character (for example, \eB
886    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
887    escape sequence" error is generated instead.
888    .P
889  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
890  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
891  \\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
892  first or last character matches \\w, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
893    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
894  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
895  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
896  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
897  multiline mode.  of a word.
898    .P
899  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
900  \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
901  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
902  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
903  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
904  string, whereas \\z matches only at the end.  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
905    argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
906  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
907  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
908  \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.
909  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  .P
910    The \eG assertion is true only when the current matching position is at the
911    start point of the match, as specified by the \fIstartoffset\fP argument of
912    \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
913    non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
914  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
915  implementation where \\G can be useful.  implementation where \eG can be useful.
916    .P
917  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
918  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
919  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
920  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
921  reproduce this behaviour.  reproduce this behaviour.
922    .P
923  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
924  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
925  regular expression.  regular expression.
926    .
927  .SH CIRCUMFLEX AND DOLLAR  .
928    .SH "CIRCUMFLEX AND DOLLAR"
929  .rs  .rs
930  .sp  .sp
931  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
932  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
933  at the start of the subject string. If the \fIstartoffset\fR argument of  at the start of the subject string. If the \fIstartoffset\fP argument of
934  \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
935  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
936  meaning (see below).  meaning
937    .\" HTML <a href="#characterclass">
938    .\" </a>
939    (see below).
940    .\"
941    .P
942  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
943  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
944  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 946  possible alternatives start with a circu
946  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
947  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
948  to be anchored.)  to be anchored.)
949    .P
950  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
951  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
952  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
953  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
954  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
955  Dollar has no special meaning in a character class.  character class.
956    .P
957  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
958  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
959  does not affect the \\Z assertion.  does not affect the \eZ assertion.
960    .P
961  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
962  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
963  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
964  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
965  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
966  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
967  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
968  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
969  \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
970  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
971    patterns that are anchored in single line mode because all branches start with
972  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
973    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
974    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
975    .P
976    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
977  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
978  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
979    .
980  .SH FULL STOP (PERIOD, DOT)  .
981    .\" HTML <a name="fullstopdot"></a>
982    .SH "FULL STOP (PERIOD, DOT) AND \eN"
983  .rs  .rs
984  .sp  .sp
985  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
986  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
987  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line.
988  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
989  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
990  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
991  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
992    (including isolated CRs and LFs). When any Unicode line endings are being
993  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
994  .rs  characters.
995  .sp  .P
996  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
997  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
998  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
999  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
1000  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
1001    The handling of dot is entirely independent of the handling of circumflex and
1002  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
1003  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
1004    .P
1005  .SH SQUARE BRACKETS  The escape sequence \eN behaves like a dot, except that it is not affected by
1006    the PCRE_DOTALL option. In other words, it matches any character except one
1007    that signifies the end of a line. Perl also uses \eN to match characters by
1008    name; PCRE does not support this.
1009    .
1010    .
1011    .SH "MATCHING A SINGLE DATA UNIT"
1012    .rs
1013    .sp
1014    Outside a character class, the escape sequence \eC matches any one data unit,
1015    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1016    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1017    matches line-ending characters. The feature is provided in Perl in order to
1018    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1019    used. Because \eC breaks up characters into individual data units, matching one
1020    unit with \eC in a UTF mode means that the rest of the string may start with a
1021    malformed UTF character. This has undefined results, because PCRE assumes that
1022    it is dealing with valid UTF strings (and by default it checks this at the
1023    start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1024    is used).
1025    .P
1026    PCRE does not allow \eC to appear in lookbehind assertions
1027    .\" HTML <a href="#lookbehind">
1028    .\" </a>
1029    (described below)
1030    .\"
1031    in a UTF mode, because this would make it impossible to calculate the length of
1032    the lookbehind.
1033    .P
1034    In general, the \eC escape sequence is best avoided. However, one
1035    way of using it that avoids the problem of malformed UTF characters is to use a
1036    lookahead to check the length of the next character, as in this pattern, which
1037    could be used with a UTF-8 string (ignore white space and line breaks):
1038    .sp
1039      (?| (?=[\ex00-\ex7f])(\eC) |
1040          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1041          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1042          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1043    .sp
1044    A group that starts with (?| resets the capturing parentheses numbers in each
1045    alternative (see
1046    .\" HTML <a href="#dupsubpatternnumber">
1047    .\" </a>
1048    "Duplicate Subpattern Numbers"
1049    .\"
1050    below). The assertions at the start of each branch check the next UTF-8
1051    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1052    character's individual bytes are then captured by the appropriate number of
1053    groups.
1054    .
1055    .
1056    .\" HTML <a name="characterclass"></a>
1057    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
1058  .rs  .rs
1059  .sp  .sp
1060  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1061  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.
1062  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
1063  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
1064  escaped with a backslash.  a member of the class, it should be the first data character in the class
1065    (after an initial circumflex, if present) or escaped with a backslash.
1066  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1067  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
1068  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
1069  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
1070  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
1071  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
1072    member of the class, ensure it is not the first character, or escape it with a
1073  backslash.  backslash.
1074    .P
1075  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1076  [^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
1077  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
1078  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
1079  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
1080  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
1081    string.
1082  In UTF-8 mode, characters with values greater than 255 can be included in a  .P
1083  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1084    included in a class as a literal string of data units, or by using the \ex{
1085    escaping mechanism.
1086    .P
1087  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
1088  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
1089  "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
1090  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
1091  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
1092    always possible. For characters with higher values, the concept of case is
1093  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.
1094  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
1095  such as [^a] will always match a newline.  above, you must ensure that PCRE is compiled with Unicode property support as
1096    well as with UTF support.
1097    .P
1098    Characters that might indicate line breaks are never treated in any special way
1099    when matching character classes, whatever line-ending sequence is in use, and
1100    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1101    such as [^a] always matches one of these characters.
1102    .P
1103  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
1104  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,
1105  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
1106  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
1107  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.
1108    .P
1109  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
1110  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
1111  ("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
1112  "-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
1113  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
1114  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
1115  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
1116    .P
1117  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
1118  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1119  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
1120  example [\\x{100}-\\x{2ff}].  .P
   
1121  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
1122  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
1123  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1124  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
1125    characters in both cases. In UTF modes, PCRE supports the concept of case for
1126  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
1127  character class, and add the characters that they match to the class. For  property support.
1128  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
1129  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,
1130  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
1131  the class [^\\W_] matches any letter or digit, but not underscore.  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1132    digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1133  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
1134  terminating ] are non-special in character classes, but it does no harm if they  character class, as described in the section entitled
1135  are escaped.  .\" HTML <a href="#genericchartypes">
1136    .\" </a>
1137  .SH POSIX CHARACTER CLASSES  "Generic character types"
1138    .\"
1139    above. The escape sequence \eb has a different meaning inside a character
1140    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1141    are not special inside a character class. Like any other unrecognized escape
1142    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1143    default, but cause an error if the PCRE_EXTRA option is set.
1144    .P
1145    A circumflex can conveniently be used with the upper case character types to
1146    specify a more restricted set of characters than the matching lower case type.
1147    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1148    whereas [\ew] includes underscore. A positive character class should be read as
1149    "something OR something OR ..." and a negative class as "NOT something AND NOT
1150    something AND NOT ...".
1151    .P
1152    The only metacharacters that are recognized in character classes are backslash,
1153    hyphen (only where it can be interpreted as specifying a range), circumflex
1154    (only at the start), opening square bracket (only when it can be interpreted as
1155    introducing a POSIX class name - see the next section), and the terminating
1156    closing square bracket. However, escaping other non-alphanumeric characters
1157    does no harm.
1158    .
1159    .
1160    .SH "POSIX CHARACTER CLASSES"
1161  .rs  .rs
1162  .sp  .sp
1163  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
1164  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1165  this notation. For example,  this notation. For example,
1166    .sp
1167    [01[:alpha:]%]    [01[:alpha:]%]
1168    .sp
1169  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1170  are  are:
1171    .sp
1172    alnum    letters and digits    alnum    letters and digits
1173    alpha    letters    alpha    letters
1174    ascii    character codes 0 - 127    ascii    character codes 0 - 127
1175    blank    space or tab only    blank    space or tab only
1176    cntrl    control characters    cntrl    control characters
1177    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
1178    graph    printing characters, excluding space    graph    printing characters, excluding space
1179    lower    lower case letters    lower    lower case letters
1180    print    printing characters, including space    print    printing characters, including space
1181    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1182    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
1183    upper    upper case letters    upper    upper case letters
1184    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
1185    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1186    .sp
1187  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
1188  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
1189  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
1190  compatibility).  compatibility).
1191    .P
1192  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
1193  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
1194  after the colon. For example,  after the colon. For example,
1195    .sp
1196    [12[:^digit:]]    [12[:^digit:]]
1197    .sp
1198  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
1199  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
1200  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1201    .P
1202  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
1203  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1204    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1205  .SH VERTICAL BAR  character properties are used. This is achieved by replacing the POSIX classes
1206    by other sequences, as follows:
1207    .sp
1208      [:alnum:]  becomes  \ep{Xan}
1209      [:alpha:]  becomes  \ep{L}
1210      [:blank:]  becomes  \eh
1211      [:digit:]  becomes  \ep{Nd}
1212      [:lower:]  becomes  \ep{Ll}
1213      [:space:]  becomes  \ep{Xps}
1214      [:upper:]  becomes  \ep{Lu}
1215      [:word:]   becomes  \ep{Xwd}
1216    .sp
1217    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1218    classes are unchanged, and match only characters with code points less than
1219    128.
1220    .
1221    .
1222    .SH "VERTICAL BAR"
1223  .rs  .rs
1224  .sp  .sp
1225  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1226  the pattern  the pattern
1227    .sp
1228    gilbert|sullivan    gilbert|sullivan
1229    .sp
1230  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1231  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1232  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
1233  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
1234  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
1235  pattern as well as the alternative in the subpattern.  .\" </a>
1236    (defined below),
1237  .SH INTERNAL OPTION SETTING  .\"
1238    "succeeds" means matching the rest of the main pattern as well as the
1239    alternative in the subpattern.
1240    .
1241    .
1242    .SH "INTERNAL OPTION SETTING"
1243  .rs  .rs
1244  .sp  .sp
1245  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1246  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
1247  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1248    The option letters are
1249    .sp
1250    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1251    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
1252    s  for PCRE_DOTALL    s  for PCRE_DOTALL
1253    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
1254    .sp
1255  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
1256  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
1257  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1258  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1259  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
1260  unset.  unset.
1261    .P
1262  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
1263  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
1264  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1265  the global options (and it will therefore show up in data extracted by the  .P
1266  \fBpcre_fullinfo()\fR function).  When one of these option changes occurs at top level (that is, not inside
1267    subpattern parentheses), the change applies to the remainder of the pattern
1268  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
1269  pattern that follows it, so  extracts it into the global options (and it will therefore show up in data
1270    extracted by the \fBpcre_fullinfo()\fP function).
1271    .P
1272    An option change within a subpattern (see below for a description of
1273    subpatterns) affects only that part of the subpattern that follows it, so
1274    .sp
1275    (a(?i)b)c    (a(?i)b)c
1276    .sp
1277  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).
1278  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
1279  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
1280  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
1281    .sp
1282    (a(?i)b|c)    (a(?i)b|c)
1283    .sp
1284  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
1285  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
1286  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1287  behaviour otherwise.  behaviour otherwise.
1288    .P
1289  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
1290  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
1291  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
1292  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
1293  when it is at top level. It is best put at the start.  the section entitled
1294    .\" HTML <a href="#newlineseq">
1295    .\" </a>
1296    "Newline sequences"
1297    .\"
1298    above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1299    can be used to set UTF and Unicode property modes; they are equivalent to
1300    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1301    .
1302    .
1303    .\" HTML <a name="subpattern"></a>
1304  .SH SUBPATTERNS  .SH SUBPATTERNS
1305  .rs  .rs
1306  .sp  .sp
1307  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1308  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1309    .sp
1310  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
1311    .sp
1312    cat(aract|erpillar|)    cat(aract|erpillar|)
1313    .sp
1314  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1315  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1316    .sp
1317  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
1318  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
1319  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
1320  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1321  from 1) to obtain the numbers of the capturing subpatterns.  the DFA matching functions do not support capturing.)
1322    .P
1323  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
1324    numbers for the capturing subpatterns. For example, if the string "the red
1325    king" is matched against the pattern
1326    .sp
1327    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1328    .sp
1329  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,
1330  2, and 3, respectively.  2, and 3, respectively.
1331    .P
1332  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
1333  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
1334  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
1335  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
1336  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1337  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1338    .sp
1339    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1340    .sp
1341  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
1342  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1343  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1344  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
1345  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1346  the ":". Thus the two patterns  the ":". Thus the two patterns
1347    .sp
1348    (?i:saturday|sunday)    (?i:saturday|sunday)
1349    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1350    .sp
1351  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1352  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
1353  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
1354  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1355    .
1356  .SH NAMED SUBPATTERNS  .
1357    .\" HTML <a name="dupsubpatternnumber"></a>
1358    .SH "DUPLICATE SUBPATTERN NUMBERS"
1359    .rs
1360    .sp
1361    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1362    the same numbers for its capturing parentheses. Such a subpattern starts with
1363    (?| and is itself a non-capturing subpattern. For example, consider this
1364    pattern:
1365    .sp
1366      (?|(Sat)ur|(Sun))day
1367    .sp
1368    Because the two alternatives are inside a (?| group, both sets of capturing
1369    parentheses are numbered one. Thus, when the pattern matches, you can look
1370    at captured substring number one, whichever alternative matched. This construct
1371    is useful when you want to capture part, but not all, of one of a number of
1372    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1373    number is reset at the start of each branch. The numbers of any capturing
1374    parentheses that follow the subpattern start after the highest number used in
1375    any branch. The following example is taken from the Perl documentation. The
1376    numbers underneath show in which buffer the captured content will be stored.
1377    .sp
1378      # before  ---------------branch-reset----------- after
1379      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1380      # 1            2         2  3        2     3     4
1381    .sp
1382    A back reference to a numbered subpattern uses the most recent value that is
1383    set for that number by any subpattern. The following pattern matches "abcabc"
1384    or "defdef":
1385    .sp
1386      /(?|(abc)|(def))\e1/
1387    .sp
1388    In contrast, a subroutine call to a numbered subpattern always refers to the
1389    first one in the pattern with the given number. The following pattern matches
1390    "abcabc" or "defabc":
1391    .sp
1392      /(?|(abc)|(def))(?1)/
1393    .sp
1394    If a
1395    .\" HTML <a href="#conditions">
1396    .\" </a>
1397    condition test
1398    .\"
1399    for a subpattern's having matched refers to a non-unique number, the test is
1400    true if any of the subpatterns of that number have matched.
1401    .P
1402    An alternative approach to using this "branch reset" feature is to use
1403    duplicate named subpatterns, as described in the next section.
1404    .
1405    .
1406    .SH "NAMED SUBPATTERNS"
1407  .rs  .rs
1408  .sp  .sp
1409  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
1410  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1411  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
1412  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1413  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
1414  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
1415    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1416  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1417  PCRE API provides function calls for extracting the name-to-number translation  .P
1418  table from a compiled pattern. For further details see the  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1419    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1420    parentheses from other parts of the pattern, such as
1421    .\" HTML <a href="#backreferences">
1422    .\" </a>
1423    back references,
1424    .\"
1425    .\" HTML <a href="#recursion">
1426    .\" </a>
1427    recursion,
1428    .\"
1429    and
1430    .\" HTML <a href="#conditions">
1431    .\" </a>
1432    conditions,
1433    .\"
1434    can be made by name as well as by number.
1435    .P
1436    Names consist of up to 32 alphanumeric characters and underscores. Named
1437    capturing parentheses are still allocated numbers as well as names, exactly as
1438    if the names were not present. The PCRE API provides function calls for
1439    extracting the name-to-number translation table from a compiled pattern. There
1440    is also a convenience function for extracting a captured substring by name.
1441    .P
1442    By default, a name must be unique within a pattern, but it is possible to relax
1443    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1444    names are also always permitted for subpatterns with the same number, set up as
1445    described in the previous section.) Duplicate names can be useful for patterns
1446    where only one instance of the named parentheses can match. Suppose you want to
1447    match the name of a weekday, either as a 3-letter abbreviation or as the full
1448    name, and in both cases you want to extract the abbreviation. This pattern
1449    (ignoring the line breaks) does the job:
1450    .sp
1451      (?<DN>Mon|Fri|Sun)(?:day)?|
1452      (?<DN>Tue)(?:sday)?|
1453      (?<DN>Wed)(?:nesday)?|
1454      (?<DN>Thu)(?:rsday)?|
1455      (?<DN>Sat)(?:urday)?
1456    .sp
1457    There are five capturing substrings, but only one is ever set after a match.
1458    (An alternative way of solving this problem is to use a "branch reset"
1459    subpattern, as described in the previous section.)
1460    .P
1461    The convenience function for extracting the data by name returns the substring
1462    for the first (and in this example, the only) subpattern of that name that
1463    matched. This saves searching to find which numbered subpattern it was.
1464    .P
1465    If you make a back reference to a non-unique named subpattern from elsewhere in
1466    the pattern, the one that corresponds to the first occurrence of the name is
1467    used. In the absence of duplicate numbers (see the previous section) this is
1468    the one with the lowest number. If you use a named reference in a condition
1469    test (see the
1470    .\"
1471    .\" HTML <a href="#conditions">
1472    .\" </a>
1473    section about conditions
1474    .\"
1475    below), either to check whether a subpattern has matched, or to check for
1476    recursion, all subpatterns with the same name are tested. If the condition is
1477    true for any one of them, the overall condition is true. This is the same
1478    behaviour as testing by number. For further details of the interfaces for
1479    handling named subpatterns, see the
1480  .\" HREF  .\" HREF
1481  \fBpcreapi\fR  \fBpcreapi\fP
1482  .\"  .\"
1483  documentation.  documentation.
1484    .P
1485    \fBWarning:\fP You cannot use different names to distinguish between two
1486    subpatterns with the same number because PCRE uses only the numbers when
1487    matching. For this reason, an error is given at compile time if different names
1488    are given to subpatterns with the same number. However, you can give the same
1489    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1490    .
1491    .
1492  .SH REPETITION  .SH REPETITION
1493  .rs  .rs
1494  .sp  .sp
1495  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1496  items:  items:
1497    .sp
1498    a literal data character    a literal data character
1499    the . metacharacter    the dot metacharacter
1500    the \\C escape sequence    the \eC escape sequence
1501    escapes such as \\d that match single characters    the \eX escape sequence
1502      the \eR escape sequence
1503      an escape such as \ed or \epL that matches a single character
1504    a character class    a character class
1505    a back reference (see next section)    a back reference (see next section)
1506    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1507      a subroutine call to a subpattern (recursive or otherwise)
1508    .sp
1509  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1510  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1511  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
1512  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1513    .sp
1514    z{2,4}    z{2,4}
1515    .sp
1516  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
1517  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
1518  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
1519  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1520    .sp
1521    [aeiou]{3,}    [aeiou]{3,}
1522    .sp
1523  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1524    .sp
1525    \\d{8}    \ed{8}
1526    .sp
1527  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
1528  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
1529  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
1530  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1531    .P
1532  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
1533  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
1534  which is represented by a two-byte sequence.  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1535    \eX{3} matches three Unicode extended sequences, each of which may be several
1536    data units long (and they may be of different lengths).
1537    .P
1538  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
1539  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1540    subpatterns that are referenced as
1541  For convenience (and historical compatibility) the three most common  .\" HTML <a href="#subpatternsassubroutines">
1542  quantifiers have single-character abbreviations:  .\" </a>
1543    subroutines
1544    .\"
1545    from elsewhere in the pattern (but see also the section entitled
1546    .\" HTML <a href="#subdefine">
1547    .\" </a>
1548    "Defining subpatterns for use by reference only"
1549    .\"
1550    below). Items other than subpatterns that have a {0} quantifier are omitted
1551    from the compiled pattern.
1552    .P
1553    For convenience, the three most common quantifiers have single-character
1554    abbreviations:
1555    .sp
1556    *    is equivalent to {0,}    *    is equivalent to {0,}
1557    +    is equivalent to {1,}    +    is equivalent to {1,}
1558    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1559    .sp
1560  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
1561  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:
1562    .sp
1563    (a?)*    (a?)*
1564    .sp
1565  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
1566  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
1567  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
1568  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1569    .P
1570  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
1571  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1572  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
1573  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 */
1574  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1575  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1576    .sp
1577    /\\*.*\\*/    /\e*.*\e*/
1578    .sp
1579  to the string  to the string
1580    .sp
1581    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1582    .sp
1583  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 .*
1584  item.  item.
1585    .P
1586  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
1587  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1588  pattern  pattern
1589    .sp
1590    /\\*.*?\\*/    /\e*.*?\e*/
1591    .sp
1592  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
1593  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1594  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
1595  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
1596    .sp
1597    \\d??\\d    \ed??\ed
1598    .sp
1599  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
1600  way the rest of the pattern matches.  way the rest of the pattern matches.
1601    .P
1602  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),
1603  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
1604  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
1605  default behaviour.  default behaviour.
1606    .P
1607  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1608  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
1609  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1610    .P
1611  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
1612  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
1613  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1614  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
1615  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
1616  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1617    .P
1618  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
1619  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1620  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1621    .P
1622  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1623  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1624  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
1625  succeed. Consider, for example:  succeeds. Consider, for example:
1626    .sp
1627    (.*)abc\\1    (.*)abc\e1
1628    .sp
1629  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
1630  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1631    .P
1632  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1633  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1634    .sp
1635    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1636    .sp
1637  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1638  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1639  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1640  example, after  example, after
1641    .sp
1642    /(a|(b))+/    /(a|(b))+/
1643    .sp
1644  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1645    .
1646  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1647    .\" HTML <a name="atomicgroup"></a>
1648    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1649  .rs  .rs
1650  .sp  .sp
1651  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1652  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
1653  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
1654  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
1655  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
1656  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1657    .P
1658  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
1659    .sp
1660    123456bar    123456bar
1661    .sp
1662  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
1663  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+
1664  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"
1665  (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
1666  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.
1667    .P
1668  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
1669  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
1670  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1671    .sp
1672    (?>\\d+)bar    (?>\ed+)foo
1673    .sp
1674  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
1675  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
1676  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1677  normal.  normal.
1678    .P
1679  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
1680  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1681  the current point in the subject string.  the current point in the subject string.
1682    .P
1683  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1684  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
1685  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
1686  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,
1687  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1688    .P
1689  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1690  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1691  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
1692  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1693  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1694  previous example can be rewritten as  previous example can be rewritten as
1695    .sp
1696    \\d++bar    \ed++foo
1697    .sp
1698    Note that a possessive quantifier can be used with an entire group, for
1699    example:
1700    .sp
1701      (abc|xyz){2,3}+
1702    .sp
1703  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1704  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
1705  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
1706  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1707    difference; possessive quantifiers should be slightly faster.
1708  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1709  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1710    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1711    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1712    package, and PCRE copied it from there. It ultimately found its way into Perl
1713    at release 5.10.
1714    .P
1715    PCRE has an optimization that automatically "possessifies" certain simple
1716    pattern constructs. For example, the sequence A+B is treated as A++B because
1717    there is no point in backtracking into a sequence of A's when B must follow.
1718    .P
1719  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
1720  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
1721  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
1722  pattern  pattern
1723    .sp
1724    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1725    .sp
1726  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
1727  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1728  quickly. However, if it is applied to  quickly. However, if it is applied to
1729    .sp
1730    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1731    .sp
1732  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
1733  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
1734  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
1735  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
1736  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
1737  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
1738  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1739    an atomic group, like this:
1740    ((?>\\D+)|<\\d+>)*[!?]  .sp
1741      ((?>\eD+)|<\ed+>)*[!?]
1742    .sp
1743  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1744    .
1745  .SH BACK REFERENCES  .
1746    .\" HTML <a name="backreferences"></a>
1747    .SH "BACK REFERENCES"
1748  .rs  .rs
1749  .sp  .sp
1750  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
1751  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1752  (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
1753  previous capturing left parentheses.  previous capturing left parentheses.
1754    .P
1755  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
1756  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
1757  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
1758  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
1759  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
1760  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1761    in an earlier iteration.
1762    .P
1763    It is not possible to have a numerical "forward back reference" to a subpattern
1764    whose number is 10 or more using this syntax because a sequence such as \e50 is
1765    interpreted as a character defined in octal. See the subsection entitled
1766    "Non-printing characters"
1767    .\" HTML <a href="#digitsafterbackslash">
1768    .\" </a>
1769    above
1770    .\"
1771    for further details of the handling of digits following a backslash. There is
1772    no such problem when named parentheses are used. A back reference to any
1773    subpattern is possible using named parentheses (see below).
1774    .P
1775    Another way of avoiding the ambiguity inherent in the use of digits following a
1776    backslash is to use the \eg escape sequence. This escape must be followed by an
1777    unsigned number or a negative number, optionally enclosed in braces. These
1778    examples are all identical:
1779    .sp
1780      (ring), \e1
1781      (ring), \eg1
1782      (ring), \eg{1}
1783    .sp
1784    An unsigned number specifies an absolute reference without the ambiguity that
1785    is present in the older syntax. It is also useful when literal digits follow
1786    the reference. A negative number is a relative reference. Consider this
1787    example:
1788    .sp
1789      (abc(def)ghi)\eg{-1}
1790    .sp
1791    The sequence \eg{-1} is a reference to the most recently started capturing
1792    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1793    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1794    can be helpful in long patterns, and also in patterns that are created by
1795    joining together fragments that contain references within themselves.
1796    .P
1797  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1798  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1799  itself (see  itself (see
# Line 832  itself (see Line 1802  itself (see
1802  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1803  .\"  .\"
1804  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1805    .sp
1806    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1807    .sp
1808  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1809  "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
1810  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1811    .sp
1812    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1813    .sp
1814  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
1815  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1816    .P
1817  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1818  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1819    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1820    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1821    references, is also supported. We could rewrite the above example in any of
1822    the following ways:
1823    .sp
1824      (?<p1>(?i)rah)\es+\ek<p1>
1825      (?'p1'(?i)rah)\es+\ek{p1}
1826      (?P<p1>(?i)rah)\es+(?P=p1)
1827      (?<p1>(?i)rah)\es+\eg{p1}
1828    .sp
1829    A subpattern that is referenced by name may appear in the pattern before or
1830    after the reference.
1831    .P
1832  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
1833  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1834  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1835    .sp
1836    (a|(bc))\\2    (a|(bc))\e2
1837    .sp
1838  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
1839  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
1840  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1841  with a digit character, some delimiter must be used to terminate the back  .P
1842  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1843  Otherwise an empty comment can be used.  following a backslash are taken as part of a potential back reference number.
1844    If the pattern continues with a digit character, some delimiter must be used to
1845    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1846    white space. Otherwise, the \eg{ syntax or an empty comment (see
1847    .\" HTML <a href="#comments">
1848    .\" </a>
1849    "Comments"
1850    .\"
1851    below) can be used.
1852    .
1853    .SS "Recursive back references"
1854    .rs
1855    .sp
1856  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
1857  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.
1858  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1859  example, the pattern  example, the pattern
1860    .sp
1861    (a|b\\1)+    (a|b\e1)+
1862    .sp
1863  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
1864  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1865  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
1866  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
1867  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
1868  minimum of zero.  minimum of zero.
1869    .P
1870    Back references of this type cause the group that they reference to be treated
1871    as an
1872    .\" HTML <a href="#atomicgroup">
1873    .\" </a>
1874    atomic group.
1875    .\"
1876    Once the whole group has been matched, a subsequent matching failure cannot
1877    cause backtracking into the middle of the group.
1878    .
1879    .
1880    .\" HTML <a name="bigassertions"></a>
1881  .SH ASSERTIONS  .SH ASSERTIONS
1882  .rs  .rs
1883  .sp  .sp
1884  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
1885  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1886  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
1887    .\" HTML <a href="#smallassertions">
1888    .\" </a>
1889    above.
1890    .\"
1891    .P
1892  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1893  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
1894  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1895    except that it does not cause the current matching position to be changed.
1896  An assertion subpattern is matched in the normal way, except that it does not  .P
1897  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns. If such an assertion
1898  with (?= for positive assertions and (?! for negative assertions. For example,  contains capturing subpatterns within it, these are counted for the purposes of
1899    numbering the capturing subpatterns in the whole pattern. However, substring
1900    \\w+(?=;)  capturing is carried out only for positive assertions, because it does not make
1901    sense for negative assertions.
1902    .P
1903    For compatibility with Perl, assertion subpatterns may be repeated; though
1904    it makes no sense to assert the same thing several times, the side effect of
1905    capturing parentheses may occasionally be useful. In practice, there only three
1906    cases:
1907    .sp
1908    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1909    However, it may contain internal capturing parenthesized groups that are called
1910    from elsewhere via the
1911    .\" HTML <a href="#subpatternsassubroutines">
1912    .\" </a>
1913    subroutine mechanism.
1914    .\"
1915    .sp
1916    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1917    were {0,1}. At run time, the rest of the pattern match is tried with and
1918    without the assertion, the order depending on the greediness of the quantifier.
1919    .sp
1920    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1921    The assertion is obeyed just once when encountered during matching.
1922    .
1923    .
1924    .SS "Lookahead assertions"
1925    .rs
1926    .sp
1927    Lookahead assertions start with (?= for positive assertions and (?! for
1928    negative assertions. For example,
1929    .sp
1930      \ew+(?=;)
1931    .sp
1932  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
1933  the match, and  the match, and
1934    .sp
1935    foo(?!bar)    foo(?!bar)
1936    .sp
1937  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
1938  apparently similar pattern  apparently similar pattern
1939    .sp
1940    (?!foo)bar    (?!foo)bar
1941    .sp
1942  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
1943  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1944  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1945  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1946    .P
1947  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
1948  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
1949  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.
1950    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1951    .
1952    .
1953    .\" HTML <a name="lookbehind"></a>
1954    .SS "Lookbehind assertions"
1955    .rs
1956    .sp
1957  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1958  negative assertions. For example,  negative assertions. For example,
1959    .sp
1960    (?<!foo)bar    (?<!foo)bar
1961    .sp
1962  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
1963  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
1964  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
1965  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1966    .sp
1967    (?<=bullock|donkey)    (?<=bullock|donkey)
1968    .sp
1969  is permitted, but  is permitted, but
1970    .sp
1971    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1972    .sp
1973  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1974  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
1975  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
1976  match the same length of string. An assertion such as  length of string. An assertion such as
1977    .sp
1978    (?<=ab(c|de))    (?<=ab(c|de))
1979    .sp
1980  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
1981  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
1982    branches:
1983    .sp
1984    (?<=abc|abde)    (?<=abc|abde)
1985    .sp
1986    In some cases, the escape sequence \eK
1987    .\" HTML <a href="#resetmatchstart">
1988    .\" </a>
1989    (see above)
1990    .\"
1991    can be used instead of a lookbehind assertion to get round the fixed-length
1992    restriction.
1993    .P
1994  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1995  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
1996  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1997  match is deemed to fail.  assertion fails.
1998    .P
1999  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
2000  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
2001  the length of the lookbehind.  it impossible to calculate the length of the lookbehind. The \eX and \eR
2002    escapes, which can match different numbers of data units, are also not
2003  Atomic groups can be used in conjunction with lookbehind assertions to specify  permitted.
2004  efficient matching at the end of the subject string. Consider a simple pattern  .P
2005  such as  .\" HTML <a href="#subpatternsassubroutines">
2006    .\" </a>
2007    "Subroutine"
2008    .\"
2009    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2010    as the subpattern matches a fixed-length string.
2011    .\" HTML <a href="#recursion">
2012    .\" </a>
2013    Recursion,
2014    .\"
2015    however, is not supported.
2016    .P
2017    Possessive quantifiers can be used in conjunction with lookbehind assertions to
2018    specify efficient matching of fixed-length strings at the end of subject
2019    strings. Consider a simple pattern such as
2020    .sp
2021    abcd$    abcd$
2022    .sp
2023  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
2024  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
2025  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
2026    .sp
2027    ^.*abcd$    ^.*abcd$
2028    .sp
2029  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
2030  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,
2031  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"
2032  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,
2033  if the pattern is written as  if the pattern is written as
2034    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
2035    ^.*+(?<=abcd)    ^.*+(?<=abcd)
2036    .sp
2037  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
2038  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
2039  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2040  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
2041    .
2042    .
2043    .SS "Using multiple assertions"
2044    .rs
2045    .sp
2046  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
2047    .sp
2048    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
2049    .sp
2050  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
2051  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
2052  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
2053  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".
2054  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
2055  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
2056  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
2057    .sp
2058    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
2059    .sp
2060  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
2061  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
2062  preceding three characters are not "999".  preceding three characters are not "999".
2063    .P
2064  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
2065    .sp
2066    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
2067    .sp
2068  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
2069  preceded by "foo", while  preceded by "foo", while
2070    .sp
2071    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
2072    .sp
2073  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
2074  characters that are not "999".  characters that are not "999".
2075    .
2076  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
2077  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
2078  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  
2079  .rs  .rs
2080  .sp  .sp
2081  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2082  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2083  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2084  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2085    .sp
2086    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2087    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2088    .sp
2089  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2090  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
2091  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2092    itself contain nested subpatterns of any form, including conditional
2093  There are three kinds of condition. If the text between the parentheses  subpatterns; the restriction to two alternatives applies only at the level of
2094  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
2095  subpattern of that number has previously matched. The number must be greater  complex:
2096  than zero. Consider the following pattern, which contains non-significant white  .sp
2097  space to make it more readable (assume the PCRE_EXTENDED option) and to divide    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2098  it into three parts for ease of discussion:  .sp
2099    .P
2100    ( \\( )?    [^()]+    (?(1) \\) )  There are four kinds of condition: references to subpatterns, references to
2101    recursion, a pseudo-condition called DEFINE, and assertions.
2102    .
2103    .SS "Checking for a used subpattern by number"
2104    .rs
2105    .sp
2106    If the text between the parentheses consists of a sequence of digits, the
2107    condition is true if a capturing subpattern of that number has previously
2108    matched. If there is more than one capturing subpattern with the same number
2109    (see the earlier
2110    .\"
2111    .\" HTML <a href="#recursion">
2112    .\" </a>
2113    section about duplicate subpattern numbers),
2114    .\"
2115    the condition is true if any of them have matched. An alternative notation is
2116    to precede the digits with a plus or minus sign. In this case, the subpattern
2117    number is relative rather than absolute. The most recently opened parentheses
2118    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2119    loops it can also make sense to refer to subsequent groups. The next
2120    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2121    zero in any of these forms is not used; it provokes a compile-time error.)
2122    .P
2123    Consider the following pattern, which contains non-significant white space to
2124    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2125    three parts for ease of discussion:
2126    .sp
2127      ( \e( )?    [^()]+    (?(1) \e) )
2128    .sp
2129  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2130  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
2131  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
2132  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2133  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,
2134  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
2135  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2136  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2137  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2138    .P
2139  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
2140  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2141  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2142      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2143  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
2144    This makes the fragment independent of the parentheses in the larger pattern.
2145    .
2146    .SS "Checking for a used subpattern by name"
2147    .rs
2148    .sp
2149    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2150    subpattern by name. For compatibility with earlier versions of PCRE, which had
2151    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2152    there is a possible ambiguity with this syntax, because subpattern names may
2153    consist entirely of digits. PCRE looks first for a named subpattern; if it
2154    cannot find one and the name consists entirely of digits, PCRE looks for a
2155    subpattern of that number, which must be greater than zero. Using subpattern
2156    names that consist entirely of digits is not recommended.
2157    .P
2158    Rewriting the above example to use a named subpattern gives this:
2159    .sp
2160      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2161    .sp
2162    If the name used in a condition of this kind is a duplicate, the test is
2163    applied to all subpatterns of the same name, and is true if any one of them has
2164    matched.
2165    .
2166    .SS "Checking for pattern recursion"
2167    .rs
2168    .sp
2169    If the condition is the string (R), and there is no subpattern with the name R,
2170    the condition is true if a recursive call to the whole pattern or any
2171    subpattern has been made. If digits or a name preceded by ampersand follow the
2172    letter R, for example:
2173    .sp
2174      (?(R3)...) or (?(R&name)...)
2175    .sp
2176    the condition is true if the most recent recursion is into a subpattern whose
2177    number or name is given. This condition does not check the entire recursion
2178    stack. If the name used in a condition of this kind is a duplicate, the test is
2179    applied to all subpatterns of the same name, and is true if any one of them is
2180    the most recent recursion.
2181    .P
2182    At "top level", all these recursion test conditions are false.
2183    .\" HTML <a href="#recursion">
2184    .\" </a>
2185    The syntax for recursive patterns
2186    .\"
2187    is described below.
2188    .
2189    .\" HTML <a name="subdefine"></a>
2190    .SS "Defining subpatterns for use by reference only"
2191    .rs
2192    .sp
2193    If the condition is the string (DEFINE), and there is no subpattern with the
2194    name DEFINE, the condition is always false. In this case, there may be only one
2195    alternative in the subpattern. It is always skipped if control reaches this
2196    point in the pattern; the idea of DEFINE is that it can be used to define
2197    subroutines that can be referenced from elsewhere. (The use of
2198    .\" HTML <a href="#subpatternsassubroutines">
2199    .\" </a>
2200    subroutines
2201    .\"
2202    is described below.) For example, a pattern to match an IPv4 address such as
2203    "192.168.23.245" could be written like this (ignore white space and line
2204    breaks):
2205    .sp
2206      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2207      \eb (?&byte) (\e.(?&byte)){3} \eb
2208    .sp
2209    The first part of the pattern is a DEFINE group inside which a another group
2210    named "byte" is defined. This matches an individual component of an IPv4
2211    address (a number less than 256). When matching takes place, this part of the
2212    pattern is skipped because DEFINE acts like a false condition. The rest of the
2213    pattern uses references to the named group to match the four dot-separated
2214    components of an IPv4 address, insisting on a word boundary at each end.
2215    .
2216    .SS "Assertion conditions"
2217    .rs
2218    .sp
2219    If the condition is not in any of the above formats, it must be an assertion.
2220  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2221  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2222  alternatives on the second line:  alternatives on the second line:
2223    .sp
2224    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
2225    \\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} )
2226    .sp
2227  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
2228  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
2229  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
2230  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
2231  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
2232  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.
2233    .
2234    .
2235    .\" HTML <a name="comments"></a>
2236  .SH COMMENTS  .SH COMMENTS
2237  .rs  .rs
2238  .sp  .sp
2239  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
2240  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,
2241  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
2242    subpattern name or number. The characters that make up a comment play no part
2243  If the PCRE_EXTENDED option is set, an unescaped # character outside a  in the pattern matching.
2244  character class introduces a comment that continues up to the next newline  .P
2245  character in the pattern.  The sequence (?# marks the start of a comment that continues up to the next
2246    closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2247  .SH RECURSIVE PATTERNS  option is set, an unescaped # character also introduces a comment, which in
2248    this case continues to immediately after the next newline character or
2249    character sequence in the pattern. Which characters are interpreted as newlines
2250    is controlled by the options passed to a compiling function or by a special
2251    sequence at the start of the pattern, as described in the section entitled
2252    .\" HTML <a href="#newlines">
2253    .\" </a>
2254    "Newline conventions"
2255    .\"
2256    above. Note that the end of this type of comment is a literal newline sequence
2257    in the pattern; escape sequences that happen to represent a newline do not
2258    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2259    default newline convention is in force:
2260    .sp
2261      abc #comment \en still comment
2262    .sp
2263    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2264    a newline in the pattern. The sequence \en is still literal at this stage, so
2265    it does not terminate the comment. Only an actual character with the code value
2266    0x0a (the default newline) does so.
2267    .
2268    .
2269    .\" HTML <a name="recursion"></a>
2270    .SH "RECURSIVE PATTERNS"
2271  .rs  .rs
2272  .sp  .sp
2273  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2274  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2275  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
2276  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
2277  experimental facility that allows regular expressions to recurse (amongst other  .P
2278  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
2279  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
2280  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2281    pattern using code interpolation to solve the parentheses problem can be
2282    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
2283    .sp
2284      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2285    .sp
2286  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
2287  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2288  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2289  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2290    supports special syntax for recursion of the entire pattern, and also for
2291  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2292  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.
2293  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2294  "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
2295  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive subroutine call of the subpattern of the
2296    given number, provided that it occurs inside that subpattern. (If not, it is a
2297  For example, this PCRE pattern solves the nested parentheses problem (assume  .\" HTML <a href="#subpatternsassubroutines">
2298  the PCRE_EXTENDED option is set so that white space is ignored):  .\" </a>
2299    non-recursive subroutine
2300    \\( ( (?>[^()]+) | (?R) )* \\)  .\"
2301    call, which is described in the next section.) The special item (?R) or (?0) is
2302    a recursive call of the entire regular expression.
2303    .P
2304    This PCRE pattern solves the nested parentheses problem (assume the
2305    PCRE_EXTENDED option is set so that white space is ignored):
2306    .sp
2307      \e( ( [^()]++ | (?R) )* \e)
2308    .sp
2309  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2310  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
2311  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2312  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2313    to avoid backtracking into sequences of non-parentheses.
2314    .P
2315  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
2316  pattern, so instead you could use this:  pattern, so instead you could use this:
2317    .sp
2318    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( [^()]++ | (?1) )* \e) )
2319    .sp
2320  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
2321  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2322  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2323  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
2324  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
2325  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2326    parentheses preceding the recursion. In other words, a negative number counts
2327    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  capturing parentheses leftwards from the point at which it is encountered.
2328    .P
2329  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2330  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2331  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
2332  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2333    .\" </a>
2334    non-recursive subroutine
2335    .\"
2336    calls, as described in the next section.
2337    .P
2338    An alternative approach is to use named parentheses instead. The Perl syntax
2339    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2340    could rewrite the above example as follows:
2341    .sp
2342      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2343    .sp
2344    If there is more than one subpattern with the same name, the earliest one is
2345    used.
2346    .P
2347    This particular example pattern that we have been looking at contains nested
2348    unlimited repeats, and so the use of a possessive quantifier for matching
2349    strings of non-parentheses is important when applying the pattern to strings
2350    that do not match. For example, when this pattern is applied to
2351    .sp
2352    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2353    .sp
2354  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,
2355  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
2356  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
2357  before failure can be reported.  before failure can be reported.
2358    .P
2359  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
2360  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
2361  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  
2362  .\" HREF  .\" HREF
2363  \fBpcrecallout\fR  \fBpcrecallout\fP
2364  .\"  .\"
2365  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2366    .sp
2367    (ab(cd)ef)    (ab(cd)ef)
2368    .sp
2369  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
2370  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
2371    matched at the top level, its final captured value is unset, even if it was
2372    \\( ( ( (?>[^()]+) | (?R) )* ) \\)  (temporarily) set at a deeper level during the matching process.
2373       ^                        ^  .P
2374       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2375    obtain extra memory to store data during a recursion, which it does by using
2376  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
2377  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2378  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.  
   
2379  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.
2380  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
2381  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2382  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
2383    .sp
2384    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
2385    .sp
2386  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
2387  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2388  is the actual recursive call.  is the actual recursive call.
2389    .
2390    .
2391    .\" HTML <a name="recursiondifference"></a>
2392    .SS "Differences in recursion processing between PCRE and Perl"
2393    .rs
2394    .sp
2395    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2396    (like Python, but unlike Perl), a recursive subpattern call is always treated
2397    as an atomic group. That is, once it has matched some of the subject string, it
2398    is never re-entered, even if it contains untried alternatives and there is a
2399    subsequent matching failure. This can be illustrated by the following pattern,
2400    which purports to match a palindromic string that contains an odd number of
2401    characters (for example, "a", "aba", "abcba", "abcdcba"):
2402    .sp
2403      ^(.|(.)(?1)\e2)$
2404    .sp
2405    The idea is that it either matches a single character, or two identical
2406    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2407    it does not if the pattern is longer than three characters. Consider the
2408    subject string "abcba":
2409    .P
2410    At the top level, the first character is matched, but as it is not at the end
2411    of the string, the first alternative fails; the second alternative is taken
2412    and the recursion kicks in. The recursive call to subpattern 1 successfully
2413    matches the next character ("b"). (Note that the beginning and end of line
2414    tests are not part of the recursion).
2415    .P
2416    Back at the top level, the next character ("c") is compared with what
2417    subpattern 2 matched, which was "a". This fails. Because the recursion is
2418    treated as an atomic group, there are now no backtracking points, and so the
2419    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2420    try the second alternative.) However, if the pattern is written with the
2421    alternatives in the other order, things are different:
2422    .sp
2423      ^((.)(?1)\e2|.)$
2424    .sp
2425    This time, the recursing alternative is tried first, and continues to recurse
2426    until it runs out of characters, at which point the recursion fails. But this
2427    time we do have another alternative to try at the higher level. That is the big
2428    difference: in the previous case the remaining alternative is at a deeper
2429    recursion level, which PCRE cannot use.
2430    .P
2431    To change the pattern so that it matches all palindromic strings, not just
2432    those with an odd number of characters, it is tempting to change the pattern to
2433    this:
2434    .sp
2435      ^((.)(?1)\e2|.?)$
2436    .sp
2437    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2438    deeper recursion has matched a single character, it cannot be entered again in
2439    order to match an empty string. The solution is to separate the two cases, and
2440    write out the odd and even cases as alternatives at the higher level:
2441    .sp
2442      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2443    .sp
2444    If you want to match typical palindromic phrases, the pattern has to ignore all
2445    non-word characters, which can be done like this:
2446    .sp
2447      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2448    .sp
2449    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2450    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2451    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2452    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2453    more) to match typical phrases, and Perl takes so long that you think it has
2454    gone into a loop.
2455    .P
2456    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2457    string does not start with a palindrome that is shorter than the entire string.
2458    For example, although "abcba" is correctly matched, if the subject is "ababa",
2459    PCRE finds the palindrome "aba" at the start, then fails at top level because
2460    the end of the string does not follow. Once again, it cannot jump back into the
2461    recursion to try other alternatives, so the entire match fails.
2462    .P
2463    The second way in which PCRE and Perl differ in their recursion processing is
2464    in the handling of captured values. In Perl, when a subpattern is called
2465    recursively or as a subpattern (see the next section), it has no access to any
2466    values that were captured outside the recursion, whereas in PCRE these values
2467    can be referenced. Consider this pattern:
2468    .sp
2469      ^(.)(\e1|a(?2))
2470    .sp
2471    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2472    then in the second group, when the back reference \e1 fails to match "b", the
2473    second alternative matches "a" and then recurses. In the recursion, \e1 does
2474    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2475    match because inside the recursive call \e1 cannot access the externally set
2476    value.
2477    .
2478    .
2479  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2480  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
2481  .rs  .rs
2482  .sp  .sp
2483  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
2484  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
2485  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2486  pattern  before or after the reference. A numbered reference can be absolute or
2487    relative, as in these examples:
2488    (sens|respons)e and \\1ibility  .sp
2489      (...(absolute)...)...(?2)...
2490      (...(relative)...)...(?-1)...
2491      (...(?+1)...(relative)...
2492    .sp
2493    An earlier example pointed out that the pattern
2494    .sp
2495      (sens|respons)e and \e1ibility
2496    .sp
2497  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
2498  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
2499    .sp
2500    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2501    .sp
2502  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
2503  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2504  refer.  .P
2505    All subroutine calls, whether recursive or not, are always treated as atomic
2506    groups. That is, once a subroutine has matched some of the subject string, it
2507    is never re-entered, even if it contains untried alternatives and there is a
2508    subsequent matching failure. Any capturing parentheses that are set during the
2509    subroutine call revert to their previous values afterwards.
2510    .P
2511    Processing options such as case-independence are fixed when a subpattern is
2512    defined, so if it is used as a subroutine, such options cannot be changed for
2513    different calls. For example, consider this pattern:
2514    .sp
2515      (abc)(?i:(?-1))
2516    .sp
2517    It matches "abcabc". It does not match "abcABC" because the change of
2518    processing option does not affect the called subpattern.
2519    .
2520    .
2521    .\" HTML <a name="onigurumasubroutines"></a>
2522    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2523    .rs
2524    .sp
2525    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2526    a number enclosed either in angle brackets or single quotes, is an alternative
2527    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2528    are two of the examples used above, rewritten using this syntax:
2529    .sp
2530      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2531      (sens|respons)e and \eg'1'ibility
2532    .sp
2533    PCRE supports an extension to Oniguruma: if a number is preceded by a
2534    plus or a minus sign it is taken as a relative reference. For example:
2535    .sp
2536      (abc)(?i:\eg<-1>)
2537    .sp
2538    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2539    synonymous. The former is a back reference; the latter is a subroutine call.
2540    .
2541    .
2542  .SH CALLOUTS  .SH CALLOUTS
2543  .rs  .rs
2544  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 2546  Perl has a feature whereby using the seq
2546  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
2547  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
2548  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
2549    .P
2550  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
2551  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
2552  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
2553  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2554    variable contains NULL, which disables all calling out.
2555    .P
2556  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2557  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
2558  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.
2559  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2560    .sp
2561    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2562    .sp
2563  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
2564  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
2565  callout, and, optionally, one item of data originally supplied by the caller of  255.
2566  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  .P
2567  fail altogether. A complete description of the interface to the callout  During matching, when PCRE reaches a callout point, the external function is
2568  function is given in the  called. It is provided with the number of the callout, the position in the
2569    pattern, and, optionally, one item of data originally supplied by the caller of
2570    the matching function. The callout function may cause matching to proceed, to
2571    backtrack, or to fail altogether. A complete description of the interface to
2572    the callout function is given in the
2573  .\" HREF  .\" HREF
2574  \fBpcrecallout\fR  \fBpcrecallout\fP
2575  .\"  .\"
2576  documentation.  documentation.
2577    .
2578  .in 0  .
2579  Last updated: 03 February 2003  .\" HTML <a name="backtrackcontrol"></a>
2580  .br  .SH "BACKTRACKING CONTROL"
2581  Copyright (c) 1997-2003 University of Cambridge.  .rs
2582    .sp
2583    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2584    are described in the Perl documentation as "experimental and subject to change
2585    or removal in a future version of Perl". It goes on to say: "Their usage in
2586    production code should be noted to avoid problems during upgrades." The same
2587    remarks apply to the PCRE features described in this section.
2588    .P
2589    Since these verbs are specifically related to backtracking, most of them can be
2590    used only when the pattern is to be matched using one of the traditional
2591    matching functions, which use a backtracking algorithm. With the exception of
2592    (*FAIL), which behaves like a failing negative assertion, they cause an error
2593    if encountered by a DFA matching function.
2594    .P
2595    If any of these verbs are used in an assertion or in a subpattern that is
2596    called as a subroutine (whether or not recursively), their effect is confined
2597    to that subpattern; it does not extend to the surrounding pattern, with one
2598    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2599    a successful positive assertion \fIis\fP passed back when a match succeeds
2600    (compare capturing parentheses in assertions). Note that such subpatterns are
2601    processed as anchored at the point where they are tested. Note also that Perl's
2602    treatment of subroutines and assertions is different in some cases.
2603    .P
2604    The new verbs make use of what was previously invalid syntax: an opening
2605    parenthesis followed by an asterisk. They are generally of the form
2606    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2607    depending on whether or not an argument is present. A name is any sequence of
2608    characters that does not include a closing parenthesis. The maximum length of
2609    name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2610    is empty, that is, if the closing parenthesis immediately follows the colon,
2611    the effect is as if the colon were not there. Any number of these verbs may
2612    occur in a pattern.
2613    .
2614    .
2615    .\" HTML <a name="nooptimize"></a>
2616    .SS "Optimizations that affect backtracking verbs"
2617    .rs
2618    .sp
2619    PCRE contains some optimizations that are used to speed up matching by running
2620    some checks at the start of each match attempt. For example, it may know the
2621    minimum length of matching subject, or that a particular character must be
2622    present. When one of these optimizations suppresses the running of a match, any
2623    included backtracking verbs will not, of course, be processed. You can suppress
2624    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2625    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2626    pattern with (*NO_START_OPT). There is more discussion of this option in the
2627    section entitled
2628    .\" HTML <a href="pcreapi.html#execoptions">
2629    .\" </a>
2630    "Option bits for \fBpcre_exec()\fP"
2631    .\"
2632    in the
2633    .\" HREF
2634    \fBpcreapi\fP
2635    .\"
2636    documentation.
2637    .P
2638    Experiments with Perl suggest that it too has similar optimizations, sometimes
2639    leading to anomalous results.
2640    .
2641    .
2642    .SS "Verbs that act immediately"
2643    .rs
2644    .sp
2645    The following verbs act as soon as they are encountered. They may not be
2646    followed by a name.
2647    .sp
2648       (*ACCEPT)
2649    .sp
2650    This verb causes the match to end successfully, skipping the remainder of the
2651    pattern. However, when it is inside a subpattern that is called as a
2652    subroutine, only that subpattern is ended successfully. Matching then continues
2653    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2654    far is captured. For example:
2655    .sp
2656      A((?:A|B(*ACCEPT)|C)D)
2657    .sp
2658    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2659    the outer parentheses.
2660    .sp
2661      (*FAIL) or (*F)
2662    .sp
2663    This verb causes a matching failure, forcing backtracking to occur. It is
2664    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2665    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2666    Perl features that are not present in PCRE. The nearest equivalent is the
2667    callout feature, as for example in this pattern:
2668    .sp
2669      a+(?C)(*FAIL)
2670    .sp
2671    A match with the string "aaaa" always fails, but the callout is taken before
2672    each backtrack happens (in this example, 10 times).
2673    .
2674    .
2675    .SS "Recording which path was taken"
2676    .rs
2677    .sp
2678    There is one verb whose main purpose is to track how a match was arrived at,
2679    though it also has a secondary use in conjunction with advancing the match
2680    starting point (see (*SKIP) below).
2681    .sp
2682      (*MARK:NAME) or (*:NAME)
2683    .sp
2684    A name is always required with this verb. There may be as many instances of
2685    (*MARK) as you like in a pattern, and their names do not have to be unique.
2686    .P
2687    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2688    path is passed back to the caller as described in the section entitled
2689    .\" HTML <a href="pcreapi.html#extradata">
2690    .\" </a>
2691    "Extra data for \fBpcre_exec()\fP"
2692    .\"
2693    in the
2694    .\" HREF
2695    \fBpcreapi\fP
2696    .\"
2697    documentation. Here is an example of \fBpcretest\fP output, where the /K
2698    modifier requests the retrieval and outputting of (*MARK) data:
2699    .sp
2700        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2701      data> XY
2702       0: XY
2703      MK: A
2704      XZ
2705       0: XZ
2706      MK: B
2707    .sp
2708    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2709    indicates which of the two alternatives matched. This is a more efficient way
2710    of obtaining this information than putting each alternative in its own
2711    capturing parentheses.
2712    .P
2713    If (*MARK) is encountered in a positive assertion, its name is recorded and
2714    passed back if it is the last-encountered. This does not happen for negative
2715    assertions.
2716    .P
2717    After a partial match or a failed match, the name of the last encountered
2718    (*MARK) in the entire match process is returned. For example:
2719    .sp
2720        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2721      data> XP
2722      No match, mark = B
2723    .sp
2724    Note that in this unanchored example the mark is retained from the match
2725    attempt that started at the letter "X" in the subject. Subsequent match
2726    attempts starting at "P" and then with an empty string do not get as far as the
2727    (*MARK) item, but nevertheless do not reset it.
2728    .P
2729    If you are interested in (*MARK) values after failed matches, you should
2730    probably set the PCRE_NO_START_OPTIMIZE option
2731    .\" HTML <a href="#nooptimize">
2732    .\" </a>
2733    (see above)
2734    .\"
2735    to ensure that the match is always attempted.
2736    .
2737    .
2738    .SS "Verbs that act after backtracking"
2739    .rs
2740    .sp
2741    The following verbs do nothing when they are encountered. Matching continues
2742    with what follows, but if there is no subsequent match, causing a backtrack to
2743    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2744    the verb. However, when one of these verbs appears inside an atomic group, its
2745    effect is confined to that group, because once the group has been matched,
2746    there is never any backtracking into it. In this situation, backtracking can
2747    "jump back" to the left of the entire atomic group. (Remember also, as stated
2748    above, that this localization also applies in subroutine calls and assertions.)
2749    .P
2750    These verbs differ in exactly what kind of failure occurs when backtracking
2751    reaches them.
2752    .sp
2753      (*COMMIT)
2754    .sp
2755    This verb, which may not be followed by a name, causes the whole match to fail
2756    outright if the rest of the pattern does not match. Even if the pattern is
2757    unanchored, no further attempts to find a match by advancing the starting point
2758    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2759    finding a match at the current starting point, or not at all. For example:
2760    .sp
2761      a+(*COMMIT)b
2762    .sp
2763    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2764    dynamic anchor, or "I've started, so I must finish." The name of the most
2765    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2766    match failure.
2767    .P
2768    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2769    unless PCRE's start-of-match optimizations are turned off, as shown in this
2770    \fBpcretest\fP example:
2771    .sp
2772        re> /(*COMMIT)abc/
2773      data> xyzabc
2774       0: abc
2775      xyzabc\eY
2776      No match
2777    .sp
2778    PCRE knows that any match must start with "a", so the optimization skips along
2779    the subject to "a" before running the first match attempt, which succeeds. When
2780    the optimization is disabled by the \eY escape in the second subject, the match
2781    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2782    starting points.
2783    .sp
2784      (*PRUNE) or (*PRUNE:NAME)
2785    .sp
2786    This verb causes the match to fail at the current starting position in the
2787    subject if the rest of the pattern does not match. If the pattern is
2788    unanchored, the normal "bumpalong" advance to the next starting character then
2789    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2790    reached, or when matching to the right of (*PRUNE), but if there is no match to
2791    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2792    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2793    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2794    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2795    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2796    .sp
2797      (*SKIP)
2798    .sp
2799    This verb, when given without a name, is like (*PRUNE), except that if the
2800    pattern is unanchored, the "bumpalong" advance is not to the next character,
2801    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2802    signifies that whatever text was matched leading up to it cannot be part of a
2803    successful match. Consider:
2804    .sp
2805      a+(*SKIP)b
2806    .sp
2807    If the subject is "aaaac...", after the first match attempt fails (starting at
2808    the first character in the string), the starting point skips on to start the
2809    next attempt at "c". Note that a possessive quantifer does not have the same
2810    effect as this example; although it would suppress backtracking during the
2811    first match attempt, the second attempt would start at the second character
2812    instead of skipping on to "c".
2813    .sp
2814      (*SKIP:NAME)
2815    .sp
2816    When (*SKIP) has an associated name, its behaviour is modified. If the
2817    following pattern fails to match, the previous path through the pattern is
2818    searched for the most recent (*MARK) that has the same name. If one is found,
2819    the "bumpalong" advance is to the subject position that corresponds to that
2820    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2821    matching name is found, the (*SKIP) is ignored.
2822    .sp
2823      (*THEN) or (*THEN:NAME)
2824    .sp
2825    This verb causes a skip to the next innermost alternative if the rest of the
2826    pattern does not match. That is, it cancels pending backtracking, but only
2827    within the current alternative. Its name comes from the observation that it can
2828    be used for a pattern-based if-then-else block:
2829    .sp
2830      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2831    .sp
2832    If the COND1 pattern matches, FOO is tried (and possibly further items after
2833    the end of the group if FOO succeeds); on failure, the matcher skips to the
2834    second alternative and tries COND2, without backtracking into COND1. The
2835    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2836    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2837    .P
2838    Note that a subpattern that does not contain a | character is just a part of
2839    the enclosing alternative; it is not a nested alternation with only one
2840    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2841    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2842    pattern fragments that do not contain any | characters at this level:
2843    .sp
2844      A (B(*THEN)C) | D
2845    .sp
2846    If A and B are matched, but there is a failure in C, matching does not
2847    backtrack into A; instead it moves to the next alternative, that is, D.
2848    However, if the subpattern containing (*THEN) is given an alternative, it
2849    behaves differently:
2850    .sp
2851      A (B(*THEN)C | (*FAIL)) | D
2852    .sp
2853    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2854    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2855    because there are no more alternatives to try. In this case, matching does now
2856    backtrack into A.
2857    .P
2858    Note also that a conditional subpattern is not considered as having two
2859    alternatives, because only one is ever used. In other words, the | character in
2860    a conditional subpattern has a different meaning. Ignoring white space,
2861    consider:
2862    .sp
2863      ^.*? (?(?=a) a | b(*THEN)c )
2864    .sp
2865    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2866    it initially matches zero characters. The condition (?=a) then fails, the
2867    character "b" is matched, but "c" is not. At this point, matching does not
2868    backtrack to .*? as might perhaps be expected from the presence of the |
2869    character. The conditional subpattern is part of the single alternative that
2870    comprises the whole pattern, and so the match fails. (If there was a backtrack
2871    into .*?, allowing it to match "b", the match would succeed.)
2872    .P
2873    The verbs just described provide four different "strengths" of control when
2874    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2875    next alternative. (*PRUNE) comes next, failing the match at the current
2876    starting position, but allowing an advance to the next character (for an
2877    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2878    than one character. (*COMMIT) is the strongest, causing the entire match to
2879    fail.
2880    .P
2881    If more than one such verb is present in a pattern, the "strongest" one wins.
2882    For example, consider this pattern, where A, B, etc. are complex pattern
2883    fragments:
2884    .sp
2885      (A(*COMMIT)B(*THEN)C|D)
2886    .sp
2887    Once A has matched, PCRE is committed to this match, at the current starting
2888    position. If subsequently B matches, but C does not, the normal (*THEN) action
2889    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2890    overrides.
2891    .
2892    .
2893    .SH "SEE ALSO"
2894    .rs
2895    .sp
2896    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2897    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2898    .
2899    .
2900    .SH AUTHOR
2901    .rs
2902    .sp
2903    .nf
2904    Philip Hazel
2905    University Computing Service
2906    Cambridge CB2 3QH, England.
2907    .fi
2908    .
2909    .
2910    .SH REVISION
2911    .rs
2912    .sp
2913    .nf
2914    Last updated: 01 June 2012
2915    Copyright (c) 1997-2012 University of Cambridge.
2916    .fi

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