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revision 535 by ph10, Thu Jun 3 19:18:24 2010 UTC revision 758 by ph10, Mon Nov 21 12:05:36 2011 UTC
# Line 32  Starting a pattern with this sequence is Line 32  Starting a pattern with this sequence is
32  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33  pattern matching is mentioned in several places below. There is also a summary  pattern matching is mentioned in several places below. There is also a summary
34  of UTF-8 features in the  of UTF-8 features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
35  .\" HREF  .\" HREF
36  \fBpcre\fP  \fBpcreunicode\fP
37  .\"  .\"
38  page.  page.
39  .P  .P
# Line 52  such as \ed and \ew to use Unicode prope Line 47  such as \ed and \ew to use Unicode prope
47  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
48  table.  table.
49  .P  .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 66  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
71  .rs  .rs
72  .sp  .sp
# Line 181  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  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
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 191  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 210  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223  .  .
224  .  .
225  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 223  but when a pattern is being prepared by Line 233  but when a pattern is being prepared by
233  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
234  .sp  .sp
235    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
236    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
237    \ee        escape (hex 1B)    \ee        escape (hex 1B)
238    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
239    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 231  one of the following escape sequences th Line 241  one of the following escape sequences th
241    \et        tab (hex 09)    \et        tab (hex 09)
242    \eddd      character with octal code ddd, or back reference    \eddd      character with octal code ddd, or back reference
243    \exhh      character with hex code hh    \exhh      character with hex code hh
244    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
245      \euhhhh    character with hex code hhhh (JavaScript mode only)
246  .sp  .sp
247  The precise effect of \ecx 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
248  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.
249  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
250  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
252  After \ex, from zero to two hexadecimal digits are read (letters can be in  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253  upper or lower case). Any number of hexadecimal digits may appear between \ex{  values are valid. A lower case letter is converted to upper case, and then the
254  and }, but the value of the character code must be less than 256 in non-UTF-8  0xc0 bits are flipped.)
255  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  .P
256  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  By default, after \ex, from zero to two hexadecimal digits are read (letters
257  point, which is 10FFFF.  can be in upper or lower case). Any number of hexadecimal digits may appear
258    between \ex{ and }, but the value of the character code must be less than 256
259    in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
260    value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
261    Unicode code point, which is 10FFFF.
262  .P  .P
263  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
264  there is no terminating }, this form of escape is not recognized. Instead, the  there is no terminating }, this form of escape is not recognized. Instead, the
265  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
266  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
267  .P  .P
268    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
269    as just described only when it is followed by two hexadecimal digits.
270    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
271    code points greater than 256 is provided by \eu, which must be followed by
272    four hexadecimal digits; otherwise it matches a literal "u" character.
273    .P
274  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
275  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
276  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
277    \eu00dc in JavaScript mode).
278  .P  .P
279  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
280  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 306  Note that octal values of 100 or greater Line 328  Note that octal values of 100 or greater
328  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
329  .P  .P
330  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
331  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
332  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
333  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
334  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
335  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
336  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
337    error if the PCRE_EXTRA option is set. Outside a character class, these
338    sequences have different meanings.
339    .
340    .
341    .SS "Unsupported escape sequences"
342    .rs
343    .sp
344    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
345    handler and used to modify the case of following characters. By default, PCRE
346    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
347    option is set, \eU matches a "U" character, and \eu can be used to define a
348    character by code point, as described in the previous section.
349  .  .
350  .  .
351  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 373  This is the same as Line 407  This is the same as
407  .\" </a>  .\" </a>
408  the "." metacharacter  the "." metacharacter
409  .\"  .\"
410  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
411    PCRE does not support this.
412  .P  .P
413  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
414  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 422  any Unicode letter, and underscore. Note Line 457  any Unicode letter, and underscore. Note
457  \eB because they are defined in terms of \ew and \eW. Matching these sequences  \eB because they are defined in terms of \ew and \eW. Matching these sequences
458  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
459  .P  .P
460  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
461  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
462  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is  characters by default, these always match certain high-valued codepoints in
463  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
464    are:
465  .sp  .sp
466    U+0009     Horizontal tab    U+0009     Horizontal tab
467    U+0020     Space    U+0020     Space
# Line 463  The vertical space characters are: Line 499  The vertical space characters are:
499  .rs  .rs
500  .sp  .sp
501  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
502  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
503  .sp  .sp
504    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
505  .sp  .sp
# Line 738  Characters with the "mark" property are Line 773  Characters with the "mark" property are
773  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
774  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
775  .P  .P
776    Note that recent versions of Perl have changed \eX to match what Unicode calls
777    an "extended grapheme cluster", which has a more complicated definition.
778    .P
779  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
780  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
781  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
# Line 772  same characters as Xan, plus underscore. Line 810  same characters as Xan, plus underscore.
810  .SS "Resetting the match start"  .SS "Resetting the match start"
811  .rs  .rs
812  .sp  .sp
813  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
814  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
815  .sp  .sp
816    foo\eKbar    foo\eKbar
817  .sp  .sp
# Line 946  The handling of dot is entirely independ Line 983  The handling of dot is entirely independ
983  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
984  special meaning in a character class.  special meaning in a character class.
985  .P  .P
986  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not  The escape sequence \eN behaves like a dot, except that it is not affected by
987  set. In other words, it matches any one character except one that signifies the  the PCRE_DOTALL option. In other words, it matches any character except one
988  end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
989    name; PCRE does not support this.
990  .  .
991  .  .
992  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
993  .rs  .rs
994  .sp  .sp
995  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
996  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
997  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
998  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
999  what remains in the string may be a malformed UTF-8 string. For this reason,  breaks up characters into individual bytes, matching one byte with \eC in UTF-8
1000  the \eC escape sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
1001    character. This has undefined results, because PCRE assumes that it is dealing
1002    with valid UTF-8 strings (and by default it checks this at the start of
1003    processing unless the PCRE_NO_UTF8_CHECK option is used).
1004  .P  .P
1005  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1006  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1007  .\" </a>  .\" </a>
1008  (described below),  (described below)
1009  .\"  .\"
1010  because in UTF-8 mode this would make it impossible to calculate the length of  in UTF-8 mode, because this would make it impossible to calculate the length of
1011  the lookbehind.  the lookbehind.
1012    .P
1013    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1014    way of using it that avoids the problem of malformed UTF-8 characters is to
1015    use a lookahead to check the length of the next character, as in this pattern
1016    (ignore white space and line breaks):
1017    .sp
1018      (?| (?=[\ex00-\ex7f])(\eC) |
1019          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1020          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1021          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1022    .sp
1023    A group that starts with (?| resets the capturing parentheses numbers in each
1024    alternative (see
1025    .\" HTML <a href="#dupsubpatternnumber">
1026    .\" </a>
1027    "Duplicate Subpattern Numbers"
1028    .\"
1029    below). The assertions at the start of each branch check the next UTF-8
1030    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1031    character's individual bytes are then captured by the appropriate number of
1032    groups.
1033  .  .
1034  .  .
1035  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 1043  characters in both cases. In UTF-8 mode, Line 1105  characters in both cases. In UTF-8 mode,
1105  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1106  property support.  property support.
1107  .P  .P
1108  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1109  \eW may also appear in a character class, and add the characters that they  \eV, \ew, and \eW may appear in a character class, and add the characters that
1110  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1111  circumflex can conveniently be used with the upper case character types to  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1112    and their upper case partners, just as it does when they appear outside a
1113    character class, as described in the section entitled
1114    .\" HTML <a href="#genericchartypes">
1115    .\" </a>
1116    "Generic character types"
1117    .\"
1118    above. The escape sequence \eb has a different meaning inside a character
1119    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1120    are not special inside a character class. Like any other unrecognized escape
1121    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1122    default, but cause an error if the PCRE_EXTRA option is set.
1123    .P
1124    A circumflex can conveniently be used with the upper case character types to
1125  specify a more restricted set of characters than the matching lower case type.  specify a more restricted set of characters than the matching lower case type.
1126  For example, the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore,
1127    whereas [\ew] includes underscore. A positive character class should be read as
1128    "something OR something OR ..." and a negative class as "NOT something AND NOT
1129    something AND NOT ...".
1130  .P  .P
1131  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1132  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 1171  extracts it into the global options (and Line 1249  extracts it into the global options (and
1249  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1250  .P  .P
1251  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1252  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1253  .sp  .sp
1254    (a(?i)b)c    (a(?i)b)c
1255  .sp  .sp
# Line 1212  Turning part of a pattern into a subpatt Line 1290  Turning part of a pattern into a subpatt
1290  .sp  .sp
1291    cat(aract|erpillar|)    cat(aract|erpillar|)
1292  .sp  .sp
1293  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1294  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1295  .sp  .sp
1296  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1297  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1298  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1299  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1300  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1301  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1302  .sp  .sp
1303    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1304  .sp  .sp
# Line 1270  at captured substring number one, whiche Line 1347  at captured substring number one, whiche
1347  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1348  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1349  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1350  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1351  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1352  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1353  .sp  .sp
1354    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1355    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# Line 1285  or "defdef": Line 1361  or "defdef":
1361  .sp  .sp
1362    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1363  .sp  .sp
1364  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1365  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1366  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1367  .sp  .sp
1368    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1369  .sp  .sp
# Line 1400  items: Line 1476  items:
1476    the \eC escape sequence    the \eC escape sequence
1477    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1478    the \eR escape sequence    the \eR escape sequence
1479    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1480    a character class    a character class
1481    a back reference (see next section)    a back reference (see next section)
1482    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1483    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1484  .sp  .sp
1485  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1486  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1442  subpatterns that are referenced as Line 1518  subpatterns that are referenced as
1518  .\" </a>  .\" </a>
1519  subroutines  subroutines
1520  .\"  .\"
1521  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1522  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1523    .\" </a>
1524    "Defining subpatterns for use by reference only"
1525    .\"
1526    below). Items other than subpatterns that have a {0} quantifier are omitted
1527    from the compiled pattern.
1528  .P  .P
1529  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1530  abbreviations:  abbreviations:
# Line 1668  no such problem when named parentheses a Line 1749  no such problem when named parentheses a
1749  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1750  .P  .P
1751  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1752  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1753  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1754  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1755  .sp  .sp
1756    (ring), \e1    (ring), \e1
1757    (ring), \eg1    (ring), \eg1
# Line 1684  example: Line 1765  example:
1765    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1766  .sp  .sp
1767  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1768  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1769  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1770  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1771  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1772  .P  .P
1773  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1774  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1789  those that look ahead of the current pos Line 1870  those that look ahead of the current pos
1870  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1871  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1872  .P  .P
1873  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1874  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1875  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1876  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1877  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1878  because it does not make sense for negative assertions.  .P
1879    For compatibility with Perl, assertion subpatterns may be repeated; though
1880    it makes no sense to assert the same thing several times, the side effect of
1881    capturing parentheses may occasionally be useful. In practice, there only three
1882    cases:
1883    .sp
1884    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1885    However, it may contain internal capturing parenthesized groups that are called
1886    from elsewhere via the
1887    .\" HTML <a href="#subpatternsassubroutines">
1888    .\" </a>
1889    subroutine mechanism.
1890    .\"
1891    .sp
1892    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1893    were {0,1}. At run time, the rest of the pattern match is tried with and
1894    without the assertion, the order depending on the greediness of the quantifier.
1895    .sp
1896    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1897    The assertion is obeyed just once when encountered during matching.
1898  .  .
1899  .  .
1900  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1823  lookbehind assertion is needed to achiev Line 1923  lookbehind assertion is needed to achiev
1923  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
1924  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
1925  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.
1926  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1927  .  .
1928  .  .
1929  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1849  is permitted, but Line 1948  is permitted, but
1948  .sp  .sp
1949  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1950  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
1951  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1952  match the same length of string. An assertion such as  length of string. An assertion such as
1953  .sp  .sp
1954    (?<=ab(c|de))    (?<=ab(c|de))
1955  .sp  .sp
# Line 1860  branches: Line 1959  branches:
1959  .sp  .sp
1960    (?<=abc|abde)    (?<=abc|abde)
1961  .sp  .sp
1962  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1963  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1964  .\" </a>  .\" </a>
1965  (see above)  (see above)
# Line 1873  temporarily move the current position ba Line 1972  temporarily move the current position ba
1972  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1973  assertion fails.  assertion fails.
1974  .P  .P
1975  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,
1976  to appear in lookbehind assertions, because it makes it impossible to calculate  even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
1977  the length of the lookbehind. The \eX and \eR escapes, which can match  impossible to calculate the length of the lookbehind. The \eX and \eR escapes,
1978  different numbers of bytes, are also not permitted.  which can match different numbers of bytes, are also not permitted.
1979  .P  .P
1980  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
1981  .\" </a>  .\" </a>
# Line 1964  already been matched. The two possible f Line 2063  already been matched. The two possible f
2063  .sp  .sp
2064  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2065  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
2066  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2067    itself contain nested subpatterns of any form, including conditional
2068    subpatterns; the restriction to two alternatives applies only at the level of
2069    the condition. This pattern fragment is an example where the alternatives are
2070    complex:
2071    .sp
2072      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2073    .sp
2074  .P  .P
2075  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2076  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1981  matched. If there is more than one captu Line 2087  matched. If there is more than one captu
2087  .\" </a>  .\" </a>
2088  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2089  .\"  .\"
2090  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2091  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2092  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2093  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2094  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2095  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2096    zero in any of these forms is not used; it provokes a compile-time error.)
2097  .P  .P
2098  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2099  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1997  three parts for ease of discussion: Line 2104  three parts for ease of discussion:
2104  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2105  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
2106  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
2107  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2108  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,
2109  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
2110  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2111  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 2054  The syntax for recursive patterns Line 2161  The syntax for recursive patterns
2161  .\"  .\"
2162  is described below.  is described below.
2163  .  .
2164    .\" HTML <a name="subdefine"></a>
2165  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2166  .rs  .rs
2167  .sp  .sp
# Line 2061  If the condition is the string (DEFINE), Line 2169  If the condition is the string (DEFINE),
2169  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2170  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2171  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2172  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2173  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2174  .\" </a>  .\" </a>
2175  "subroutines"  subroutines
2176  .\"  .\"
2177  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2178  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2179    breaks):
2180  .sp  .sp
2181    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2182    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2102  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2211  dd-aaa-dd or dd-dd-dd, where aaa are let
2211  .SH COMMENTS  .SH COMMENTS
2212  .rs  .rs
2213  .sp  .sp
2214  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2215  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,
2216  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
2217    subpattern name or number. The characters that make up a comment play no part
2218    in the pattern matching.
2219  .P  .P
2220  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2221  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2222  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2223    this case continues to immediately after the next newline character or
2224    character sequence in the pattern. Which characters are interpreted as newlines
2225    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2226    sequence at the start of the pattern, as described in the section entitled
2227    .\" HTML <a href="#newlines">
2228    .\" </a>
2229    "Newline conventions"
2230    .\"
2231    above. Note that the end of this type of comment is a literal newline sequence
2232    in the pattern; escape sequences that happen to represent a newline do not
2233    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2234    default newline convention is in force:
2235    .sp
2236      abc #comment \en still comment
2237    .sp
2238    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2239    a newline in the pattern. The sequence \en is still literal at this stage, so
2240    it does not terminate the comment. Only an actual character with the code value
2241    0x0a (the default newline) does so.
2242  .  .
2243  .  .
2244  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2137  individual subpattern recursion. After i Line 2267  individual subpattern recursion. After i
2267  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2268  .P  .P
2269  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2270  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2271  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2272  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2273  .\" </a>  .\" </a>
2274  "subroutine"  non-recursive subroutine
2275  .\"  .\"
2276  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2277  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2166  We have put the pattern into parentheses Line 2296  We have put the pattern into parentheses
2296  them instead of the whole pattern.  them instead of the whole pattern.
2297  .P  .P
2298  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2299  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2300  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2301  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2302  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2303  .P  .P
2304  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2305  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2306  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2307  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2308  .\" </a>  .\" </a>
2309  "subroutine"  non-recursive subroutine
2310  .\"  .\"
2311  calls, as described in the next section.  calls, as described in the next section.
2312  .P  .P
# Line 2214  documentation). If the pattern above is Line 2343  documentation). If the pattern above is
2343  .sp  .sp
2344  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2345  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2346  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2347  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2348  .P  .P
2349  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2350  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2235  is the actual recursive call. Line 2364  is the actual recursive call.
2364  .  .
2365  .  .
2366  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2367  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2368  .rs  .rs
2369  .sp  .sp
2370  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2371  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2372  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2373  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2374  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2375  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2376    characters (for example, "a", "aba", "abcba", "abcdcba"):
2377  .sp  .sp
2378    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2379  .sp  .sp
# Line 2273  time we do have another alternative to t Line 2403  time we do have another alternative to t
2403  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2404  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2405  .P  .P
2406  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2407  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2408    this:
2409  .sp  .sp
2410    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2411  .sp  .sp
# Line 2303  For example, although "abcba" is correct Line 2434  For example, although "abcba" is correct
2434  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2435  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2436  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2437    .P
2438    The second way in which PCRE and Perl differ in their recursion processing is
2439    in the handling of captured values. In Perl, when a subpattern is called
2440    recursively or as a subpattern (see the next section), it has no access to any
2441    values that were captured outside the recursion, whereas in PCRE these values
2442    can be referenced. Consider this pattern:
2443    .sp
2444      ^(.)(\e1|a(?2))
2445    .sp
2446    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2447    then in the second group, when the back reference \e1 fails to match "b", the
2448    second alternative matches "a" and then recurses. In the recursion, \e1 does
2449    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2450    match because inside the recursive call \e1 cannot access the externally set
2451    value.
2452  .  .
2453  .  .
2454  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2455  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2456  .rs  .rs
2457  .sp  .sp
2458  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
2459  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
2460  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2461  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2462  relative, as in these examples:  relative, as in these examples:
2463  .sp  .sp
# Line 2331  matches "sense and sensibility" and "res Line 2477  matches "sense and sensibility" and "res
2477  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
2478  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2479  .P  .P
2480  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2481  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2482  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2483  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2484  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2485  .P  .P
2486  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2487  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2488  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2489  .sp  .sp
2490    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2491  .sp  .sp
# Line 2420  a backtracking algorithm. With the excep Line 2566  a backtracking algorithm. With the excep
2566  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2567  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2568  .P  .P
2569  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2570  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2571  it does not extend to the surrounding pattern. Note that such subpatterns are  to that subpattern; it does not extend to the surrounding pattern, with one
2572  processed as anchored at the point where they are tested.  exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2573    back (compare capturing parentheses in assertions). Note that such subpatterns
2574    are processed as anchored at the point where they are tested. Note also that
2575    Perl's treatment of subroutines is different in some cases.
2576  .P  .P
2577  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2578  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2579  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2580  depending on whether or not an argument is present. An name is a sequence of  depending on whether or not an argument is present. A name is any sequence of
2581  letters, digits, and underscores. If the name is empty, that is, if the closing  characters that does not include a closing parenthesis. If the name is empty,
2582  parenthesis immediately follows the colon, the effect is as if the colon were  that is, if the closing parenthesis immediately follows the colon, the effect
2583  not there. Any number of these verbs may occur in a pattern.  is as if the colon were not there. Any number of these verbs may occur in a
2584    pattern.
2585  .P  .P
2586  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2587  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
# Line 2439  minimum length of matching subject, or t Line 2589  minimum length of matching subject, or t
2589  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations suppresses the running of a match, any
2590  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2591  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2592  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2593    pattern with (*NO_START_OPT).
2594  .  .
2595  .  .
2596  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2451  followed by a name. Line 2602  followed by a name.
2602     (*ACCEPT)     (*ACCEPT)
2603  .sp  .sp
2604  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2605  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2606  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2607  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2608    far is captured. For example:
2609  .sp  .sp
2610    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2611  .sp  .sp
# Line 2462  the outer parentheses. Line 2614  the outer parentheses.
2614  .sp  .sp
2615    (*FAIL) or (*F)    (*FAIL) or (*F)
2616  .sp  .sp
2617  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2618  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2619  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2620  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2513  indicates which of the two alternatives Line 2665  indicates which of the two alternatives
2665  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2666  capturing parentheses.  capturing parentheses.
2667  .P  .P
2668    If (*MARK) is encountered in a positive assertion, its name is recorded and
2669    passed back if it is the last-encountered. This does not happen for negative
2670    assertions.
2671    .P
2672  A name may also be returned after a failed match if the final path through the  A name may also be returned after a failed match if the final path through the
2673  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2674  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
# Line 2625  following pattern fails to match, the pr Line 2781  following pattern fails to match, the pr
2781  searched for the most recent (*MARK) that has the same name. If one is found,  searched for the most recent (*MARK) that has the same name. If one is found,
2782  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2783  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2784  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, normal "bumpalong" of one character happens (that is,
2785  (*SKIP) is ignored).  the (*SKIP) is ignored).
2786  .sp  .sp
2787    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2788  .sp  .sp
2789  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next innermost alternative if the rest of the
2790  not match. That is, it cancels pending backtracking, but only within the  pattern does not match. That is, it cancels pending backtracking, but only
2791  current alternation. Its name comes from the observation that it can be used  within the current alternative. Its name comes from the observation that it can
2792  for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2793  .sp  .sp
2794    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2795  .sp  .sp
2796  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2797  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2798  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2799  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2800  overall match fails. If (*THEN) is not directly inside an alternation, it acts  overall match fails. If (*THEN) is not inside an alternation, it acts like
2801  like (*PRUNE).  (*PRUNE).
2802    .P
2803    Note that a subpattern that does not contain a | character is just a part of
2804    the enclosing alternative; it is not a nested alternation with only one
2805    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2806    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2807    pattern fragments that do not contain any | characters at this level:
2808    .sp
2809      A (B(*THEN)C) | D
2810    .sp
2811    If A and B are matched, but there is a failure in C, matching does not
2812    backtrack into A; instead it moves to the next alternative, that is, D.
2813    However, if the subpattern containing (*THEN) is given an alternative, it
2814    behaves differently:
2815    .sp
2816      A (B(*THEN)C | (*FAIL)) | D
2817    .sp
2818    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2819    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2820    because there are no more alternatives to try. In this case, matching does now
2821    backtrack into A.
2822    .P
2823    Note also that a conditional subpattern is not considered as having two
2824    alternatives, because only one is ever used. In other words, the | character in
2825    a conditional subpattern has a different meaning. Ignoring white space,
2826    consider:
2827    .sp
2828      ^.*? (?(?=a) a | b(*THEN)c )
2829    .sp
2830    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2831    it initially matches zero characters. The condition (?=a) then fails, the
2832    character "b" is matched, but "c" is not. At this point, matching does not
2833    backtrack to .*? as might perhaps be expected from the presence of the |
2834    character. The conditional subpattern is part of the single alternative that
2835    comprises the whole pattern, and so the match fails. (If there was a backtrack
2836    into .*?, allowing it to match "b", the match would succeed.)
2837    .P
2838    The verbs just described provide four different "strengths" of control when
2839    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2840    next alternative. (*PRUNE) comes next, failing the match at the current
2841    starting position, but allowing an advance to the next character (for an
2842    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2843    than one character. (*COMMIT) is the strongest, causing the entire match to
2844    fail.
2845    .P
2846    If more than one such verb is present in a pattern, the "strongest" one wins.
2847    For example, consider this pattern, where A, B, etc. are complex pattern
2848    fragments:
2849    .sp
2850      (A(*COMMIT)B(*THEN)C|D)
2851    .sp
2852    Once A has matched, PCRE is committed to this match, at the current starting
2853    position. If subsequently B matches, but C does not, the normal (*THEN) action
2854    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2855    overrides.
2856  .  .
2857  .  .
2858  .SH "SEE ALSO"  .SH "SEE ALSO"
# Line 2666  Cambridge CB2 3QH, England. Line 2876  Cambridge CB2 3QH, England.
2876  .rs  .rs
2877  .sp  .sp
2878  .nf  .nf
2879  Last updated: 18 May 2010  Last updated: 19 November 2011
2880  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2881  .fi  .fi

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