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revision 557 by ph10, Tue Oct 26 15:26:12 2010 UTC revision 745 by ph10, Mon Nov 14 11:41:03 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 182  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 192  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 211  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.  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 225  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 233  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 316  unrecognized escape sequences, they are Line 336  unrecognized escape sequences, they are
336  set. Outside a character class, these sequences have different meanings.  set. Outside a character class, these sequences have different meanings.
337  .  .
338  .  .
339    .SS "Unsupported escape sequences"
340    .rs
341    .sp
342    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
343    handler and used to modify the case of following characters. By default, PCRE
344    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
345    option is set, \eU matches a "U" character, and \eu can be used to define a
346    character by code point, as described in the previous section.
347    .
348    .
349  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
350  .rs  .rs
351  .sp  .sp
# Line 375  This is the same as Line 405  This is the same as
405  .\" </a>  .\" </a>
406  the "." metacharacter  the "." metacharacter
407  .\"  .\"
408  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
409    PCRE does not support this.
410  .P  .P
411  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
412  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 424  any Unicode letter, and underscore. Note Line 455  any Unicode letter, and underscore. Note
455  \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
456  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
457  .P  .P
458  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
459  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
460  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
461  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
462    are:
463  .sp  .sp
464    U+0009     Horizontal tab    U+0009     Horizontal tab
465    U+0020     Space    U+0020     Space
# Line 465  The vertical space characters are: Line 497  The vertical space characters are:
497  .rs  .rs
498  .sp  .sp
499  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
500  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:  
501  .sp  .sp
502    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
503  .sp  .sp
# Line 740  Characters with the "mark" property are Line 771  Characters with the "mark" property are
771  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
772  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
773  .P  .P
774    Note that recent versions of Perl have changed \eX to match what Unicode calls
775    an "extended grapheme cluster", which has a more complicated definition.
776    .P
777  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
778  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
779  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 774  same characters as Xan, plus underscore. Line 808  same characters as Xan, plus underscore.
808  .SS "Resetting the match start"  .SS "Resetting the match start"
809  .rs  .rs
810  .sp  .sp
811  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
812  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:  
813  .sp  .sp
814    foo\eKbar    foo\eKbar
815  .sp  .sp
# Line 948  The handling of dot is entirely independ Line 981  The handling of dot is entirely independ
981  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
982  special meaning in a character class.  special meaning in a character class.
983  .P  .P
984  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
985  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
986  end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
987    name; PCRE does not support this.
988  .  .
989  .  .
990  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
991  .rs  .rs
992  .sp  .sp
993  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
994  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
995  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
996  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
997  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
998  the \eC escape sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
999    character. This has undefined results, because PCRE assumes that it is dealing
1000    with valid UTF-8 strings (and by default it checks this at the start of
1001    processing unless the PCRE_NO_UTF8_CHECK option is used).
1002  .P  .P
1003  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1004  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 970  PCRE does not allow \eC to appear in loo Line 1007  PCRE does not allow \eC to appear in loo
1007  .\"  .\"
1008  because in UTF-8 mode this would make it impossible to calculate the length of  because in UTF-8 mode this would make it impossible to calculate the length of
1009  the lookbehind.  the lookbehind.
1010    .P
1011    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1012    way of using it that avoids the problem of malformed UTF-8 characters is to
1013    use a lookahead to check the length of the next character, as in this pattern
1014    (ignore white space and line breaks):
1015    .sp
1016      (?| (?=[\ex00-\ex7f])(\eC) |
1017          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1018          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1019          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1020    .sp
1021    A group that starts with (?| resets the capturing parentheses numbers in each
1022    alternative (see
1023    .\" HTML <a href="#dupsubpatternnumber">
1024    .\" </a>
1025    "Duplicate Subpattern Numbers"
1026    .\"
1027    below). The assertions at the start of each branch check the next UTF-8
1028    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1029    character's individual bytes are then captured by the appropriate number of
1030    groups.
1031  .  .
1032  .  .
1033  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 1045  characters in both cases. In UTF-8 mode, Line 1103  characters in both cases. In UTF-8 mode,
1103  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
1104  property support.  property support.
1105  .P  .P
1106  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,
1107  \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
1108  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1109  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
1110    and their upper case partners, just as it does when they appear outside a
1111    character class, as described in the section entitled
1112    .\" HTML <a href="#genericchartypes">
1113    .\" </a>
1114    "Generic character types"
1115    .\"
1116    above. The escape sequence \eb has a different meaning inside a character
1117    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1118    are not special inside a character class. Like any other unrecognized escape
1119    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1120    default, but cause an error if the PCRE_EXTRA option is set.
1121    .P
1122    A circumflex can conveniently be used with the upper case character types to
1123  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.
1124  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,
1125    whereas [\ew] includes underscore. A positive character class should be read as
1126    "something OR something OR ..." and a negative class as "NOT something AND NOT
1127    something AND NOT ...".
1128  .P  .P
1129  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1130  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 1173  extracts it into the global options (and Line 1247  extracts it into the global options (and
1247  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1248  .P  .P
1249  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1250  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
1251  .sp  .sp
1252    (a(?i)b)c    (a(?i)b)c
1253  .sp  .sp
# Line 1214  Turning part of a pattern into a subpatt Line 1288  Turning part of a pattern into a subpatt
1288  .sp  .sp
1289    cat(aract|erpillar|)    cat(aract|erpillar|)
1290  .sp  .sp
1291  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1292  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1293  .sp  .sp
1294  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
1295  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
1296  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
1297  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1298  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1299  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1300  .sp  .sp
1301    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1302  .sp  .sp
# Line 1272  at captured substring number one, whiche Line 1345  at captured substring number one, whiche
1345  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
1346  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1347  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
1348  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
1349  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1350  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.  
1351  .sp  .sp
1352    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1353    / ( 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 1287  or "defdef": Line 1359  or "defdef":
1359  .sp  .sp
1360    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1361  .sp  .sp
1362  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
1363  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
1364  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1365  .sp  .sp
1366    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1367  .sp  .sp
# Line 1402  items: Line 1474  items:
1474    the \eC escape sequence    the \eC escape sequence
1475    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1476    the \eR escape sequence    the \eR escape sequence
1477    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1478    a character class    a character class
1479    a back reference (see next section)    a back reference (see next section)
1480    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1481    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1482  .sp  .sp
1483  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1484  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1444  subpatterns that are referenced as Line 1516  subpatterns that are referenced as
1516  .\" </a>  .\" </a>
1517  subroutines  subroutines
1518  .\"  .\"
1519  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1520  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1521    .\" </a>
1522    "Defining subpatterns for use by reference only"
1523    .\"
1524    below). Items other than subpatterns that have a {0} quantifier are omitted
1525    from the compiled pattern.
1526  .P  .P
1527  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1528  abbreviations:  abbreviations:
# Line 1670  no such problem when named parentheses a Line 1747  no such problem when named parentheses a
1747  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1748  .P  .P
1749  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
1750  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
1751  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
1752  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1753  .sp  .sp
1754    (ring), \e1    (ring), \e1
1755    (ring), \eg1    (ring), \eg1
# Line 1686  example: Line 1763  example:
1763    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1764  .sp  .sp
1765  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
1766  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.
1767  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
1768  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
1769  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1770  .P  .P
1771  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1772  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1791  those that look ahead of the current pos Line 1868  those that look ahead of the current pos
1868  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,
1869  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.
1870  .P  .P
1871  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1872  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
1873  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1874  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1875  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1876  because it does not make sense for negative assertions.  .P
1877    For compatibility with Perl, assertion subpatterns may be repeated; though
1878    it makes no sense to assert the same thing several times, the side effect of
1879    capturing parentheses may occasionally be useful. In practice, there only three
1880    cases:
1881    .sp
1882    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1883    However, it may contain internal capturing parenthesized groups that are called
1884    from elsewhere via the
1885    .\" HTML <a href="#subpatternsassubroutines">
1886    .\" </a>
1887    subroutine mechanism.
1888    .\"
1889    .sp
1890    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1891    were {0,1}. At run time, the rest of the pattern match is tried with and
1892    without the assertion, the order depending on the greediness of the quantifier.
1893    .sp
1894    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1895    The assertion is obeyed just once when encountered during matching.
1896  .  .
1897  .  .
1898  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1825  lookbehind assertion is needed to achiev Line 1921  lookbehind assertion is needed to achiev
1921  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
1922  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
1923  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.
1924  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 (?!).  
1925  .  .
1926  .  .
1927  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1851  is permitted, but Line 1946  is permitted, but
1946  .sp  .sp
1947  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1948  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
1949  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
1950  match the same length of string. An assertion such as  length of string. An assertion such as
1951  .sp  .sp
1952    (?<=ab(c|de))    (?<=ab(c|de))
1953  .sp  .sp
# Line 1862  branches: Line 1957  branches:
1957  .sp  .sp
1958    (?<=abc|abde)    (?<=abc|abde)
1959  .sp  .sp
1960  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1961  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1962  .\" </a>  .\" </a>
1963  (see above)  (see above)
# Line 1967  already been matched. The two possible f Line 2062  already been matched. The two possible f
2062  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2063  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
2064  subpattern, a compile-time error occurs. Each of the two alternatives may  subpattern, a compile-time error occurs. Each of the two alternatives may
2065  itself contain nested subpatterns of any form, including conditional  itself contain nested subpatterns of any form, including conditional
2066  subpatterns; the restriction to two alternatives applies only at the level of  subpatterns; the restriction to two alternatives applies only at the level of
2067  the condition. This pattern fragment is an example where the alternatives are  the condition. This pattern fragment is an example where the alternatives are
2068  complex:  complex:
2069  .sp  .sp
2070    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
# Line 1990  matched. If there is more than one captu Line 2085  matched. If there is more than one captu
2085  .\" </a>  .\" </a>
2086  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2087  .\"  .\"
2088  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
2089  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
2090  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2091  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
2092  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
2093  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2094    zero in any of these forms is not used; it provokes a compile-time error.)
2095  .P  .P
2096  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2097  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 2006  three parts for ease of discussion: Line 2102  three parts for ease of discussion:
2102  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2103  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
2104  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
2105  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2106  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,
2107  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
2108  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2109  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 2063  The syntax for recursive patterns Line 2159  The syntax for recursive patterns
2159  .\"  .\"
2160  is described below.  is described below.
2161  .  .
2162    .\" HTML <a name="subdefine"></a>
2163  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2164  .rs  .rs
2165  .sp  .sp
# Line 2070  If the condition is the string (DEFINE), Line 2167  If the condition is the string (DEFINE),
2167  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
2168  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2169  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
2170  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2171  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2172  .\" </a>  .\" </a>
2173  "subroutines"  subroutines
2174  .\"  .\"
2175  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
2176  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2177    breaks):
2178  .sp  .sp
2179    (?(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) )
2180    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2111  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2209  dd-aaa-dd or dd-dd-dd, where aaa are let
2209  .SH COMMENTS  .SH COMMENTS
2210  .rs  .rs
2211  .sp  .sp
2212  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
2213  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,
2214  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
2215    subpattern name or number. The characters that make up a comment play no part
2216    in the pattern matching.
2217  .P  .P
2218  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
2219  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2220  next newline character or character sequence in the pattern. Which characters  option is set, an unescaped # character also introduces a comment, which in
2221  are interpreted as newlines is controlled by the options passed to  this case continues to immediately after the next newline character or
2222  \fBpcre_compile()\fP or by a special sequence at the start of the pattern, as  character sequence in the pattern. Which characters are interpreted as newlines
2223  described in the section entitled  is controlled by the options passed to \fBpcre_compile()\fP or by a special
2224  .\" HTML <a href="#recursion">  sequence at the start of the pattern, as described in the section entitled
2225    .\" HTML <a href="#newlines">
2226  .\" </a>  .\" </a>
2227  "Newline conventions"  "Newline conventions"
2228  .\"  .\"
2229  above. Note that end of a comment is a literal newline sequence in the pattern;  above. Note that the end of this type of comment is a literal newline sequence
2230  escape sequences that happen to represent a newline do not terminate a comment.  in the pattern; escape sequences that happen to represent a newline do not
2231  For example, consider this pattern when PCRE_EXTENDED is set, and the default  count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2232  newline convention is in force:  default newline convention is in force:
2233  .sp  .sp
2234    abc #comment \en still comment    abc #comment \en still comment
2235  .sp  .sp
2236  On encountering the # character, \fBpcre_compile()\fP skips along, looking for  On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2237  a newline in the pattern. The sequence \en is still literal at this stage, so  a newline in the pattern. The sequence \en is still literal at this stage, so
2238  it does not terminate the comment. Only an actual character with the code value  it does not terminate the comment. Only an actual character with the code value
2239  0x0a does so.  0x0a (the default newline) does so.
2240  .  .
2241  .  .
2242  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2164  individual subpattern recursion. After i Line 2265  individual subpattern recursion. After i
2265  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.
2266  .P  .P
2267  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
2268  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
2269  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
2270  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2271  .\" </a>  .\" </a>
2272  "subroutine"  non-recursive subroutine
2273  .\"  .\"
2274  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
2275  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2193  We have put the pattern into parentheses Line 2294  We have put the pattern into parentheses
2294  them instead of the whole pattern.  them instead of the whole pattern.
2295  .P  .P
2296  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
2297  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
2298  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
2299  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2300  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.  
2301  .P  .P
2302  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2303  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2304  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2305  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2306  .\" </a>  .\" </a>
2307  "subroutine"  non-recursive subroutine
2308  .\"  .\"
2309  calls, as described in the next section.  calls, as described in the next section.
2310  .P  .P
# Line 2241  documentation). If the pattern above is Line 2341  documentation). If the pattern above is
2341  .sp  .sp
2342  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
2343  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
2344  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
2345  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2346  .P  .P
2347  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
2348  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 2262  is the actual recursive call. Line 2362  is the actual recursive call.
2362  .  .
2363  .  .
2364  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2365  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2366  .rs  .rs
2367  .sp  .sp
2368  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
2369  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
2370  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
2371  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
2372  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2373  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2374    characters (for example, "a", "aba", "abcba", "abcdcba"):
2375  .sp  .sp
2376    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2377  .sp  .sp
# Line 2300  time we do have another alternative to t Line 2401  time we do have another alternative to t
2401  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2402  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2403  .P  .P
2404  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
2405  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
2406    this:
2407  .sp  .sp
2408    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2409  .sp  .sp
# Line 2330  For example, although "abcba" is correct Line 2432  For example, although "abcba" is correct
2432  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
2433  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
2434  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2435    .P
2436    The second way in which PCRE and Perl differ in their recursion processing is
2437    in the handling of captured values. In Perl, when a subpattern is called
2438    recursively or as a subpattern (see the next section), it has no access to any
2439    values that were captured outside the recursion, whereas in PCRE these values
2440    can be referenced. Consider this pattern:
2441    .sp
2442      ^(.)(\e1|a(?2))
2443    .sp
2444    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2445    then in the second group, when the back reference \e1 fails to match "b", the
2446    second alternative matches "a" and then recurses. In the recursion, \e1 does
2447    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2448    match because inside the recursive call \e1 cannot access the externally set
2449    value.
2450  .  .
2451  .  .
2452  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2453  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2454  .rs  .rs
2455  .sp  .sp
2456  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
2457  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
2458  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2459  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2460  relative, as in these examples:  relative, as in these examples:
2461  .sp  .sp
# Line 2358  matches "sense and sensibility" and "res Line 2475  matches "sense and sensibility" and "res
2475  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
2476  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2477  .P  .P
2478  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2479  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
2480  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
2481  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2482  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2483  .P  .P
2484  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2485  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
2486  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2487  .sp  .sp
2488    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2489  .sp  .sp
# Line 2447  a backtracking algorithm. With the excep Line 2564  a backtracking algorithm. With the excep
2564  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2565  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2566  .P  .P
2567  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
2568  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2569  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
2570  processed as anchored at the point where they are tested.  exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2571    back (compare capturing parentheses in assertions). Note that such subpatterns
2572    are processed as anchored at the point where they are tested. Note also that
2573    Perl's treatment of subroutines is different in some cases.
2574  .P  .P
2575  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
2576  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2577  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2578  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
2579  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,
2580  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
2581  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
2582    pattern.
2583  .P  .P
2584  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
2585  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 2466  minimum length of matching subject, or t Line 2587  minimum length of matching subject, or t
2587  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
2588  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2589  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
2590  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2591    pattern with (*NO_START_OPT).
2592  .  .
2593  .  .
2594  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2478  followed by a name. Line 2600  followed by a name.
2600     (*ACCEPT)     (*ACCEPT)
2601  .sp  .sp
2602  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
2603  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2604  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2605  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
2606    far is captured. For example:
2607  .sp  .sp
2608    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2609  .sp  .sp
# Line 2489  the outer parentheses. Line 2612  the outer parentheses.
2612  .sp  .sp
2613    (*FAIL) or (*F)    (*FAIL) or (*F)
2614  .sp  .sp
2615  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
2616  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
2617  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2618  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 2540  indicates which of the two alternatives Line 2663  indicates which of the two alternatives
2663  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2664  capturing parentheses.  capturing parentheses.
2665  .P  .P
2666    If (*MARK) is encountered in a positive assertion, its name is recorded and
2667    passed back if it is the last-encountered. This does not happen for negative
2668    assertions.
2669    .P
2670  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
2671  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2672  (*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 2652  following pattern fails to match, the pr Line 2779  following pattern fails to match, the pr
2779  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,
2780  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2781  (*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
2782  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, normal "bumpalong" of one character happens (that is,
2783  (*SKIP) is ignored).  the (*SKIP) is ignored).
2784  .sp  .sp
2785    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2786  .sp  .sp
2787  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next innermost alternative if the rest of the
2788  group if the rest of the pattern does not match. That is, it cancels pending  pattern does not match. That is, it cancels pending backtracking, but only
2789  backtracking, but only within the current alternation. Its name comes from the  within the current alternative. Its name comes from the observation that it can
2790  observation that it can be used for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2791  .sp  .sp
2792    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2793  .sp  .sp
2794  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
2795  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
2796  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2797  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
2798  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
2799  like (*PRUNE).  (*PRUNE).
 .  
 .P  
 The above verbs provide four different "strengths" of control when subsequent  
 matching fails. (*THEN) is the weakest, carrying on the match at the next  
 alternation. (*PRUNE) comes next, failing the match at the current starting  
 position, but allowing an advance to the next character (for an unanchored  
 pattern). (*SKIP) is similar, except that the advance may be more than one  
 character. (*COMMIT) is the strongest, causing the entire match to fail.  
2800  .P  .P
2801  If more than one is present in a pattern, the "stongest" one wins. For example,  Note that a subpattern that does not contain a | character is just a part of
2802  consider this pattern, where A, B, etc. are complex pattern fragments:  the enclosing alternative; it is not a nested alternation with only one
2803    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2804    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2805    pattern fragments that do not contain any | characters at this level:
2806    .sp
2807      A (B(*THEN)C) | D
2808    .sp
2809    If A and B are matched, but there is a failure in C, matching does not
2810    backtrack into A; instead it moves to the next alternative, that is, D.
2811    However, if the subpattern containing (*THEN) is given an alternative, it
2812    behaves differently:
2813    .sp
2814      A (B(*THEN)C | (*FAIL)) | D
2815    .sp
2816    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2817    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2818    because there are no more alternatives to try. In this case, matching does now
2819    backtrack into A.
2820    .P
2821    Note also that a conditional subpattern is not considered as having two
2822    alternatives, because only one is ever used. In other words, the | character in
2823    a conditional subpattern has a different meaning. Ignoring white space,
2824    consider:
2825    .sp
2826      ^.*? (?(?=a) a | b(*THEN)c )
2827    .sp
2828    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2829    it initially matches zero characters. The condition (?=a) then fails, the
2830    character "b" is matched, but "c" is not. At this point, matching does not
2831    backtrack to .*? as might perhaps be expected from the presence of the |
2832    character. The conditional subpattern is part of the single alternative that
2833    comprises the whole pattern, and so the match fails. (If there was a backtrack
2834    into .*?, allowing it to match "b", the match would succeed.)
2835    .P
2836    The verbs just described provide four different "strengths" of control when
2837    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2838    next alternative. (*PRUNE) comes next, failing the match at the current
2839    starting position, but allowing an advance to the next character (for an
2840    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2841    than one character. (*COMMIT) is the strongest, causing the entire match to
2842    fail.
2843    .P
2844    If more than one such verb is present in a pattern, the "strongest" one wins.
2845    For example, consider this pattern, where A, B, etc. are complex pattern
2846    fragments:
2847  .sp  .sp
2848    (A(*COMMIT)B(*THEN)C|D)    (A(*COMMIT)B(*THEN)C|D)
2849  .sp  .sp
2850  Once A has matched, PCRE is committed to this match, at the current starting  Once A has matched, PCRE is committed to this match, at the current starting
2851  position. If subsequently B matches, but C does not, the normal (*THEN) action  position. If subsequently B matches, but C does not, the normal (*THEN) action
2852  of trying the next alternation (that is, D) does not happen because (*COMMIT)  of trying the next alternative (that is, D) does not happen because (*COMMIT)
2853  overrides.  overrides.
2854  .  .
2855  .  .
# Line 2711  Cambridge CB2 3QH, England. Line 2874  Cambridge CB2 3QH, England.
2874  .rs  .rs
2875  .sp  .sp
2876  .nf  .nf
2877  Last updated: 26 October 2010  Last updated: 14 November 2011
2878  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2879  .fi  .fi

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