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revision 574 by ph10, Sat Nov 20 17:47:27 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 184  The following sections describe the use Line 184  The following sections describe the use
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  character that is not a number or a letter, it takes away any special meaning  character that is not a number or a letter, it takes away any special meaning
186  that character may have. This use of backslash as an escape character applies  that character may have. This use of backslash as an escape character applies
187  both inside and 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 193  non-alphanumeric with backslash to speci Line 193  non-alphanumeric with backslash to speci
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  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  backslash. All other characters (in particular, those whose codepoints are
197  greater than 127) are treated as literals.  greater than 127) are treated as literals.
198  .P  .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
# Line 215  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 237  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 (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
250  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251  than 127, a compile-time error occurs. This locks out non-ASCII characters in  than 127, a compile-time error occurs. This locks out non-ASCII characters in
252  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253  values are valid. A lower case letter is converted to upper case, and then the  values are valid. A lower case letter is converted to upper case, and then the
254  0xc0 bits are flipped.)  0xc0 bits are flipped.)
255  .P  .P
256  After \ex, from zero to two hexadecimal digits are read (letters can be in  By default, after \ex, from zero to two hexadecimal digits are read (letters
257  upper or lower case). Any number of hexadecimal digits may appear between \ex{  can be in upper or lower case). Any number of hexadecimal digits may appear
258  and }, but the value of the character code must be less than 256 in non-UTF-8  between \ex{ and }, but the value of the character code must be less than 256
259  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
260  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
261  point, which is 10FFFF.  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 324  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 383  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 432  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 features that were added to Perl at  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
459  release 5.10. In contrast to the other sequences, which match only ASCII  release 5.10. In contrast to the other sequences, which match only ASCII
460  characters by default, these always match certain high-valued codepoints in  characters by default, these always match certain high-valued codepoints in
461  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
# Line 748  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 955  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 behaves like a dot, except that it is not affected by  The escape sequence \eN behaves like a dot, except that it is not affected by
985  the PCRE_DOTALL option. In other words, it matches any character except one  the PCRE_DOTALL option. In other words, it matches any character except one
986  that signifies the 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, the  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
997  rest of the string may start with a malformed UTF-8 character. 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 977  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 1052  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 1292  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 1410  items: Line 1477  items:
1477    an escape such as \ed or \epL 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 1801  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 1976  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 2003  the condition is true if any of them hav Line 2089  the condition is true if any of them hav
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. Inside  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2092  loops it can also make sense to refer to subsequent groups. The next  loops it can also make sense to refer to subsequent groups. The next
2093  parentheses to be opened can be referenced as (?(+1), and so on. (The value  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.)  zero in any of these forms is not used; it provokes a compile-time error.)
2095  .P  .P
# Line 2081  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 such as  is described below.) For example, a pattern to match an IPv4 address such as
2176  "192.168.23.245" could be written like this (ignore whitespace and line  "192.168.23.245" could be written like this (ignore whitespace and line
# Line 2123  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  There are two ways of including comments in patterns that are processed by  There are two ways of including comments in patterns that are processed by
2213  PCRE. In both cases, the start of the comment must not be in a character class,  PCRE. In both cases, the start of the comment must not be in a character class,
2214  nor in the middle of any other sequence of related characters such as (?: or a  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  subpattern name or number. The characters that make up a comment play no part
# Line 2147  default newline convention is in force: Line 2233  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 (the default newline) does so.  0x0a (the default newline) does so.
# Line 2179  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 2218  references such as (?+2). However, these Line 2304  references such as (?+2). However, these
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 2255  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 2276  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 2345  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 2373  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 2462  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 2481  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 2493  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 2504  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 2555  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 2667  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 2726  Cambridge CB2 3QH, England. Line 2874  Cambridge CB2 3QH, England.
2874  .rs  .rs
2875  .sp  .sp
2876  .nf  .nf
2877  Last updated: 20 November 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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