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revision 630 by ph10, Fri Jul 22 10:00:10 2011 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 220  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218  An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed  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  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  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  a character class, this causes an error, because the character class is not
222  terminated.  terminated.
# Line 246  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.
# Line 257  both byte mode and UTF-8 mode. (When PCR Line 253  both byte mode and UTF-8 mode. (When PCR
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 333  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 392  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 757  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  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.  an "extended grapheme cluster", which has a more complicated definition.
776  .P  .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
# Line 969  special meaning in a character class. Line 983  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 989  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 1320  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 1438  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 1829  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 2109  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 2207  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 2246  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 2283  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 2304  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 2373  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 2401  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 2490  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, with one exception: a *MARK that  to that subpattern; it does not extend to the surrounding pattern, with one
2570  is encountered in a positive assertion \fIis\fP passed back (compare capturing  exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2571  parentheses in assertions). Note that such subpatterns are processed as  back (compare capturing parentheses in assertions). Note that such subpatterns
2572  anchored at the point where they are tested.  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 2524  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 2535  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 2586  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  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  passed back if it is the last-encountered. This does not happen for negative
2668  assetions.  assertions.
2669  .P  .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
# Line 2702  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 2761  Cambridge CB2 3QH, England. Line 2874  Cambridge CB2 3QH, England.
2874  .rs  .rs
2875  .sp  .sp
2876  .nf  .nf
2877  Last updated: 22 July 2011  Last updated: 14 November 2011
2878  Copyright (c) 1997-2011 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2879  .fi  .fi

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