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revision 628 by ph10, Wed Jul 20 18:03:20 2011 UTC revision 788 by ph10, Tue Dec 6 15:38:01 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 325  Note that octal values of 100 or greater Line 328  Note that octal values of 100 or greater
328  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
329  .P  .P
330  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
331  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
332  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
333  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
334  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
335  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
336  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
337    error if the PCRE_EXTRA option is set. Outside a character class, these
338    sequences have different meanings.
339    .
340    .
341    .SS "Unsupported escape sequences"
342    .rs
343    .sp
344    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
345    handler and used to modify the case of following characters. By default, PCRE
346    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
347    option is set, \eU matches a "U" character, and \eu can be used to define a
348    character by code point, as described in the previous section.
349  .  .
350  .  .
351  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 392  This is the same as Line 407  This is the same as
407  .\" </a>  .\" </a>
408  the "." metacharacter  the "." metacharacter
409  .\"  .\"
410  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
411    PCRE does not support this.
412  .P  .P
413  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
414  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 757  Characters with the "mark" property are Line 773  Characters with the "mark" property are
773  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
774  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
775  .P  .P
776  Note that recent versions of Perl have changed \eX to match what Unicode calls  Note that recent versions of Perl have changed \eX to match what Unicode calls
777  an "extended grapheme cluster", which has a more complicated definition.  an "extended grapheme cluster", which has a more complicated definition.
778  .P  .P
779  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
# Line 969  special meaning in a character class. Line 985  special meaning in a character class.
985  .P  .P
986  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
987  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
988  that signifies the end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
989    name; PCRE does not support this.
990  .  .
991  .  .
992  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
993  .rs  .rs
994  .sp  .sp
995  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
996  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
997  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
998  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
999  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
1000  the \eC escape sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
1001    character. This has undefined results, because PCRE assumes that it is dealing
1002    with valid UTF-8 strings (and by default it checks this at the start of
1003    processing unless the PCRE_NO_UTF8_CHECK option is used).
1004  .P  .P
1005  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1006  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1007  .\" </a>  .\" </a>
1008  (described below),  (described below)
1009  .\"  .\"
1010  because in UTF-8 mode this would make it impossible to calculate the length of  in UTF-8 mode, because this would make it impossible to calculate the length of
1011  the lookbehind.  the lookbehind.
1012    .P
1013    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1014    way of using it that avoids the problem of malformed UTF-8 characters is to
1015    use a lookahead to check the length of the next character, as in this pattern
1016    (ignore white space and line breaks):
1017    .sp
1018      (?| (?=[\ex00-\ex7f])(\eC) |
1019          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1020          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1021          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1022    .sp
1023    A group that starts with (?| resets the capturing parentheses numbers in each
1024    alternative (see
1025    .\" HTML <a href="#dupsubpatternnumber">
1026    .\" </a>
1027    "Duplicate Subpattern Numbers"
1028    .\"
1029    below). The assertions at the start of each branch check the next UTF-8
1030    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1031    character's individual bytes are then captured by the appropriate number of
1032    groups.
1033  .  .
1034  .  .
1035  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 1320  or "defdef": Line 1361  or "defdef":
1361  .sp  .sp
1362    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1363  .sp  .sp
1364  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1365  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1366  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1367  .sp  .sp
1368    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1369  .sp  .sp
# Line 1438  items: Line 1479  items:
1479    an escape such as \ed or \epL that matches a single character    an escape such as \ed or \epL that matches a single character
1480    a character class    a character class
1481    a back reference (see next section)    a back reference (see next section)
1482    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1483    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1484  .sp  .sp
1485  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1486  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1829  those that look ahead of the current pos Line 1870  those that look ahead of the current pos
1870  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1871  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1872  .P  .P
1873  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1874  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1875  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1876  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1877  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1878  because it does not make sense for negative assertions.  .P
1879    For compatibility with Perl, assertion subpatterns may be repeated; though
1880    it makes no sense to assert the same thing several times, the side effect of
1881    capturing parentheses may occasionally be useful. In practice, there only three
1882    cases:
1883    .sp
1884    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1885    However, it may contain internal capturing parenthesized groups that are called
1886    from elsewhere via the
1887    .\" HTML <a href="#subpatternsassubroutines">
1888    .\" </a>
1889    subroutine mechanism.
1890    .\"
1891    .sp
1892    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1893    were {0,1}. At run time, the rest of the pattern match is tried with and
1894    without the assertion, the order depending on the greediness of the quantifier.
1895    .sp
1896    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1897    The assertion is obeyed just once when encountered during matching.
1898  .  .
1899  .  .
1900  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1912  temporarily move the current position ba Line 1972  temporarily move the current position ba
1972  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1973  assertion fails.  assertion fails.
1974  .P  .P
1975  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,
1976  to appear in lookbehind assertions, because it makes it impossible to calculate  even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
1977  the length of the lookbehind. The \eX and \eR escapes, which can match  impossible to calculate the length of the lookbehind. The \eX and \eR escapes,
1978  different numbers of bytes, are also not permitted.  which can match different numbers of bytes, are also not permitted.
1979  .P  .P
1980  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
1981  .\" </a>  .\" </a>
# Line 2109  If the condition is the string (DEFINE), Line 2169  If the condition is the string (DEFINE),
2169  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2170  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2171  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2172  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2173  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2174  .\" </a>  .\" </a>
2175  "subroutines"  subroutines
2176  .\"  .\"
2177  is described below.) For example, a pattern to match an IPv4 address such as  is described below.) For example, a pattern to match an IPv4 address such as
2178  "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 2267  individual subpattern recursion. After i
2267  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2268  .P  .P
2269  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2270  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2271  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2272  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2273  .\" </a>  .\" </a>
2274  "subroutine"  non-recursive subroutine
2275  .\"  .\"
2276  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2277  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2246  references such as (?+2). However, these Line 2306  references such as (?+2). However, these
2306  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2307  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2308  .\" </a>  .\" </a>
2309  "subroutine"  non-recursive subroutine
2310  .\"  .\"
2311  calls, as described in the next section.  calls, as described in the next section.
2312  .P  .P
# Line 2283  documentation). If the pattern above is Line 2343  documentation). If the pattern above is
2343  .sp  .sp
2344  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2345  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2346  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2347  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2348  .P  .P
2349  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2350  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2304  is the actual recursive call. Line 2364  is the actual recursive call.
2364  .  .
2365  .  .
2366  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2367  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2368  .rs  .rs
2369  .sp  .sp
2370  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2371  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2372  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2373  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2374  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2375  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2376    characters (for example, "a", "aba", "abcba", "abcdcba"):
2377  .sp  .sp
2378    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2379  .sp  .sp
# Line 2373  For example, although "abcba" is correct Line 2434  For example, although "abcba" is correct
2434  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2435  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2436  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2437    .P
2438    The second way in which PCRE and Perl differ in their recursion processing is
2439    in the handling of captured values. In Perl, when a subpattern is called
2440    recursively or as a subpattern (see the next section), it has no access to any
2441    values that were captured outside the recursion, whereas in PCRE these values
2442    can be referenced. Consider this pattern:
2443    .sp
2444      ^(.)(\e1|a(?2))
2445    .sp
2446    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2447    then in the second group, when the back reference \e1 fails to match "b", the
2448    second alternative matches "a" and then recurses. In the recursion, \e1 does
2449    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2450    match because inside the recursive call \e1 cannot access the externally set
2451    value.
2452  .  .
2453  .  .
2454  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2455  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2456  .rs  .rs
2457  .sp  .sp
2458  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2459  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2460  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2461  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2462  relative, as in these examples:  relative, as in these examples:
2463  .sp  .sp
# Line 2401  matches "sense and sensibility" and "res Line 2477  matches "sense and sensibility" and "res
2477  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2478  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2479  .P  .P
2480  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2481  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2482  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2483  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2484  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2485  .P  .P
2486  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2487  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2488  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2489  .sp  .sp
2490    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2491  .sp  .sp
# Line 2490  a backtracking algorithm. With the excep Line 2566  a backtracking algorithm. With the excep
2566  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2567  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2568  .P  .P
2569  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2570  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2571  it does not extend to the surrounding pattern. Note that such subpatterns are  to that subpattern; it does not extend to the surrounding pattern, with one
2572  processed as anchored at the point where they are tested.  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2573    a successful positive assertion \fIis\fP passed back when a match succeeds
2574    (compare capturing parentheses in assertions). Note that such subpatterns are
2575    processed as anchored at the point where they are tested. Note also that Perl's
2576    treatment of subroutines is different in some cases.
2577  .P  .P
2578  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
2579  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2580  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2581  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
2582  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,
2583  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
2584  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
2585    pattern.
2586  .P  .P
2587  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
2588  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 2511  included backtracking verbs will not, of Line 2592  included backtracking verbs will not, of
2592  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
2593  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2594  pattern with (*NO_START_OPT).  pattern with (*NO_START_OPT).
2595    .P
2596    Experiments with Perl suggest that it too has similar optimizations, sometimes
2597    leading to anomalous results.
2598  .  .
2599  .  .
2600  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2522  followed by a name. Line 2606  followed by a name.
2606     (*ACCEPT)     (*ACCEPT)
2607  .sp  .sp
2608  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
2609  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2610  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2611  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
2612    far is captured. For example:
2613  .sp  .sp
2614    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2615  .sp  .sp
# Line 2533  the outer parentheses. Line 2618  the outer parentheses.
2618  .sp  .sp
2619    (*FAIL) or (*F)    (*FAIL) or (*F)
2620  .sp  .sp
2621  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
2622  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
2623  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2624  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 2557  starting point (see (*SKIP) below). Line 2642  starting point (see (*SKIP) below).
2642  A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
2643  (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
2644  .P  .P
2645  When a match succeeds, the name of the last-encountered (*MARK) is passed back  When a match succeeds, the name of the last-encountered (*MARK) on the matching
2646  to the caller via the \fIpcre_extra\fP data structure, as described in the  path is passed back to the caller via the \fIpcre_extra\fP data structure, as
2647    described in the
2648  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2649  .\" </a>  .\" </a>
2650  section on \fIpcre_extra\fP  section on \fIpcre_extra\fP
# Line 2567  in the Line 2653  in the
2653  .\" HREF  .\" HREF
2654  \fBpcreapi\fP  \fBpcreapi\fP
2655  .\"  .\"
2656  documentation. No data is returned for a partial match. Here is an example of  documentation. Here is an example of \fBpcretest\fP output, where the /K
2657  \fBpcretest\fP output, where the /K modifier requests the retrieval and  modifier requests the retrieval and outputting of (*MARK) data:
 outputting of (*MARK) data:  
2658  .sp  .sp
2659    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2660    XY    data> XY
2661     0: XY     0: XY
2662    MK: A    MK: A
2663    XZ    XZ
# Line 2584  indicates which of the two alternatives Line 2669  indicates which of the two alternatives
2669  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2670  capturing parentheses.  capturing parentheses.
2671  .P  .P
2672  A name may also be returned after a failed match if the final path through the  If (*MARK) is encountered in a positive assertion, its name is recorded and
2673  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  passed back if it is the last-encountered. This does not happen for negative
2674  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  assertions.
2675  starting point for matching is advanced, the final check is often with an empty  .P
2676  string, causing a failure before (*MARK) is reached. For example:  After a partial match or a failed match, the name of the last encountered
2677  .sp  (*MARK) in the entire match process is returned. For example:
   /X(*MARK:A)Y|X(*MARK:B)Z/K  
   XP  
   No match  
 .sp  
 There are three potential starting points for this match (starting with X,  
 starting with P, and with an empty string). If the pattern is anchored, the  
 result is different:  
2678  .sp  .sp
2679    /^X(*MARK:A)Y|^X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2680    XP    data> XP
2681    No match, mark = B    No match, mark = B
2682  .sp  .sp
2683  PCRE's start-of-match optimizations can also interfere with this. For example,  Note that in this unanchored example the mark is retained from the match
2684  if, as a result of a call to \fBpcre_study()\fP, it knows the minimum  attempt that started at the letter "X". Subsequent match attempts starting at
2685  subject length for a match, a shorter subject will not be scanned at all.  "P" and then with an empty string do not get as far as the (*MARK) item, but
2686  .P  nevertheless do not reset it.
 Note that similar anomalies (though different in detail) exist in Perl, no  
 doubt for the same reasons. The use of (*MARK) data after a failed match of an  
 unanchored pattern is not recommended, unless (*COMMIT) is involved.  
2687  .  .
2688  .  .
2689  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2645  Note that (*COMMIT) at the start of a pa Line 2720  Note that (*COMMIT) at the start of a pa
2720  unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
2721  \fBpcretest\fP example:  \fBpcretest\fP example:
2722  .sp  .sp
2723    /(*COMMIT)abc/      re> /(*COMMIT)abc/
2724    xyzabc    data> xyzabc
2725     0: abc     0: abc
2726    xyzabc\eY    xyzabc\eY
2727    No match    No match
# Line 2667  reached, or when matching to the right o Line 2742  reached, or when matching to the right o
2742  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2743  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2744  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2745  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2746  match fails completely; the name is passed back if this is the final attempt.  anchored pattern (*PRUNE) has the same effect as (*COMMIT).
 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored  
 pattern (*PRUNE) has the same effect as (*COMMIT).  
2747  .sp  .sp
2748    (*SKIP)    (*SKIP)
2749  .sp  .sp
# Line 2696  following pattern fails to match, the pr Line 2769  following pattern fails to match, the pr
2769  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,
2770  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2771  (*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
2772  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, the (*SKIP) is ignored.
 (*SKIP) is ignored).  
2773  .sp  .sp
2774    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2775  .sp  .sp
2776  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
2777  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
2778  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
2779  observation that it can be used for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2780  .sp  .sp
2781    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2782  .sp  .sp
2783  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
2784  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
2785  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2786  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).
2787  overall match fails. If (*THEN) is not directly inside an alternation, it acts  If (*THEN) is not inside an alternation, it acts like (*PRUNE).
 like (*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.  
2788  .P  .P
2789  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
2790  consider this pattern, where A, B, etc. are complex pattern fragments:  the enclosing alternative; it is not a nested alternation with only one
2791    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2792    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2793    pattern fragments that do not contain any | characters at this level:
2794    .sp
2795      A (B(*THEN)C) | D
2796    .sp
2797    If A and B are matched, but there is a failure in C, matching does not
2798    backtrack into A; instead it moves to the next alternative, that is, D.
2799    However, if the subpattern containing (*THEN) is given an alternative, it
2800    behaves differently:
2801    .sp
2802      A (B(*THEN)C | (*FAIL)) | D
2803    .sp
2804    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2805    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2806    because there are no more alternatives to try. In this case, matching does now
2807    backtrack into A.
2808    .P
2809    Note also that a conditional subpattern is not considered as having two
2810    alternatives, because only one is ever used. In other words, the | character in
2811    a conditional subpattern has a different meaning. Ignoring white space,
2812    consider:
2813    .sp
2814      ^.*? (?(?=a) a | b(*THEN)c )
2815    .sp
2816    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2817    it initially matches zero characters. The condition (?=a) then fails, the
2818    character "b" is matched, but "c" is not. At this point, matching does not
2819    backtrack to .*? as might perhaps be expected from the presence of the |
2820    character. The conditional subpattern is part of the single alternative that
2821    comprises the whole pattern, and so the match fails. (If there was a backtrack
2822    into .*?, allowing it to match "b", the match would succeed.)
2823    .P
2824    The verbs just described provide four different "strengths" of control when
2825    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2826    next alternative. (*PRUNE) comes next, failing the match at the current
2827    starting position, but allowing an advance to the next character (for an
2828    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2829    than one character. (*COMMIT) is the strongest, causing the entire match to
2830    fail.
2831    .P
2832    If more than one such verb is present in a pattern, the "strongest" one wins.
2833    For example, consider this pattern, where A, B, etc. are complex pattern
2834    fragments:
2835  .sp  .sp
2836    (A(*COMMIT)B(*THEN)C|D)    (A(*COMMIT)B(*THEN)C|D)
2837  .sp  .sp
2838  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
2839  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
2840  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)
2841  overrides.  overrides.
2842  .  .
2843  .  .
# Line 2755  Cambridge CB2 3QH, England. Line 2862  Cambridge CB2 3QH, England.
2862  .rs  .rs
2863  .sp  .sp
2864  .nf  .nf
2865  Last updated: 20 July 2011  Last updated: 29 November 2011
2866  Copyright (c) 1997-2011 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2867  .fi  .fi

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