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revision 208 by ph10, Mon Aug 6 15:23:29 2007 UTC revision 210 by ph10, Wed Aug 8 14:24:50 2007 UTC
# Line 1966  description of the interface to the call Line 1966  description of the interface to the call
1966  documentation.  documentation.
1967  .  .
1968  .  .
1970    .rs
1971    .sp
1972    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1973    are described in the Perl documentation as "experimental and subject to change
1974    or removal in a future version of Perl". It goes on to say: "Their usage in
1975    production code should be noted to avoid problems during upgrades." The same
1976    remarks apply to the PCRE features described in this section.
1977    .P
1978    Since these verbs are specifically related to backtracking, they can be used
1979    only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
1980    backtracking algorithm. They cause an error if encountered by
1981    \fBpcre_dfa_exec()\fP.
1982    .P
1983    The new verbs make use of what was previously invalid syntax: an opening
1984    parenthesis followed by an asterisk. In Perl, they are generally of the form
1985    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
1986    form is just (*VERB). Any number of these verbs may occur in a pattern. There
1987    are two kinds:
1988    .
1989    .SS "Verbs that act immediately"
1990    .rs
1991    .sp
1992    The following verbs act as soon as they are encountered:
1993    .sp
1994       (*ACCEPT)
1995    .sp
1996    This verb causes the match to end successfully, skipping the remainder of the
1997    pattern. When inside a recursion, only the innermost pattern is ended
1998    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
1999    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2000    captured. For example:
2001    .sp
2002      A(A|B(*ACCEPT)|C)D
2003    .sp
2004    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2005    captured.
2006    .sp
2007      (*FAIL) or (*F)
2008    .sp
2009    This verb causes the match to fail, forcing backtracking to occur. It is
2010    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2011    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2012    Perl features that are not present in PCRE. The nearest equivalent is the
2013    callout feature, as for example in this pattern:
2014    .sp
2015      a+(?C)(*FAIL)
2016    .sp
2017    A match with the string "aaaa" always fails, but the callout is taken before
2018    each backtrack happens (in this example, 10 times).
2019    .
2020    .SS "Verbs that act after backtracking"
2021    .rs
2022    .sp
2023    The following verbs do nothing when they are encountered. Matching continues
2024    with what follows, but if there is no subsequent match, a failure is forced.
2025    The verbs differ in exactly what kind of failure occurs.
2026    .sp
2027      (*COMMIT)
2028    .sp
2029    This verb causes the whole match to fail outright if the rest of the pattern
2030    does not match. Even if the pattern is unanchored, no further attempts to find
2031    a match by advancing the start point take place. Once (*COMMIT) has been
2032    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2033    starting point, or not at all. For example:
2034    .sp
2035      a+(*COMMIT)b
2036    .sp
2037    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2038    dynamic anchor, or "I've started, so I must finish."
2039    .sp
2040      (*PRUNE)
2041    .sp
2042    This verb causes the match to fail at the current position if the rest of the
2043    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2044    advance to the next starting character then happens. Backtracking can occur as
2045    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2046    if there is no match to the right, backtracking cannot cross (*PRUNE).
2047    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2048    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2049    be expressed in any other way.
2050    .sp
2051      (*SKIP)
2052    .sp
2053    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2054    "bumpalong" advance is not to the next character, but to the position in the
2055    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2056    was matched leading up to it cannot be part of a successful match. Consider:
2057    .sp
2058      a+(*SKIP)b
2059    .sp
2060    If the subject is "aaaac...", after the first match attempt fails (starting at
2061    the first character in the string), the starting point skips on to start the
2062    next attempt at "c". Note that a possessive quantifer does not have the same
2063    effect in this example; although it would suppress backtracking during the
2064    first match attempt, the second attempt would start at the second character
2065    instead of skipping on to "c".
2066    .sp
2067      (*THEN)
2068    .sp
2069    This verb causes a skip to the next alternation if the rest of the pattern does
2070    not match. That is, it cancels pending backtracking, but only within the
2071    current alternation. Its name comes from the observation that it can be used
2072    for a pattern-based if-then-else block:
2073    .sp
2074      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2075    .sp
2076    If the COND1 pattern matches, FOO is tried (and possibly further items after
2077    the end of the group if FOO succeeds); on failure the matcher skips to the
2078    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2079    is used outside of any alternation, it acts exactly like (*PRUNE).
2080    .
2081    .
2082  .SH "SEE ALSO"  .SH "SEE ALSO"
2083  .rs  .rs
2084  .sp  .sp
# Line 1986  Cambridge CB2 3QH, England. Line 2099  Cambridge CB2 3QH, England.
2099  .rs  .rs
2100  .sp  .sp
2101  .nf  .nf
2102  Last updated: 06 August 2007  Last updated: 08 August 2007
2103  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
2104  .fi  .fi

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