/[pcre]/code/trunk/doc/pcrepattern.3
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revision 509 by ph10, Wed Mar 10 16:08:01 2010 UTC revision 510 by ph10, Sat Mar 27 17:45:29 2010 UTC
# Line 2318  description of the interface to the call Line 2318  description of the interface to the call
2318  documentation.  documentation.
2319  .  .
2320  .  .
2321    .\" HTML <a name="backtrackcontrol"></a>
2322  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2323  .rs  .rs
2324  .sp  .sp
# Line 2339  it does not extend to the surrounding pa Line 2340  it does not extend to the surrounding pa
2340  processed as anchored at the point where they are tested.  processed as anchored at the point where they are tested.
2341  .P  .P
2342  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
2343  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2344  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2345  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. An name is a sequence of
2346  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2347    parenthesis immediately follows the colon, the effect is as if the colon were
2348    not there. Any number of these verbs may occur in a pattern.
2349    .P
2350    PCRE contains some optimizations that are used to speed up matching by running
2351    some checks at the start of each match attempt. For example, it may know the
2352    minimum length of matching subject, or that a particular character must be
2353    present. When one of these optimizations suppresses the running of a match, any
2354    included backtracking verbs will not, of course, be processed. You can suppress
2355    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2356    when calling \fBpcre_exec()\fP.
2357    .
2358  .  .
2359  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2360  .rs  .rs
2361  .sp  .sp
2362  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2363    followed by a name.
2364  .sp  .sp
2365     (*ACCEPT)     (*ACCEPT)
2366  .sp  .sp
# Line 2374  callout feature, as for example in this Line 2387  callout feature, as for example in this
2387  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2388  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2389  .  .
2390    .
2391    .SS "Recording which path was taken"
2392    .rs
2393    .sp
2394    There is one verb whose main purpose is to track how a match was arrived at,
2395    though it also has a secondary use in conjunction with advancing the match
2396    starting point (see (*SKIP) below).
2397    .sp
2398      (*MARK:NAME) or (*:NAME)
2399    .sp
2400    A name is always required with this verb. There may be as many instances of
2401    (*MARK) as you like in a pattern, and their names do not have to be unique.
2402    .P
2403    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2404    to the caller via the \fIpcre_extra\fP data structure, as described in the
2405    .\" HTML <a href="pcreapi.html#extradata">
2406    .\" </a>
2407    section on \fIpcre_extra\fP
2408    .\"
2409    in the
2410    .\" HREF
2411    \fBpcreapi\fP
2412    .\"
2413    documentation. No data is returned for a partial match. Here is an example of
2414    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2415    outputting of (*MARK) data:
2416    .sp
2417      /X(*MARK:A)Y|X(*MARK:B)Z/K
2418      XY
2419       0: XY
2420      MK: A
2421      XZ
2422       0: XZ
2423      MK: B
2424    .sp
2425    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2426    indicates which of the two alternatives matched. This is a more efficient way
2427    of obtaining this information than putting each alternative in its own
2428    capturing parentheses.
2429    .P
2430    A name may also be returned after a failed match if the final path through the
2431    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2432    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2433    starting point for matching is advanced, the final check is often with an empty
2434    string, causing a failure before (*MARK) is reached. For example:
2435    .sp
2436      /X(*MARK:A)Y|X(*MARK:B)Z/K
2437      XP
2438      No match
2439    .sp
2440    There are three potential starting points for this match (starting with X,
2441    starting with P, and with an empty string). If the pattern is anchored, the
2442    result is different:
2443    .sp
2444      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2445      XP
2446      No match, mark = B
2447    .sp
2448    PCRE's start-of-match optimizations can also interfere with this. For example,
2449    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2450    subject length for a match, a shorter subject will not be scanned at all.
2451    .P
2452    Note that similar anomalies (though different in detail) exist in Perl, no
2453    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2454    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2455    .
2456    .
2457  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2458  .rs  .rs
2459  .sp  .sp
2460  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2461  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2462  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2463    the verb. However, when one of these verbs appears inside an atomic group, its
2464    effect is confined to that group, because once the group has been matched,
2465    there is never any backtracking into it. In this situation, backtracking can
2466    "jump back" to the left of the entire atomic group. (Remember also, as stated
2467    above, that this localization also applies in subroutine calls and assertions.)
2468    .P
2469    These verbs differ in exactly what kind of failure occurs when backtracking
2470    reaches them.
2471  .sp  .sp
2472    (*COMMIT)    (*COMMIT)
2473  .sp  .sp
2474  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2475  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2476  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2477  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2478  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2479  .sp  .sp
2480    a+(*COMMIT)b    a+(*COMMIT)b
2481  .sp  .sp
2482  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2483  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2484  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2485    (*PRUNE)  match failure.
2486  .sp  .P
2487  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2488  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2489  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2490  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2491  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2492  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2493  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2494  be expressed in any other way.    xyzabc\eY
2495      No match
2496    .sp
2497    PCRE knows that any match must start with "a", so the optimization skips along
2498    the subject to "a" before running the first match attempt, which succeeds. When
2499    the optimization is disabled by the \eY escape in the second subject, the match
2500    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2501    starting points.
2502    .sp
2503      (*PRUNE) or (*PRUNE:NAME)
2504    .sp
2505    This verb causes the match to fail at the current starting position in the
2506    subject if the rest of the pattern does not match. If the pattern is
2507    unanchored, the normal "bumpalong" advance to the next starting character then
2508    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2509    reached, or when matching to the right of (*PRUNE), but if there is no match to
2510    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2511    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2512    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2513    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2514    match fails completely; the name is passed back if this is the final attempt.
2515    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2516    pattern (*PRUNE) has the same effect as (*COMMIT).
2517  .sp  .sp
2518    (*SKIP)    (*SKIP)
2519  .sp  .sp
2520  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2521  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2522  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2523  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2524    successful match. Consider:
2525  .sp  .sp
2526    a+(*SKIP)b    a+(*SKIP)b
2527  .sp  .sp
# Line 2421  effect as this example; although it woul Line 2532  effect as this example; although it woul
2532  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2533  instead of skipping on to "c".  instead of skipping on to "c".
2534  .sp  .sp
2535    (*THEN)    (*SKIP:NAME)
2536    .sp
2537    When (*SKIP) has an associated name, its behaviour is modified. If the
2538    following pattern fails to match, the previous path through the pattern is
2539    searched for the most recent (*MARK) that has the same name. If one is found,
2540    the "bumpalong" advance is to the subject position that corresponds to that
2541    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2542    matching name is found, normal "bumpalong" of one character happens (the
2543    (*SKIP) is ignored).
2544    .sp
2545      (*THEN) or (*THEN:NAME)
2546  .sp  .sp
2547  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next alternation if the rest of the pattern does
2548  not match. That is, it cancels pending backtracking, but only within the  not match. That is, it cancels pending backtracking, but only within the
# Line 2432  for a pattern-based if-then-else block: Line 2553  for a pattern-based if-then-else block:
2553  .sp  .sp
2554  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
2555  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
2556  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2557  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2558    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2559    like (*PRUNE).
2560  .  .
2561  .  .
2562  .SH "SEE ALSO"  .SH "SEE ALSO"
# Line 2457  Cambridge CB2 3QH, England. Line 2580  Cambridge CB2 3QH, England.
2580  .rs  .rs
2581  .sp  .sp
2582  .nf  .nf
2583  Last updated: 06 March 2010  Last updated: 27 March 2010
2584  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2585  .fi  .fi

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