In normal use of PCRE, if the subject string that is passed to -pcre_exec() matches as far as it goes, but is too short to match the -entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where -it might be helpful to distinguish this case from other cases in which there is -no match. +pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is +too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There +are circumstances where it might be helpful to distinguish this case from other +cases in which there is no match.
Consider, for example, an application where a human is required to type in data @@ -41,10 +42,20 @@
PCRE supports the concept of partial matching by means of the PCRE_PARTIAL -option, which can be set when calling pcre_exec(). When this is done, the -return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any -time during the matching process the entire subject string matched part of the -pattern. No captured data is set when this occurs. +option, which can be set when calling pcre_exec() or +pcre_dfa_exec(). When this flag is set for pcre_exec(), the return +code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time +during the matching process the last part of the subject string matched part of +the pattern. Unfortunately, for non-anchored matching, it is not possible to +obtain the position of the start of the partial match. No captured data is set +when PCRE_ERROR_PARTIAL is returned. +
++When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code +PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the +subject is reached, there have been no complete matches, but there is still at +least one matching possibility. The portion of the string that provided the +partial match is set as the first matching string.
Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the @@ -54,9 +65,10 @@
-Because of the way certain internal optimizations are implemented in PCRE, the -PCRE_PARTIAL option cannot be used with all patterns. Repeated single -characters such as +Because of the way certain internal optimizations are implemented in the +pcre_exec() function, the PCRE_PARTIAL option cannot be used with all +patterns. These restrictions do not apply when pcre_dfa_exec() is used. +For pcre_exec(), repeated single characters such as
a{2,4}
@@ -100,12 +112,94 @@
The first data string is matched completely, so pcretest shows the
matched substrings. The remaining four strings do not match the complete
-pattern, but the first two are partial matches.
+pattern, but the first two are partial matches. The same test, using DFA
+matching (by means of the \D escape sequence), produces the following output:
++ re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 25jun04\P\D + 0: 25jun04 + data> 23dec3\P\D + Partial match: 23dec3 + data> 3ju\P\D + Partial match: 3ju + data> 3juj\P\D + No match + data> j\P\D + No match ++Notice that in this case the portion of the string that was matched is made +available. + +
+When a partial match has been found using pcre_dfa_exec(), it is possible +to continue the match by providing additional subject data and calling +pcre_dfa_exec() again with the PCRE_DFA_RESTART option and the same +working space (where details of the previous partial match are stored). Here is +an example using pcretest, where the \R escape sequence sets the +PCRE_DFA_RESTART option and the \D escape sequence requests the use of +pcre_dfa_exec(): +
+ re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 23ja\P\D + Partial match: 23ja + data> n05\R\D + 0: n05 ++The first call has "23ja" as the subject, and requests partial matching; the +second call has "n05" as the subject for the continued (restarted) match. +Notice that when the match is complete, only the last part is shown; PCRE does +not retain the previously partially-matched string. It is up to the calling +program to do that if it needs to. + +
+This facility can be used to pass very long subject strings to +pcre_dfa_exec(). However, some care is needed for certain types of +pattern. +
++1. If the pattern contains tests for the beginning or end of a line, you need +to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the +subject string for any call does not contain the beginning or end of a line. +
++2. If the pattern contains backward assertions (including \b or \B), you need +to arrange for some overlap in the subject strings to allow for this. For +example, you could pass the subject in chunks that were 500 bytes long, but in +a buffer of 700 bytes, with the starting offset set to 200 and the previous 200 +bytes at the start of the buffer. +
++3. Matching a subject string that is split into multiple segments does not +always produce exactly the same result as matching over one single long string. +The difference arises when there are multiple matching possibilities, because a +partial match result is given only when there are no completed matches in a +call to fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has +been found, continuation to a new subject segment is no longer possible. +Consider this pcretest example: +
+ re> /dog(sbody)?/ + data> do\P\D + Partial match: do + data> gsb\R\P\D + 0: g + data> dogsbody\D + 0: dogsbody + 1: dog ++The pattern matches the words "dog" or "dogsbody". When the subject is +presented in several parts ("do" and "gsb" being the first two) the match stops +when "dog" has been found, and it is not possible to continue. On the other +hand, if "dogsbody" is presented as a single string, both matches are found. + +
+Because of this phenomenon, it does not usually make sense to end a pattern +that is going to be matched in this way with a variable repeat.
-Last updated: 08 September 2004
+Last updated: 28 February 2005
-Copyright © 1997-2004 University of Cambridge.
+Copyright © 1997-2005 University of Cambridge.
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