373 lines
16 KiB
Groff
373 lines
16 KiB
Groff
.TH PCREPARTIAL 3
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.SH NAME
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PCRE - Perl-compatible regular expressions
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.SH "PARTIAL MATCHING IN PCRE"
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.rs
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.sp
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In normal use of PCRE, if the subject string that is passed to
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\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is
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too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
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are circumstances where it might be helpful to distinguish this case from other
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cases in which there is no match.
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.P
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Consider, for example, an application where a human is required to type in data
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for a field with specific formatting requirements. An example might be a date
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in the form \fIddmmmyy\fP, defined by this pattern:
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.sp
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^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
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.sp
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If the application sees the user's keystrokes one by one, and can check that
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what has been typed so far is potentially valid, it is able to raise an error
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as soon as a mistake is made, by beeping and not reflecting the character that
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has been typed, for example. This immediate feedback is likely to be a better
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user interface than a check that is delayed until the entire string has been
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entered. Partial matching can also sometimes be useful when the subject string
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is very long and is not all available at once.
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.P
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PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
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PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
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\fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
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for PCRE_PARTIAL_SOFT. The essential difference between the two options is
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whether or not a partial match is preferred to an alternative complete match,
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though the details differ between the two matching functions. If both options
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are set, PCRE_PARTIAL_HARD takes precedence.
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.P
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Setting a partial matching option disables two of PCRE's optimizations. PCRE
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remembers the last literal byte in a pattern, and abandons matching immediately
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if such a byte is not present in the subject string. This optimization cannot
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be used for a subject string that might match only partially. If the pattern
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was studied, PCRE knows the minimum length of a matching string, and does not
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bother to run the matching function on shorter strings. This optimization is
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also disabled for partial matching.
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.
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.
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.SH "PARTIAL MATCHING USING pcre_exec()"
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.rs
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.sp
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A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
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the subject string is reached successfully, but matching cannot continue
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because more characters are needed. However, at least one character must have
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been matched. (In other words, a partial match can never be an empty string.)
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.P
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If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
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continues as normal, and other alternatives in the pattern are tried. If no
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complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
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instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
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vector, the first of them is set to the offset of the earliest character that
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was inspected when the partial match was found. For convenience, the second
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offset points to the end of the string so that a substring can easily be
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identified.
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.P
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For the majority of patterns, the first offset identifies the start of the
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partially matched string. However, for patterns that contain lookbehind
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assertions, or \eK, or begin with \eb or \eB, earlier characters have been
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inspected while carrying out the match. For example:
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.sp
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/(?<=abc)123/
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.sp
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This pattern matches "123", but only if it is preceded by "abc". If the subject
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string is "xyzabc12", the offsets after a partial match are for the substring
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"abc12", because all these characters are needed if another match is tried
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with extra characters added.
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.P
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If there is more than one partial match, the first one that was found provides
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the data that is returned. Consider this pattern:
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.sp
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/123\ew+X|dogY/
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.sp
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If this is matched against the subject string "abc123dog", both
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alternatives fail to match, but the end of the subject is reached during
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matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
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offsets are set to 3 and 9, identifying "123dog" as the first partial match
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that was found. (In this example, there are two partial matches, because "dog"
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on its own partially matches the second alternative.)
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.P
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If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
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PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
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search for possible complete matches. The difference between the two options
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can be illustrated by a pattern such as:
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.sp
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/dog(sbody)?/
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.sp
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This matches either "dog" or "dogsbody", greedily (that is, it prefers the
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longer string if possible). If it is matched against the string "dog" with
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PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
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PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
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if the pattern is made ungreedy the result is different:
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.sp
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/dog(sbody)??/
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.sp
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In this case the result is always a complete match because \fBpcre_exec()\fP
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finds that first, and it never continues after finding a match. It might be
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easier to follow this explanation by thinking of the two patterns like this:
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.sp
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/dog(sbody)?/ is the same as /dogsbody|dog/
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/dog(sbody)??/ is the same as /dog|dogsbody/
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.sp
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The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
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used, because it will always find the shorter match first.
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.
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.
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.SH "PARTIAL MATCHING USING pcre_dfa_exec()"
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.rs
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.sp
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The \fBpcre_dfa_exec()\fP function moves along the subject string character by
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character, without backtracking, searching for all possible matches
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simultaneously. If the end of the subject is reached before the end of the
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pattern, there is the possibility of a partial match, again provided that at
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least one character has matched.
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.P
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When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
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have been no complete matches. Otherwise, the complete matches are returned.
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However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
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complete matches. The portion of the string that was inspected when the longest
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partial match was found is set as the first matching string, provided there are
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at least two slots in the offsets vector.
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.P
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Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
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there is no difference between greedy and ungreedy repetition, its behaviour is
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different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
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string "dog" matched against the ungreedy pattern shown above:
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.sp
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/dog(sbody)??/
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.sp
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Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
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"dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
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so returns that when PCRE_PARTIAL_HARD is set.
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.
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.
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.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
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.rs
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.sp
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If a pattern ends with one of sequences \eb or \eB, which test for word
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boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
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results. Consider this pattern:
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.sp
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/\ebcat\eb/
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.sp
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This matches "cat", provided there is a word boundary at either end. If the
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subject string is "the cat", the comparison of the final "t" with a following
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character cannot take place, so a partial match is found. However,
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\fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
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of the subject when the last character is a letter, thus finding a complete
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match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
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happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
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.P
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Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
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then the partial match takes precedence.
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.
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.
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.SH "FORMERLY RESTRICTED PATTERNS"
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.rs
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.sp
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For releases of PCRE prior to 8.00, because of the way certain internal
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optimizations were implemented in the \fBpcre_exec()\fP function, the
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PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
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all patterns. From release 8.00 onwards, the restrictions no longer apply, and
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partial matching with \fBpcre_exec()\fP can be requested for any pattern.
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.P
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Items that were formerly restricted were repeated single characters and
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repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
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conform to the restrictions, \fBpcre_exec()\fP returned the error code
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PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
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PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
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pattern can be used for partial matching now always returns 1.
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.
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.
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.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
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.rs
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.sp
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If the escape sequence \eP is present in a \fBpcretest\fP data line, the
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PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
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that uses the date example quoted above:
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.sp
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re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
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data> 25jun04\eP
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0: 25jun04
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1: jun
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data> 25dec3\eP
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Partial match: 23dec3
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data> 3ju\eP
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Partial match: 3ju
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data> 3juj\eP
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No match
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data> j\eP
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No match
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.sp
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The first data string is matched completely, so \fBpcretest\fP shows the
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matched substrings. The remaining four strings do not match the complete
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pattern, but the first two are partial matches. Similar output is obtained
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when \fBpcre_dfa_exec()\fP is used.
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.P
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If the escape sequence \eP is present more than once in a \fBpcretest\fP data
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line, the PCRE_PARTIAL_HARD option is set for the match.
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.
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.
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.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
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.rs
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.sp
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When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
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to continue the match by providing additional subject data and calling
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\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
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time setting the PCRE_DFA_RESTART option. You must pass the same working
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space as before, because this is where details of the previous partial match
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are stored. Here is an example using \fBpcretest\fP, using the \eR escape
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sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
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\fBpcre_dfa_exec()\fP):
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.sp
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re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
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data> 23ja\eP\eD
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Partial match: 23ja
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data> n05\eR\eD
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0: n05
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.sp
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The first call has "23ja" as the subject, and requests partial matching; the
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second call has "n05" as the subject for the continued (restarted) match.
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Notice that when the match is complete, only the last part is shown; PCRE does
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not retain the previously partially-matched string. It is up to the calling
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program to do that if it needs to.
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.P
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You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
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PCRE_DFA_RESTART to continue partial matching over multiple segments. This
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facility can be used to pass very long subject strings to
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\fBpcre_dfa_exec()\fP.
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.
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.
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.SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
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.rs
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.sp
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From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
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matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
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previous match with a new segment of data. Instead, new data must be added to
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the previous subject string, and the entire match re-run, starting from the
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point where the partial match occurred. Earlier data can be discarded.
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Consider an unanchored pattern that matches dates:
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.sp
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re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
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data> The date is 23ja\eP
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Partial match: 23ja
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.sp
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At this stage, an application could discard the text preceding "23ja", add on
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text from the next segment, and call \fBpcre_exec()\fP again. Unlike
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\fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
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the complete matching process occurs for each call, so more memory and more
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processing time is needed.
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.P
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\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
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with \eb or \eB, the string that is returned for a partial match will include
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characters that precede the partially matched string itself, because these must
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be retained when adding on more characters for a subsequent matching attempt.
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.
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.
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.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
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.rs
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.sp
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Certain types of pattern may give problems with multi-segment matching,
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whichever matching function is used.
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.P
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1. If the pattern contains tests for the beginning or end of a line, you need
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to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
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subject string for any call does not contain the beginning or end of a line.
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.P
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2. Lookbehind assertions at the start of a pattern are catered for in the
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offsets that are returned for a partial match. However, in theory, a lookbehind
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assertion later in the pattern could require even earlier characters to be
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inspected, and it might not have been reached when a partial match occurs. This
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is probably an extremely unlikely case; you could guard against it to a certain
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extent by always including extra characters at the start.
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.P
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3. Matching a subject string that is split into multiple segments may not
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always produce exactly the same result as matching over one single long string,
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especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
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Word Boundaries" above describes an issue that arises if the pattern ends with
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\eb or \eB. Another kind of difference may occur when there are multiple
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matching possibilities, because a partial match result is given only when there
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are no completed matches. This means that as soon as the shortest match has
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been found, continuation to a new subject segment is no longer possible.
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Consider again this \fBpcretest\fP example:
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.sp
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re> /dog(sbody)?/
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data> dogsb\eP
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0: dog
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data> do\eP\eD
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Partial match: do
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data> gsb\eR\eP\eD
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0: g
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data> dogsbody\eD
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0: dogsbody
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1: dog
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.sp
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The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
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PCRE_PARTIAL_SOFT option. Although the string is a partial match for
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"dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
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"dog" is a complete match. Similarly, when the subject is presented to
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\fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
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match stops when "dog" has been found, and it is not possible to continue. On
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the other hand, if "dogsbody" is presented as a single string,
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\fBpcre_dfa_exec()\fP finds both matches.
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.P
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Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
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matching multi-segment data. The example above then behaves differently:
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.sp
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re> /dog(sbody)?/
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data> dogsb\eP\eP
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Partial match: dogsb
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data> do\eP\eD
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Partial match: do
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data> gsb\eR\eP\eP\eD
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Partial match: gsb
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.sp
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.P
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4. Patterns that contain alternatives at the top level which do not all
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start with the same pattern item may not work as expected when
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PCRE_DFA_RESTART is used with \fBpcre_dfa_exec()\fP. For example, consider this
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pattern:
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.sp
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1234|3789
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.sp
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If the first part of the subject is "ABC123", a partial match of the first
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alternative is found at offset 3. There is no partial match for the second
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alternative, because such a match does not start at the same point in the
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subject string. Attempting to continue with the string "7890" does not yield a
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match because only those alternatives that match at one point in the subject
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are remembered. The problem arises because the start of the second alternative
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matches within the first alternative. There is no problem with anchored
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patterns or patterns such as:
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.sp
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1234|ABCD
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.sp
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where no string can be a partial match for both alternatives. This is not a
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problem if \fBpcre_exec()\fP is used, because the entire match has to be rerun
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each time:
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.sp
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re> /1234|3789/
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data> ABC123\eP
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Partial match: 123
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data> 1237890
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0: 3789
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.sp
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Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re-running
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the entire match can also be used with \fBpcre_dfa_exec()\fP. Another
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possibility is to work with two buffers. If a partial match at offset \fIn\fP
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in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
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the second buffer, you can then try a new match starting at offset \fIn+1\fP in
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the first buffer.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge CB2 3QH, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 19 October 2009
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Copyright (c) 1997-2009 University of Cambridge.
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.fi
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