220 lines
8.5 KiB
Groff
220 lines
8.5 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, possibly beeping and not reflecting the
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character that has been typed. 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.
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.P
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PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
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option, which can be set when calling \fBpcre_exec()\fP or
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\fBpcre_dfa_exec()\fP. When this flag is set for \fBpcre_exec()\fP, the return
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code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
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during the matching process the last part of the subject string matched part of
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the pattern. Unfortunately, for non-anchored matching, it is not possible to
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obtain the position of the start of the partial match. No captured data is set
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when PCRE_ERROR_PARTIAL is returned.
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.P
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When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, the return code
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PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the
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subject is reached, there have been no complete matches, but there is still at
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least one matching possibility. The portion of the string that provided the
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partial match is set as the first matching string.
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.P
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Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
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last literal byte in a pattern, and abandons matching immediately if such a
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byte is not present in the subject string. This optimization cannot be used
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for a subject string that might match only partially.
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.
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.
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.SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
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.rs
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.sp
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Because of the way certain internal optimizations are implemented in the
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\fBpcre_exec()\fP function, the PCRE_PARTIAL option cannot be used with all
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patterns. These restrictions do not apply when \fBpcre_dfa_exec()\fP is used.
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For \fBpcre_exec()\fP, repeated single characters such as
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.sp
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a{2,4}
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.sp
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and repeated single metasequences such as
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.sp
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\ed+
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.sp
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are not permitted if the maximum number of occurrences is greater than one.
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Optional items such as \ed? (where the maximum is one) are permitted.
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Quantifiers with any values are permitted after parentheses, so the invalid
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examples above can be coded thus:
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.sp
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(a){2,4}
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(\ed)+
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.sp
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These constructions run more slowly, but for the kinds of application that are
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envisaged for this facility, this is not felt to be a major restriction.
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.P
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If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
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\fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
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You can use the PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out
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if a compiled pattern can be used for partial matching.
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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 flag is used for the match. Here is a run of \fBpcretest\fP that
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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$/
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data> 25jun04\eP
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0: 25jun04
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1: jun
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data> 25dec3\eP
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Partial match
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data> 3ju\eP
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Partial match
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data> 3juj\eP
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No match
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data> j\eP
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No match
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.sp
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The first data string is matched completely, so \fBpcretest\fP shows the
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matched substrings. The remaining four strings do not match the complete
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pattern, but the first two are partial matches. The same test, using
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\fBpcre_dfa_exec()\fP matching (by means of the \eD escape sequence), produces
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the following output:
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.sp
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re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
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data> 25jun04\eP\eD
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0: 25jun04
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data> 23dec3\eP\eD
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Partial match: 23dec3
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data> 3ju\eP\eD
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Partial match: 3ju
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data> 3juj\eP\eD
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No match
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data> j\eP\eD
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No match
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.sp
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Notice that in this case the portion of the string that was matched is made
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available.
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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 also 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 (\eP and \eD are as 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> 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 PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching
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over multiple segments. This facility can be used to pass very long subject
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strings to \fBpcre_dfa_exec()\fP. However, some care is needed for certain
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types of pattern.
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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. If the pattern contains backward assertions (including \eb or \eB), you need
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to arrange for some overlap in the subject strings to allow for this. For
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example, you could pass the subject in chunks that are 500 bytes long, but in
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a buffer of 700 bytes, with the starting offset set to 200 and the previous 200
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bytes at the start of the buffer.
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.P
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3. Matching a subject string that is split into multiple segments does not
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always produce exactly the same result as matching over one single long string.
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The difference arises when there are multiple matching possibilities, because a
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partial match result is given only when there are no completed matches in a
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call to \fBpcre_dfa_exec()\fP. 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 this \fBpcretest\fP example:
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.sp
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re> /dog(sbody)?/
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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 pattern matches the words "dog" or "dogsbody". When the subject is
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presented in several parts ("do" and "gsb" being the first two) the match stops
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when "dog" has been found, and it is not possible to continue. On the other
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hand, if "dogsbody" is presented as a single string, both matches are found.
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.P
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Because of this phenomenon, it does not usually make sense to end a pattern
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that is going to be matched in this way with a variable repeat.
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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. For example,
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consider this 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 "789" 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.
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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: 04 June 2007
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Copyright (c) 1997-2007 University of Cambridge.
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.fi
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