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.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"
.rs
.sp
In normal use of PCRE, if the subject string that is passed to
\fBpcre_exec()\fP 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.
.P
Consider, for example, an application where a human is required to type in data
for a field with specific formatting requirements. An example might be a date
in the form \fIddmmmyy\fP, defined by this pattern:
.sp
^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
.sp
If the application sees the user's keystrokes one by one, and can check that
what has been typed so far is potentially valid, it is able to raise an error
as soon as a mistake is made, possibly beeping and not reflecting the
character that has been typed. This immediate feedback is likely to be a better
user interface than a check that is delayed until the entire string has been
entered.
.P
PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
option, which can be set when calling \fBpcre_exec()\fP. 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.
.P
Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
last literal byte in a pattern, and abandons matching immediately if such a
byte is not present in the subject string. This optimization cannot be used
for a subject string that might match only partially.
.
.
.SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
.rs
.sp
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
.sp
a{2,4}
.sp
and repeated single metasequences such as
.sp
\ed+
.sp
are not permitted if the maximum number of occurrences is greater than one.
Optional items such as \ed? (where the maximum is one) are permitted.
Quantifiers with any values are permitted after parentheses, so the invalid
examples above can be coded thus:
.sp
(a){2,4}
(\ed)+
.sp
These constructions run more slowly, but for the kinds of application that are
envisaged for this facility, this is not felt to be a major restriction.
.P
If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
\fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
.
.
.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
.rs
.sp
If the escape sequence \eP is present in a \fBpcretest\fP data line, the
PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that
uses the date example quoted above:
.sp
re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match
data> 3ju\P
Partial match
data> 3juj\P
No match
data> j\P
No match
.sp
The first data string is matched completely, so \fBpcretest\fP shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches.
.
.
.P
.in 0
Last updated: 08 September 2004
.br
Copyright (c) 1997-2004 University of Cambridge.
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