.TH PCREPARTIAL 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"
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In normal use of PCRE, if the subject string that is passed to
\fBpcre_exec()\fP or \fBpcre_dfa_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$
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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 or
\fBpcre_dfa_exec()\fP. When this flag is set for \fBpcre_exec()\fP, 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.
.P
When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, 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.
.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.
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.SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
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Because of the way certain internal optimizations are implemented in the
\fBpcre_exec()\fP function, the PCRE_PARTIAL option cannot be used with all
patterns. These restrictions do not apply when \fBpcre_dfa_exec()\fP is used.
For \fBpcre_exec()\fP, repeated single characters such as
.sp
  a{2,4}
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and repeated single metasequences such as
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  \ed+
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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).
You can use the PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out
if a compiled pattern can be used for partial matching.
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.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
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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$/
  data> 25jun04\eP
   0: 25jun04
   1: jun
  data> 25dec3\eP
  Partial match
  data> 3ju\eP
  Partial match
  data> 3juj\eP
  No match
  data> j\eP
  No match
.sp
The first data string is matched completely, so \fBpcretest\fP shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. The same test, using
\fBpcre_dfa_exec()\fP matching (by means of the \eD escape sequence), produces
the following output:
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 25jun04\eP\eD
   0: 25jun04
  data> 23dec3\eP\eD
  Partial match: 23dec3
  data> 3ju\eP\eD
  Partial match: 3ju
  data> 3juj\eP\eD
  No match
  data> j\eP\eD
  No match
.sp
Notice that in this case the portion of the string that was matched is made
available.
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.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
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When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
to continue the match by providing additional subject data and calling
\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
time setting the PCRE_DFA_RESTART option. You must also pass the same working
space as before, because this is where details of the previous partial match
are stored. Here is an example using \fBpcretest\fP, using the \eR escape
sequence to set the PCRE_DFA_RESTART option (\eP and \eD are as above):
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 23ja\eP\eD
  Partial match: 23ja
  data> n05\eR\eD
   0: n05
.sp
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.
.P
You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching
over multiple segments. This facility can be used to pass very long subject
strings to \fBpcre_dfa_exec()\fP. However, some care is needed for certain
types of pattern.
.P
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.
.P
2. If the pattern contains backward assertions (including \eb or \eB), 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 are 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.
.P
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 \fBpcretest\fP example:
.sp
    re> /dog(sbody)?/
  data> do\eP\eD
  Partial match: do
  data> gsb\eR\eP\eD
   0: g
  data> dogsbody\eD
   0: dogsbody
   1: dog
.sp
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.
.P
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.
.P
4. Patterns that contain alternatives at the top level which do not all
start with the same pattern item may not work as expected. For example,
consider this pattern:
.sp
  1234|3789
.sp
If the first part of the subject is "ABC123", a partial match of the first
alternative is found at offset 3. There is no partial match for the second
alternative, because such a match does not start at the same point in the
subject string. Attempting to continue with the string "789" does not yield a
match because only those alternatives that match at one point in the subject
are remembered. The problem arises because the start of the second alternative
matches within the first alternative. There is no problem with anchored
patterns or patterns such as:
.sp
  1234|ABCD
.sp
where no string can be a partial match for both alternatives.
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.SH AUTHOR
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Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
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.SH REVISION
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Last updated: 04 June 2007
Copyright (c) 1997-2007 University of Cambridge.
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