pcrepartial.3   [plain text]


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