PCREPARTIAL(3)PCREPARTIAL(3)NAME
PCRE - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE
In normal use of PCRE, if the subject string that is passed to
pcre_exec() or pcre_dfa_exec() 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.
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 ddmmmyy, defined by this pattern:
^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
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 immedi-
ate 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.
PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or
pcre_dfa_exec(). 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 com-
plete match, though the details differ between the two matching func-
tions. If both options are set, PCRE_PARTIAL_HARD takes precedence.
Setting a partial matching option disables two of PCRE's optimizations.
PCRE remembers the last literal byte in a pattern, and abandons match-
ing 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.
PARTIAL MATCHING USING pcre_exec()
A partial match occurs during a call to pcre_exec() whenever the end of
the subject string is reached successfully, but matching cannot con-
tinue because more characters are needed. However, at least one charac-
ter must have been matched. (In other words, a partial match can never
be an empty string.)
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, pcre_exec() returns
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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.
For the majority of patterns, the first offset identifies the start of
the partially matched string. However, for patterns that contain look-
behind assertions, or \K, or begin with \b or \B, earlier characters
have been inspected while carrying out the match. For example:
/(?<=abc)123/
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.
If there is more than one partial match, the first one that was found
provides the data that is returned. Consider this pattern:
/123\w+X|dogY/
If this is matched against the subject string "abc123dog", both alter-
natives 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.)
If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR-
TIAL 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:
/dog(sbody)?/
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 dif-
ferent:
/dog(sbody)??/
In this case the result is always a complete match because pcre_exec()
finds that first, and it never continues after finding a match. It
might be easier to follow this explanation by thinking of the two pat-
terns like this:
/dog(sbody)?/ is the same as /dogsbody|dog/
/dog(sbody)??/ is the same as /dog|dogsbody/
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The second pattern will never match "dogsbody" when pcre_exec() is
used, because it will always find the shorter match first.
PARTIAL MATCHING USING pcre_dfa_exec()
The pcre_dfa_exec() 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 pro-
vided that at least one character has matched.
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.
Because pcre_dfa_exec() always searches for all possible matches, and
there is no difference between greedy and ungreedy repetition, its
behaviour is different from pcre_exec when PCRE_PARTIAL_HARD is set.
Consider the string "dog" matched against the ungreedy pattern shown
above:
/dog(sbody)??/
Whereas pcre_exec() stops as soon as it finds the complete match for
"dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and
so returns that when PCRE_PARTIAL_HARD is set.
PARTIAL MATCHING AND WORD BOUNDARIES
If a pattern ends with one of sequences \b or \B, which test for word
boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-
intuitive results. Consider this pattern:
/\bcat\b/
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, pcre_exec() carries on with normal matching, which matches \b
at the end of the subject when the last character is a letter, thus
finding a complete match. The result, therefore, is not PCRE_ERROR_PAR-
TIAL. The same thing happens with pcre_dfa_exec(), because it also
finds the complete match.
Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL,
because then the partial match takes precedence.
FORMERLY RESTRICTED PATTERNS
For releases of PCRE prior to 8.00, because of the way certain internal
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optimizations were implemented in the pcre_exec() 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 pcre_exec() can be requested
for any pattern.
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, pcre_exec() returned the error code
PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled
pattern can be used for partial matching now always returns 1.
EXAMPLE OF PARTIAL MATCHING USING PCRETEST
If the escape sequence \P is present in a pcretest data line, the
PCRE_PARTIAL_SOFT option is used for the match. Here is a run of
pcretest that uses the date example quoted above:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match: 23dec3
data> 3ju\P
Partial match: 3ju
data> 3juj\P
No match
data> j\P
No match
The first data string is matched completely, so pcretest shows the
matched substrings. The remaining four strings do not match the com-
plete pattern, but the first two are partial matches. Similar output is
obtained when pcre_dfa_exec() is used.
If the escape sequence \P is present more than once in a pcretest data
line, the PCRE_PARTIAL_HARD option is set for the match.
MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
When a partial match has been found using pcre_dfa_exec(), it is possi-
ble to continue the match by providing additional subject data and
calling pcre_dfa_exec() again with the same compiled regular expres-
sion, this time setting the PCRE_DFA_RESTART option. You must pass the
same working space as before, because this is where details of the pre-
vious partial match are stored. Here is an example using pcretest,
using the \R escape sequence to set the PCRE_DFA_RESTART option (\D
specifies the use of pcre_dfa_exec()):
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\P\D
Partial match: 23ja
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data> n05\R\D
0: n05
The first call has "23ja" as the subject, and requests partial match-
ing; 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.
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
pcre_dfa_exec().
MULTI-SEGMENT MATCHING WITH pcre_exec()
From release 8.00, pcre_exec() can also be used to do multi-segment
matching. Unlike pcre_dfa_exec(), 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:
re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
data> The date is 23ja\P
Partial match: 23ja
At this stage, an application could discard the text preceding "23ja",
add on text from the next segment, and call pcre_exec() again. Unlike
pcre_dfa_exec(), 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.
Note: If the pattern contains lookbehind assertions, or \K, or starts
with \b or \B, 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.
ISSUES WITH MULTI-SEGMENT MATCHING
Certain types of pattern may give problems with multi-segment matching,
whichever matching function is used.
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 appropri-
ate, when the subject string for any call does not contain the begin-
ning or end of a line.
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
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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.
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 \b or \B. 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, continua-
tion to a new subject segment is no longer possible. Consider again
this pcretest example:
re> /dog(sbody)?/
data> dogsb\P
0: dog
data> do\P\D
Partial match: do
data> gsb\R\P\D
0: g
data> dogsbody\D
0: dogsbody
1: dog
The first data line passes the string "dogsb" to pcre_exec(), 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 pcre_dfa_exec() 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, pcre_dfa_exec() finds both matches.
Because of these problems, it is probably best to use PCRE_PARTIAL_HARD
when matching multi-segment data. The example above then behaves dif-
ferently:
re> /dog(sbody)?/
data> dogsb\P\P
Partial match: dogsb
data> do\P\D
Partial match: do
data> gsb\R\P\P\D
Partial match: gsb
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 pcre_dfa_exec(). For example, consider
this pattern:
1234|3789
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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:
1234|ABCD
where no string can be a partial match for both alternatives. This is
not a problem if pcre_exec() is used, because the entire match has to
be rerun each time:
re> /1234|3789/
data> ABC123\P
Partial match: 123
data> 1237890
0: 3789
Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re-
running the entire match can also be used with pcre_dfa_exec(). Another
possibility is to work with two buffers. If a partial match at offset n
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 n+1 in the first buffer.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 19 October 2009
Copyright (c) 1997-2009 University of Cambridge.
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