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 dis-
tinguish 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 ddmm-
myy, defined by this pattern:
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 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 com-
patibility, 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.
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.
PARTIAL MATCHING USING pcre_exec()
A partial match occurs during a call to pcre_exec() when the end of the subject string is
reached successfully, but matching cannot continue because more characters are needed.
However, at least one character in the subject must have been inspected. This character
need not form part of the final matched string; lookbehind assertions and the \K escape
sequence provide ways of inspecting characters before the start of a matched substring.
The requirement for inspecting at least one character exists because an empty string can
always be matched; without such a restriction there would always be a partial match of an
empty string at the end of the subject.
If there are at least two slots in the offsets vector when pcre_exec() returns with a par-
tial match, the first slot 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 subject 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 lookbehind assertions, or \K, or begin
with \b or \B, earlier characters have been inspected while carrying out the match. For
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 to the
What happens when a partial match is identified depends on which of the two partial match-
ing options are set.
PCRE_PARTIAL_SOFT with pcre_exec()
If PCRE_PARTIAL_SOFT is set when pcre_exec() identifies a partial match, 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 PCRE_ERROR_PARTIAL instead
This option is "soft" because it prefers a complete match over a partial match. All the
various matching items in a pattern behave as if the subject string is potentially com-
plete. For example, \z, \Z, and $ match at the end of the subject, as normal, and for \b
and \B the end of the subject is treated as a non-alphanumeric.
If there is more than one partial match, the first one that was found provides the data
that is returned. Consider this pattern:
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. 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.)
PCRE_PARTIAL_HARD with pcre_exec()
If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PARTIAL as soon as a
partial match is found, without continuing to search for possible complete matches. This
option is "hard" because it prefers an earlier partial match over a later complete match.
For this reason, the assumption is made that the end of the supplied subject string may
not be the true end of the available data, and so, if \z, \Z, \b, \B, or $ are encountered
at the end of the subject, the result is PCRE_ERROR_PARTIAL.
Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks UTF-8 subject strings
for validity. Normally, an invalid UTF-8 sequence causes the error PCRE_ERROR_BADUTF8.
However, in the special case of a truncated UTF-8 character at the end of the subject,
PCRE_ERROR_SHORTUTF8 is returned when PCRE_PARTIAL_HARD is set.
Comparing hard and soft partial matching
The difference between the two partial matching options can be illustrated by a pattern
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-
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 explana-
tion by thinking of the two patterns like this:
/dog(sbody)?/ is the same as /dogsbody|dog/
/dog(sbody)??/ is the same as /dog|dogsbody/
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, with-
out 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 been inspected.
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_PAR-
TIAL_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 differ-
ence 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 pat-
tern shown above:
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:
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 find-
ing a complete match. The result, therefore, is not PCRE_ERROR_PARTIAL. 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 par-
tial match takes precedence.
FORMERLY RESTRICTED PATTERNS
For releases of PCRE prior to 8.00, because of the way certain internal optimizations were
implemented in the pcre_exec() function, the PCRE_PARTIAL option (predecessor of PCRE_PAR-
TIAL_SOFT) could not be used with all patterns. From release 8.00 onwards, the restric-
tions no longer apply, and partial matching with pcre_exec() can be requested for any pat-
Items that were formerly restricted were repeated single characters and repeated metase-
quences. 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:
Partial match: 23dec3
Partial match: 3ju
The first data string is matched completely, so pcretest 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 pcre_dfa_exec() is used.
If the escape sequence \P is present more than once in a pcretest data line, the PCRE_PAR-
TIAL_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 possible to continue the
match by providing additional subject data and calling pcre_dfa_exec() 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 pcretest, using the \R escape sequence to set
the PCRE_DFA_RESTART option (\D specifies the use of pcre_dfa_exec()):
Partial match: 23ja
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.
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 dis-
carded. It is best to use PCRE_PARTIAL_HARD in this situation, because it does not treat
the end of a segment as the end of the subject when matching \z, \Z, \b, \B, and $. Con-
sider an unanchored pattern that matches dates:
data> The date is 23ja\P\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 par-
tially matched string itself, because these must be retained when adding on more charac-
ters 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 a test for the beginning of a line, you need to pass the
PCRE_NOTBOL option when the subject string for any call does start at the beginning of a
line. There is also a PCRE_NOTEOL option, but in practice when doing multi-segment match-
ing you should be using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.
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 pat-
tern 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
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_PAR-
TIAL_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 (for PCRE_PARTIAL_SOFT) 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 pos-
sible. Consider again this pcretest example:
Partial match: do
The first data line passes the string "dogsb" to pcre_exec(), setting the PCRE_PAR-
TIAL_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
Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching multi-segment
data. The example above then behaves differently:
Partial match: dogsb
Partial match: do
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:
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 pat-
terns or patterns such as:
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:
Partial match: 123
Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running the entire
match can also be used with pcre_dfa_exec(). Another possibility is to work with two buf-
fers. 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.
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Last updated: 07 November 2010
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