PCREPARTIAL(3) Library Functions Manual PCREPARTIAL(3)
PCRE - Perl-compatible regular expressions
In normal use of PCRE, if the subject string that is passed to a
matching function 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 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 any
of the matching functions. 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 types of matching function. If
both options are set, PCRE_PARTIAL_HARD takes precedence.
If you want to use partial matching with just-in-time optimized
code, you must call pcre_study(), pcre16_study() or
pcre32_study() with one or both of these options:
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_COMPILE should also be set if you are going to run
non-partial matches on the same pattern. If the appropriate JIT
study mode has not been set for a match, the interpretive
matching code is used.
Setting a partial matching option disables two of PCRE's standard
optimizations. PCRE remembers the last literal data unit in a
pattern, and abandons matching immediately if it 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.
A partial match occurs during a call to pcre_exec() or
pcre[16|32]_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 a
partial match is returned, the first slot is set to the offset of
the earliest character that was inspected. For convenience, the
second offset points to the end of the subject so that a
substring can easily be identified. If there are at least three
slots in the offsets vector, the third slot is set to the offset
of the character where matching started.
For the majority of patterns, the contents of the first and third
slots will be the same. However, for patterns that contain
lookbehind assertions, or begin with \b or \B, characters before
the one where matching started may have been inspected while
carrying out the match. For example, consider this pattern:
/(?<=abc)123/
This pattern matches "123", but only if it is preceded by "abc".
If the subject string is "xyzabc12", the first two offsets after
a partial match are for the substring "abc12", because all these
characters were inspected. However, the third offset is set to 6,
because that is the offset where matching began.
What happens when a partial match is identified depends on which
of the two partial matching options are set.
PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()
If PCRE_PARTIAL_SOFT is set when pcre_exec() or
pcre[16|32]_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_ERROR_PARTIAL is returned instead of
PCRE_ERROR_NOMATCH.
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 complete. 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:
/123\w+X|dogY/
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() OR pcre[16|32]_exec()
If PCRE_PARTIAL_HARD is set for pcre_exec() or
pcre[16|32]_exec(), PCRE_ERROR_PARTIAL is returned 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, provided that at least
one character in the subject has been inspected.
Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
subject strings are checked for validity. Normally, an invalid
sequence causes the error PCRE_ERROR_BADUTF8 or
PCRE_ERROR_BADUTF16. However, in the special case of a truncated
character at the end of the subject, PCRE_ERROR_SHORTUTF8 or
PCRE_ERROR_SHORTUTF16 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 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 different:
/dog(sbody)??/
In this case the result is always a complete match because that
is found first, and matching never continues after finding a
complete match. It might be easier to follow this explanation 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", because it will
always find the shorter match first.
PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()
The DFA functions move 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
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_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 the DFA functions always search for all possible matches,
and there is no difference between greedy and ungreedy
repetition, their behaviour is different from the standard
functions when PCRE_PARTIAL_HARD is set. Consider the string
"dog" matched against the ungreedy pattern shown above:
/dog(sbody)??/
Whereas the standard functions stop as soon as they find the
complete match for "dog", the DFA functions also find the partial
match for "dogsbody", and so return that when PCRE_PARTIAL_HARD
is set.
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, normal matching carries on, and
\b matches at the end of the subject when the last character is a
letter, so a complete match is found. The result, therefore, is
not PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does
yield PCRE_ERROR_PARTIAL, because then the partial match takes
precedence.
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_PARTIAL_SOFT) could not be used with all patterns. From
release 8.00 onwards, the restrictions no longer apply, and
partial matching with 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.
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$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match: 23dec3
data> 3ju\P
Partial match: 3ju
data> 3juj\P
No match
data> j\P
No match
The first data string is matched completely, so pcretest shows
the matched substrings. The remaining four strings do not match
the complete pattern, but the first two are partial matches.
Similar output is obtained if DFA matching is used.
If the escape sequence \P is present more than once in a pcretest
data line, the PCRE_PARTIAL_HARD option is set for the match.
MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()
When a partial match has been found using a DFA matching
function, it is possible to continue the match by providing
additional subject data and calling the function again with the
same compiled regular expression, this time setting the
PCRE_DFA_RESTART option. You must pass the same working space as
before, because this is where details of the previous partial
match are stored. Here is an example using pcretest, using the \R
escape sequence to set the PCRE_DFA_RESTART option (\D specifies
the use of the DFA matching function):
re>
/^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\P\D
Partial match: 23ja
data> n05\R\D
0: n05
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.
That means that, for an unanchored pattern, if a continued match
fails, it is not possible to try again at a new starting point.
All this facility is capable of doing is continuing with the
previous match attempt. In the previous example, if the second
set of data is "ug23" the result is no match, even though there
would be a match for "aug23" if the entire string were given at
once. Depending on the application, this may or may not be what
you want. The only way to allow for starting again at the next
character is to retain the matched part of the subject and try a
new complete match.
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 the DFA matching functions.
From release 8.00, the standard matching functions can also be
used to do multi-segment matching. Unlike the DFA functions, 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.
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 $. 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\P
Partial match: 23ja
At this stage, an application could discard the text preceding
"23ja", add on text from the next segment, and call the matching
function again. Unlike the DFA matching functions, 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 includes characters that precede the start of what would be
returned for a complete match, because it contains all the
characters that were inspected during the partial match.
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
matching you should be using PCRE_PARTIAL_HARD, which includes
the effect of PCRE_NOTEOL.
2. Lookbehind assertions that have already been obeyed are
catered for in the offsets that are returned for a partial match.
However a lookbehind assertion later in the pattern could require
even earlier characters to be inspected. You can handle this case
by using the PCRE_INFO_MAXLOOKBEHIND option of the
pcre_fullinfo() or pcre[16|32]_fullinfo() functions to obtain the
length of the longest lookbehind in the pattern. This length is
given in characters, not bytes. If you always retain at least
that many characters before the partially matched string, all
should be well. (Of course, near the start of the subject, fewer
characters may be present; in that case all characters should be
retained.)
From release 8.33, there is a more accurate way of deciding which
characters to retain. Instead of subtracting the length of the
longest lookbehind from the earliest inspected character
(offsets[0]), the match start position (offsets[2]) should be
used, and the next match attempt started at the offsets[2]
character by setting the startoffset argument of pcre_exec() or
pcre_dfa_exec().
For example, if the pattern "(?<=123)abc" is partially matched
against the string "xx123a", the three offset values returned are
2, 6, and 5. This indicates that the matching process that gave a
partial match started at offset 5, but the characters "123a" were
all inspected. The maximum lookbehind for that pattern is 3, so
taking that away from 5 shows that we need only keep "123a", and
the next match attempt can be started at offset 3 (that is, at
"a") when further characters have been added. When the match
start is not the earliest inspected character, pcretest shows it
explicitly:
re> "(?<=123)abc"
data> xx123a\P\P
Partial match at offset 5: 123a
3. Because a partial match must always contain at least one
character, what might be considered a partial match of an empty
string actually gives a "no match" result. For example:
re> /c(?<=abc)x/
data> ab\P
No match
If the next segment begins "cx", a match should be found, but
this will only happen if characters from the previous segment are
retained. For this reason, a "no match" result should be
interpreted as "partial match of an empty string" when the
pattern contains lookbehinds.
4. 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 (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 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 a standard
matching function, 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 a DFA
matching function 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, a DFA matching function finds both
matches.
Because of these problems, it is best to use PCRE_PARTIAL_HARD
when matching multi-segment data. The example above then behaves
differently:
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
5. 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. For example, consider this
pattern:
1234|3789
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 a standard matching function is used,
because the entire match has to be rerun each time:
re> /1234|3789/
data> ABC123\P\P
Partial match: 123
data> 1237890
0: 3789
Of course, instead of using PCRE_DFA_RESTART, the same technique
of re-running the entire match can also be used with the DFA
matching functions. 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.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 02 July 2013
Copyright (c) 1997-2013 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
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PCRE 8.34 02 July 2013 PCREPARTIAL(3)
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