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PCRE2PARTIAL(3) Library Functions Manual PCRE2PARTIAL(3)
PCRE2 - Perl-compatible regular expressions
In normal use of PCRE2, if there is a match up to the end of a
subject string, but more characters are needed to match the
entire pattern, PCRE2_ERROR_NOMATCH is returned, just like any
other failing match. There are circumstances where it might be
helpful to distinguish this "partial match" case.
One example is an application where the subject string is very
long, and not all available at once. The requirement here is to
be able to do the matching segment by segment, but special action
is needed when a matched substring spans the boundary between two
segments.
Another example is checking a user input string as it is typed,
to ensure that it conforms to a required format. Invalid
characters can be immediately diagnosed and rejected, giving
instant feedback.
Partial matching is a PCRE2-specific feature; it is not Perl-
compatible. It is requested by setting one of the
PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options when calling a
matching function. The 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, PCRE2_PARTIAL_HARD
takes precedence.
If you want to use partial matching with just-in-time optimized
code, as well as setting a partial match option for the matching
function, you must also call pcre2_jit_compile() with one or both
of these options:
PCRE2_JIT_PARTIAL_HARD
PCRE2_JIT_PARTIAL_SOFT
PCRE2_JIT_COMPLETE should also be set if you are going to run
non-partial matches on the same pattern. Separate code is
compiled for each mode. If the appropriate JIT mode has not been
compiled, interpretive matching code is used.
Setting a partial matching option disables two of PCRE2's
standard optimization hints. PCRE2 remembers the last literal
code 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.
PCRE2 also remembers a 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 possible partial match occurs during matching when the end of
the subject string is reached successfully, but either more
characters are needed to complete the match, or the addition of
more characters might change what is matched.
Example 1: if the pattern is /abc/ and the subject is "ab", more
characters are definitely needed to complete a match. In this
case both hard and soft matching options yield a partial match.
Example 2: if the pattern is /ab+/ and the subject is "ab", a
complete match can be found, but the addition of more characters
might change what is matched. In this case, only
PCRE2_PARTIAL_HARD returns a partial match; PCRE2_PARTIAL_SOFT
returns the complete match.
On reaching the end of the subject, when PCRE2_PARTIAL_HARD is
set, if the next pattern item is \z, \Z, \b, \B, or $ there is
always a partial match. Otherwise, for both options, the next
pattern item must be one that inspects a character, and at least
one of the following must be true:
(1) At least one character has already been inspected. An
inspected 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
string.
(2) The pattern contains one or more lookbehind assertions. This
condition exists in case there is a lookbehind that inspects
characters before the start of the match.
(3) There is a special case when the whole pattern can match an
empty string. When the starting point is at the end of the
subject, the empty string match is a possibility, and if
PCRE2_PARTIAL_SOFT is set and neither of the above conditions is
true, it is returned. However, because adding more characters
might result in a non-empty match, PCRE2_PARTIAL_HARD returns a
partial match, which in this case means "there is going to be a
match at this point, but until some more characters are added, we
do not know if it will be an empty string or something longer".
When a partial matching option is set, the result of calling
pcre2_match() can be one of the following:
A successful match
A complete match has been found, starting and ending within
this subject.
PCRE2_ERROR_NOMATCH
No match can start anywhere in this subject.
PCRE2_ERROR_PARTIAL
Adding more characters may result in a complete match that uses
one or more characters from the end of this subject.
When a partial match is returned, the first two elements in the
ovector point to the portion of the subject that was matched, but
the values in the rest of the ovector are undefined. The
appearance of \K in the pattern has no effect for a partial
match. Consider this pattern:
/abc\K123/
If it is matched against "456abc123xyz" the result is a complete
match, and the ovector defines the matched string as "123",
because \K resets the "start of match" point. However, if a
partial match is requested and the subject string is "456abc12",
a partial match is found for the string "abc12", because all
these characters are needed for a subsequent re-match with
additional characters.
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 PCRE2_ERROR_PARTIAL is returned. The offsets
are set to 3 and 9, identifying "123dog" as the first partial
match. (In this example, there are two partial matches, because
"dog" on its own partially matches the second alternative.)
How a partial match is processed by pcre2_match()
What happens when a partial match is identified depends on which
of the two partial matching options is set.
If PCRE2_PARTIAL_HARD is set, PCRE2_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 is not the true end of the available data, which
is why \z, \Z, \b, \B, and $ always give a partial match.
If PCRE2_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,
PCRE2_ERROR_PARTIAL is returned instead of PCRE2_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; \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.
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 PCRE2_PARTIAL_SOFT, it yields a complete
match for "dog". However, if PCRE2_PARTIAL_HARD is set, the
result is PCRE2_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.
Example of partial matching using pcre2test
The pcre2test data modifiers partial_hard (or ph) and
partial_soft (or ps) set PCRE2_PARTIAL_HARD and
PCRE2_PARTIAL_SOFT, respectively, when calling pcre2_match().
Here is a run of pcre2test using a pattern that matches the whole
subject in the form of a date:
re>
/^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25dec3\=ph
Partial match: 23dec3
data> 3ju\=ph
Partial match: 3ju
data> 3juj\=ph
No match
This example gives the same results for both hard and soft
partial matching options. Here is an example where there is a
difference:
re>
/^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25jun04\=ps
0: 25jun04
1: jun
data> 25jun04\=ph
Partial match: 25jun04
With PCRE2_PARTIAL_SOFT, the subject is matched completely. For
PCRE2_PARTIAL_HARD, however, the subject is assumed not to be
complete, so there is only a partial match.
PCRE was not originally designed with multi-segment matching in
mind. However, over time, features (including partial matching)
that make multi-segment matching possible have been added. A very
long string can be searched segment by segment by calling
pcre2_match() repeatedly, with the aim of achieving the same
results that would happen if the entire string was available for
searching all the time. Normally, the strings that are being
sought are much shorter than each individual segment, and are in
the middle of very long strings, so the pattern is normally not
anchored.
Special logic must be implemented to handle a matched substring
that spans a segment boundary. PCRE2_PARTIAL_HARD should be used,
because it returns a partial match at the end of a segment
whenever there is the possibility of changing the match by adding
more characters. The PCRE2_NOTBOL option should also be set for
all but the first segment.
When a partial match occurs, the next segment must be added to
the current subject and the match re-run, using the startoffset
argument of pcre2_match() to begin at the point where the partial
match started. For example:
re>
/\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
data> ...the date is 23ja\=ph
Partial match: 23ja
data> ...the date is 23jan19 and on that day...\=offset=15
0: 23jan19
1: jan
Note the use of the offset modifier to start the new match where
the partial match was found. In this example, the next segment
was added to the one in which the partial match was found. This
is the most straightforward approach, typically using a memory
buffer that is twice the size of each segment. After a partial
match, the first half of the buffer is discarded, the second half
is moved to the start of the buffer, and a new segment is added
before repeating the match as in the example above. After a no
match, the entire buffer can be discarded.
If there are memory constraints, you may want to discard text
that precedes a partial match before adding the next segment.
Unfortunately, this is not at present straightforward. In cases
such as the above, where the pattern does not contain any
lookbehinds, it is sufficient to retain only the partially
matched substring. However, if the pattern contains a lookbehind
assertion, characters that precede the start of the partial match
may have been inspected during the matching process. When
pcre2test displays a partial match, it indicates these characters
with '<' if the allusedtext modifier is set:
re> "(?<=123)abc"
data> xx123ab\=ph,allusedtext
Partial match: 123ab
<<<
However, the allusedtext modifier is not available for JIT
matching, because JIT matching does not record the first (or
last) consulted characters. For this reason, this information is
not available via the API. It is therefore not possible in
general to obtain the exact number of characters that must be
retained in order to get the right match result. If you cannot
retain the entire segment, you must find some heuristic way of
choosing.
If you know the approximate length of the matching substrings,
you can use that to decide how much text to retain. The only
lookbehind information that is currently available via the API is
the length of the longest individual lookbehind in a pattern, but
this can be misleading if there are nested lookbehinds. The value
returned by calling pcre2_pattern_info() with the
PCRE2_INFO_MAXLOOKBEHIND option is the maximum number of
characters (not code units) that any individual lookbehind moves
back when it is processed. A pattern such as "(?<=(?<!b)a)" has a
maximum lookbehind value of one, but inspects two characters
before its starting point.
In a non-UTF or a 32-bit case, moving back is just a subtraction,
but in UTF-8 or UTF-16 you have to count characters while moving
back through the code units.
The DFA 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.
When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned
only if there have been no complete matches. Otherwise, the
complete matches are returned. If PCRE2_PARTIAL_HARD is set, a
partial match takes precedence over any complete matches. The
portion of the string that was matched when the longest partial
match was found is set as the first matching string.
Because the DFA function always searches for all possible
matches, and there is no difference between greedy and ungreedy
repetition, its behaviour is different from the pcre2_match().
Consider the string "dog" matched against this ungreedy pattern:
/dog(sbody)??/
Whereas the standard function stops as soon as it finds the
complete match for "dog", the DFA function also finds the partial
match for "dogsbody", and so returns that when PCRE2_PARTIAL_HARD
is set.
When a partial match has been found using the 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
PCRE2_DFA_RESTART option. You must pass the same working space as
before, because this is where details of the previous partial
match are stored. You can set the PCRE2_PARTIAL_SOFT or
PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART to continue
partial matching over multiple segments. Here is an example using
pcre2test:
re>
/^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\=dfa,ps
Partial match: 23ja
data> n05\=dfa,dfa_restart
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; PCRE2 does not retain the
previously partially-matched string. It is up to the calling
program to do that if it needs to. This 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. 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. Depending on the application, this may or may not
be what you want.
If you do want to allow for starting again at the next character,
one way of doing it is to retain some or all of the segment and
try a new complete match, as described for pcre2_match() above.
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 PCRE2_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
Retired from University Computing Service
Cambridge, England.
Last updated: 04 September 2019
Copyright (c) 1997-2019 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
PCRE2 10.34 04 September 2019 PCRE2PARTIAL(3)
Pages that refer to this page: pcre2api(3)
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