pcre2partial(3) Library Functions Manual pcre2partial(3)
NAME
PCRE2 - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE2
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.
REQUIREMENTS FOR A PARTIAL MATCH
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".
PARTIAL MATCHING USING pcre2_match()
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 "456abc123yz" 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.
MULTI-SEGMENT MATCHING WITH pcre2_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.
PARTIAL MATCHING USING pcre2_dfa_match()
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.
MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()
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.
AUTHOR
Philip Hazel
Retired from University Computing Service
Cambridge, England.
REVISION
Last updated: 04 September 2019
Copyright (c) 1997-2019 University of Cambridge.
PCRE2 10.34 04 September 2019 pcre2partial(3)
pcre2 10.43 - Generated Sat Mar 2 11:34:08 CST 2024