PCRE2PARTIAL(3) Introduction to Library Functions PCRE2PARTIAL(3)

NAME

PCRE2 - Perl-compatible regular expressions (revised API)

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 "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.

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: 27 November 2024
Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.45 27 November 2024 PCRE2PARTIAL(3)