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pcre2partial(3)
PCRE2PARTIAL(3) Library Functions Manual PCRE2PARTIAL(3)
NAME
PCRE2 - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE2
In normal use of PCRE2, 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, PCRE2_ERROR_NOMATCH is returned. There are circum‐
stances 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 re‐
flecting 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.
PCRE2 supports partial matching by means of the PCRE2_PARTIAL_SOFT and
PCRE2_PARTIAL_HARD options, which can be set 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 op‐
tions are set, PCRE2_PARTIAL_HARD takes precedence.
If you want to use partial matching with just-in-time optimized code,
you must call pcre2_jit_compile() with one or both of these options:
PCRE2_JIT_PARTIAL_SOFT
PCRE2_JIT_PARTIAL_HARD
PCRE2_JIT_COMPLETE should also be set if you are going to run non-par‐
tial matches on the same pattern. 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 op‐
timizations. 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 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 match‐
ing.
PARTIAL MATCHING USING pcre2_match()
A partial match occurs during a call to pcre2_match() when the end of
the subject string is reached successfully, but matching cannot con‐
tinue because more characters are needed. However, at least one charac‐
ter 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 string. The requirement for inspecting at least one char‐
acter 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.
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 pat‐
tern 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 subse‐
quent re-match with additional characters.
What happens when a partial match is identified depends on which of the
two partial matching options are set.
PCRE2_PARTIAL_SOFT WITH pcre2_match()
If PCRE2_PARTIAL_SOFT is set when pcre2_match() identifies a partial
match, the partial match is remembered, but matching continues as nor‐
mal, 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 par‐
tial 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 alter‐
natives 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 that was found.
(In this example, there are two partial matches, because "dog" on its
own partially matches the second alternative.)
PCRE2_PARTIAL_HARD WITH pcre2_match()
If PCRE2_PARTIAL_HARD is set for pcre2_match(), 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 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 PCRE2_ERROR_PARTIAL, provided that at least one character in the
subject has been inspected.
Comparing hard and soft partial matching
The difference between the two partial matching options can be illus‐
trated 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_PAR‐
TIAL. 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 pcre2_dfa_match()
The DFA functions move along the subject string character by character,
without backtracking, searching for all possible matches simultane‐
ously. If the end of the subject is reached before the end of the pat‐
tern, there is the possibility of a partial match, again provided that
at least one character has been inspected.
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. However, 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 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 PCRE2_PAR‐
TIAL_HARD is set. Consider the string "dog" matched against the un‐
greedy pattern shown above:
/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.
PARTIAL MATCHING AND WORD BOUNDARIES
If a pattern ends with one of sequences \b or \B, which test for word
boundaries, partial matching with PCRE2_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 PCRE2_ERROR_PARTIAL. Using
PCRE2_PARTIAL_HARD in this case does yield PCRE2_ERROR_PARTIAL, because
then the partial match takes precedence.
EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST
If the partial_soft (or ps) modifier is present on a pcre2test data
line, the PCRE2_PARTIAL_SOFT option is used for the match. Here is a
run of pcre2test that uses the date example quoted above:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25jun04\=ps
0: 25jun04
1: jun
data> 25dec3\=ps
Partial match: 23dec3
data> 3ju\=ps
Partial match: 3ju
data> 3juj\=ps
No match
data> j\=ps
No match
The first data string is matched completely, so pcre2test shows the
matched substrings. The remaining four strings do not match the com‐
plete pattern, but the first two are partial matches. Similar output is
obtained if DFA matching is used.
If the partial_hard (or ph) modifier is present on a pcre2test data
line, the PCRE2_PARTIAL_HARD option is set for the match.
MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()
When a partial match has been found using a DFA matching function, it
is possible to continue the match by providing additional subject data
and calling the function again with the same compiled regular expres‐
sion, this time setting the PCRE2_DFA_RESTART option. You must pass the
same working space as before, because this is where details of the pre‐
vious partial match are stored. Here is an example using pcre2test:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
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 match‐
ing; 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.
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 fa‐
cility is capable of doing is continuing with the previous match at‐
tempt. 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 start‐
ing again at the next character is to retain the matched part of the
subject and try a new complete match.
You can set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with
PCRE2_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.
MULTI-SEGMENT MATCHING WITH pcre2_match()
Unlike the DFA function, it is not possible to restart the previous
match with a new segment of data when using pcre2_match(). 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. Ear‐
lier data can be discarded.
It is best to use PCRE2_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\=ph
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 function, 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.
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 PCRE2_NOTBOL option when the subject string for any call
does start at the beginning of a line. There is also a PCRE2_NOTEOL op‐
tion, but in practice when doing multi-segment matching you should be
using PCRE2_PARTIAL_HARD, which includes the effect of PCRE2_NOTEOL.
2. If a pattern contains a lookbehind assertion, characters that pre‐
cede the start of the partial match may have been inspected during the
matching process. When using pcre2_match(), sufficient characters must
be retained for the next match attempt. You can ensure that enough
characters are retained by doing the following:
Before doing any matching, find the length of the longest lookbehind in
the pattern by calling pcre2_pattern_info() with the
PCRE2_INFO_MAXLOOKBEHIND option. Note that the resulting count is in
characters, not code units. After a partial match, moving back from the
ovector[0] offset in the subject by the number of characters given for
the maximum lookbehind gets you to the earliest character that must be
retained. In a non-UTF or a 32-bit situation, 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.
Characters before the point you have now reached can be discarded, and
after the next segment has been added to what is retained, you should
run the next match with the startoffset argument set so that the match
begins at the same point as before.
For example, if the pattern "(?<=123)abc" is partially matched against
the string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maxi‐
mum lookbehind count is 3, so all characters before offset 2 can be
discarded. The value of startoffset for the next match should be 3.
When pcre2test displays a partial match, it indicates the lookbehind
characters with '<' characters:
re> "(?<=123)abc"
data> xx123ab\=ph
Partial match: 123ab
<<<
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\=ps
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 PCRE2_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
PCRE2_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 possi‐
ble. Consider this pcre2test example:
re> /dog(sbody)?/
data> dogsb\=ps
0: dog
data> do\=ps,dfa
Partial match: do
data> gsb\=ps,dfa,dfa_restart
0: g
data> dogsbody\=dfa
0: dogsbody
1: dog
The first data line passes the string "dogsb" to a standard matching
function, setting the PCRE2_PARTIAL_SOFT option. Although the string is
a partial match for "dogsbody", the result is not PCRE2_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 PCRE2_PARTIAL_HARD when
matching multi-segment data. The example above then behaves differ‐
ently:
re> /dog(sbody)?/
data> dogsb\=ph
Partial match: dogsb
data> do\=ps,dfa
Partial match: do
data> gsb\=ph,dfa,dfa_restart
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
PCRE2_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 en‐
tire match has to be rerun each time:
re> /1234|3789/
data> ABC123\=ph
Partial match: 123
data> 1237890
0: 3789
Of course, instead of using PCRE2_DFA_RESTART, the same technique of
re-running the entire match can also be used with the DFA matching
function. 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
University Computing Service
Cambridge, England.
REVISION
Last updated: 22 December 2014
Copyright (c) 1997-2014 University of Cambridge.
PCRE2 10.00 22 December 2014 PCRE2PARTIAL(3)