pcrepartial(3)
PCREPARTIAL(3) C LIBRARY FUNCTIONS PCREPARTIAL(3)
NAME
PCRE - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE
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 match-
ing 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 optim-
ized 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 optimiza-
tions. PCRE remembers the last literal data unit in a pat-
tern, 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
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function on shorter strings. This optimization is also dis-
abled for partial matching.
PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()
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 asser-
tions 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 ear-
liest 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, con-
sider 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 iden-
tified 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
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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. How-
ever, 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:
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/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 por-
tion of the string that was inspected when the longest par-
tial 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.
PARTIAL MATCHING AND WORD BOUNDARIES
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
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comparison of the final "t" with a following character can-
not take place, so a partial match is found. However, normal
matching carries on, and \b matches at the end of the sub-
ject 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.
FORMERLY RESTRICTED PATTERNS
For releases of PCRE prior to 8.00, because of the way cer-
tain 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.
EXAMPLE OF PARTIAL MATCHING USING PCRETEST
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$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match: 23dec3
data> 3ju\P
Partial match: 3ju
data> 3juj\P
No match
data> j\P
No match
The first data string is matched completely, so pcretest
shows the matched substrings. The remaining four strings do
not match the complete pattern, but the first two are par-
tial matches. Similar output is obtained if DFA matching is
used. If the escape sequence \P is present more than once
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in a pcretest data line, the PCRE_PARTIAL_HARD option is set
for the match.
MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR
pcre[16|32]_dfa_exec()
When a partial match has been found using a DFA matching
function, it is possible to continue the match by providing
additional subject data and calling the function again with
the same compiled regular expression, this time setting the
PCRE_DFA_RESTART option. You must pass the same working
space as before, because this is where details of the previ-
ous partial match are stored. Here is an example using
pcretest, using the \R escape sequence to set the
PCRE_DFA_RESTART option (\D specifies the use of the DFA
matching function):
re>
/^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\P\D
Partial match: 23ja
data> n05\R\D
0: n05
The first call has "23ja" as the subject, and requests par-
tial 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.
MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()
From release 8.00, the standard matching functions can also
be used to do multi-segment matching. Unlike the DFA func-
tions, 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,
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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 pat-
tern 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 preced-
ing "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.
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 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
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started at the offsets[2] character by setting the star-
toffset 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 look-
behind 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 sub-
ject 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
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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 dif-
ferently:
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, con-
sider 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 alterna-
tives. 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
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0: 3789
Of course, instead of using PCRE_DFA_RESTART, the same tech-
nique 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.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 02 July 2013
Copyright (c) 1997-2013 University of Cambridge.
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