Provided by: libpcre3-dev_8.31-2ubuntu2.3_amd64 
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 matching 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 optimized code, you must call
pcre_study() or pcre16_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 optimizations. PCRE
remembers the last literal data 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. If the pattern was studied, PCRE 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 matching.
PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()
A partial match occurs during a call to pcre_exec() or pcre16_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
assertions 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 earliest character that was inspected. For
convenience, the second offset points to the end of the subject so that a substring can
easily be identified.
For the majority of patterns, the first offset identifies the start of the partially
matched string. However, for patterns that contain lookbehind assertions, or \K, or begin
with \b or \B, earlier characters have been inspected while carrying out the match. For
example:
/(?<=abc)123/
This pattern matches "123", but only if it is preceded by "abc". If the subject string is
"xyzabc12", the offsets after a partial match are for the substring "abc12", because all
these characters are needed if another match is tried with extra characters added to the
subject.
What happens when a partial match is identified depends on which of the two partial
matching options are set.
PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()
If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre16_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 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 pcre16_exec()
If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre16_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. However, 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:
/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 pcre16_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
portion of the string that was inspected when the longest partial 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 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 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 certain 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$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match: 23dec3
data> 3ju\P
Partial match: 3ju
data> 3juj\P
No match
data> j\P
No match
The first data string is matched completely, so pcretest shows the matched substrings. The
remaining four strings do not match the complete pattern, but the first two are partial
matches. Similar output is obtained if DFA matching is used.
If the escape sequence \P is present more than once in a pcretest data line, the
PCRE_PARTIAL_HARD option is set for the match.
MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()
When a partial match has been found using a DFA matching function, it is possible to
continue the match by providing additional subject data and calling the function again
with the same compiled regular expression, this time setting the PCRE_DFA_RESTART option.
You must pass the same working space as before, because this is where details of the
previous partial match are stored. Here is an example using pcretest, using the \R escape
sequence to set the PCRE_DFA_RESTART option (\D specifies the use of the DFA matching
function):
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\P\D
Partial match: 23ja
data> n05\R\D
0: n05
The first call has "23ja" as the subject, and requests 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; PCRE does not retain the previously partially-
matched string. It is up to the calling program to do that if it needs to.
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 pcre16_exec()
From release 8.00, the standard matching functions can also be used to do multi-segment
matching. Unlike the DFA functions, 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, 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 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\P\P
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 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 partially
matched string itself, because these must be retained when adding on more characters for a
subsequent matching attempt. However, in some cases you may need to retain even earlier
characters, as discussed in the next section.
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 pcre16_fullinfo() functions
to obtain the length of the largest 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.)
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 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
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
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 differently:
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,
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 entire match has to be rerun each time:
re> /1234|3789/
data> ABC123\P\P
Partial match: 123
data> 1237890
0: 3789
Of course, instead of using PCRE_DFA_RESTART, the same technique 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: 24 February 2012
Copyright (c) 1997-2012 University of Cambridge.