trunk/doc/pcrepartial.3

.TH PCREPARTIAL 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"
.rs
.sp
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.
.P
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 \fIddmmmyy\fP, defined by this pattern:
.sp
  ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
.sp
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.
.P
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.
.P
Setting a partial matching option disables the use of any just-in-time code
that was set up by studying the compiled pattern with the
PCRE_STUDY_JIT_COMPILE option. It also 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.
.
.
.SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()"
.rs
.sp
A partial match occurs during a call to \fBpcre_exec()\fP or 
\fBpcre16_exec()\fP 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
\eK 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.
.P
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.
.P
For the majority of patterns, the first offset identifies the start of the
partially matched string. However, for patterns that contain lookbehind
assertions, or \eK, or begin with \eb or \eB, earlier characters have been
inspected while carrying out the match. For example:
.sp
  /(?<=abc)123/
.sp
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.
.P
What happens when a partial match is identified depends on which of the two
partial matching options are set.
.
.
.SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()"
.rs
.sp
If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP
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.
.P
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, \ez, \eZ, and $ match at the end of the
subject, as normal, and for \eb and \eB the end of the subject is treated as a
non-alphanumeric.
.P
If there is more than one partial match, the first one that was found provides
the data that is returned. Consider this pattern:
.sp
  /123\ew+X|dogY/
.sp
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.)
.
.
.SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()"
.rs
.sp
If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP,
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 \ez, \eZ, \eb, \eB,
or $ are encountered at the end of the subject, the result is
PCRE_ERROR_PARTIAL.
.P
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.
.
.
.SS "Comparing hard and soft partial matching"
.rs
.sp
The difference between the two partial matching options can be illustrated by a
pattern such as:
.sp
  /dog(sbody)?/
.sp
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:
.sp
  /dog(sbody)??/
.sp
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:
.sp
  /dog(sbody)?/    is the same as  /dogsbody|dog/
  /dog(sbody)??/   is the same as  /dog|dogsbody/
.sp
The second pattern will never match "dogsbody", because it will always find the
shorter match first.
.
.
.SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()"
.rs
.sp
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.
.P
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.
.P
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:
.sp
  /dog(sbody)??/
.sp
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.
.
.
.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
.rs
.sp
If a pattern ends with one of sequences \eb or \eB, which test for word
boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
results. Consider this pattern:
.sp
  /\ebcat\eb/
.sp
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 \eb matches at the end of the subject when the last
character is a letter, so a complete match is found. The result, therefore, is
\fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
.
.
.SH "FORMERLY RESTRICTED PATTERNS"
.rs
.sp
For releases of PCRE prior to 8.00, because of the way certain internal
optimizations were implemented in the \fBpcre_exec()\fP 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.
.P
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, \fBpcre_exec()\fP returned the error code
PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
pattern can be used for partial matching now always returns 1.
.
.
.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
.rs
.sp
If the escape sequence \eP is present in a \fBpcretest\fP data line, the
PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
that uses the date example quoted above:
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 25jun04\eP
   0: 25jun04
   1: jun
  data> 25dec3\eP
  Partial match: 23dec3
  data> 3ju\eP
  Partial match: 3ju
  data> 3juj\eP
  No match
  data> j\eP
  No match
.sp
The first data string is matched completely, so \fBpcretest\fP shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. Similar output is obtained
if DFA matching is used.
.P
If the escape sequence \eP is present more than once in a \fBpcretest\fP data
line, the PCRE_PARTIAL_HARD option is set for the match.
.
.
.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()"
.rs
.sp
When a partial match has been found using a DFA matching function, it is
possible to continue the match by providing additional subject data and calling
the function again with the same compiled regular expression, this time setting
the PCRE_DFA_RESTART option. You must pass the same working space as before,
because this is where details of the previous partial match are stored. Here is
an example using \fBpcretest\fP, using the \eR escape sequence to set the
PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 23ja\eP\eD
  Partial match: 23ja
  data> n05\eR\eD
   0: n05
.sp
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.
.P
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.
.
.
.SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()"
.rs
.sp
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.
.P
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 \ez, \eZ,
\eb, \eB, and $. Consider an unanchored pattern that matches dates:
.sp
    re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
  data> The date is 23ja\eP\eP
  Partial match: 23ja
.sp
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.
.P
\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
with \eb or \eB, 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.
.
.
.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
.rs
.sp
Certain types of pattern may give problems with multi-segment matching,
whichever matching function is used.
.P
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.
.P
2. Lookbehind assertions at the start of a pattern are catered for in the
offsets that are returned for a partial match. However, in theory, a lookbehind
assertion later in the pattern could require even earlier characters to be
inspected, and it might not have been reached when a partial match occurs. This
is probably an extremely unlikely case; you could guard against it to a certain
extent by always including extra characters at the start.
.P
3. 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
\eb or \eB. 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 \fBpcretest\fP example:
.sp
    re> /dog(sbody)?/
  data> dogsb\eP
   0: dog
  data> do\eP\eD
  Partial match: do
  data> gsb\eR\eP\eD
   0: g
  data> dogsbody\eD
   0: dogsbody
   1: dog
.sp
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.
.P
Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
multi-segment data. The example above then behaves differently:
.sp
    re> /dog(sbody)?/
  data> dogsb\eP\eP
  Partial match: dogsb
  data> do\eP\eD
  Partial match: do
  data> gsb\eR\eP\eP\eD
  Partial match: gsb
.sp
4. 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:
.sp
  1234|3789
.sp
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:
.sp
  1234|ABCD
.sp
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:
.sp
    re> /1234|3789/
  data> ABC123\eP\eP
  Partial match: 123
  data> 1237890
   0: 3789
.sp
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 \fIn\fP
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 \fIn+1\fP in
the first buffer.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 08 January 2012
Copyright (c) 1997-2012 University of Cambridge.
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