MaxScale/pcre2/doc/pcre2partial.3

.TH PCRE2PARTIAL 3 "22 December 2014" "PCRE2 10.00"
.SH NAME
PCRE2 - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE2"
.rs
.sp
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 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
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 options are set, PCRE2_PARTIAL_HARD
takes precedence.
.P
If you want to use partial matching with just-in-time optimized code, you must
call \fBpcre2_jit_compile()\fP with one or both of these options:
.sp
  PCRE2_JIT_PARTIAL_SOFT
  PCRE2_JIT_PARTIAL_HARD
.sp
PCRE2_JIT_COMPLETE should also be set if you are going to run non-partial
matches on the same pattern. If the appropriate JIT mode has not been compiled,
interpretive matching code is used.
.P
Setting a partial matching option disables two of PCRE2's standard
optimizations. 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 matching.
.
.
.SH "PARTIAL MATCHING USING pcre2_match()"
.rs
.sp
A partial match occurs during a call to \fBpcre2_match()\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 string. 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
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 \eK in the pattern has no effect
for a partial match. Consider this pattern:
.sp
  /abc\eK123/
.sp
If it is matched against "456abc123xyz" the result is a complete match, and the
ovector defines the matched string as "123", because \eK 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.
.P
What happens when a partial match is identified depends on which of the two
partial matching options are set.
.
.
.SS "PCRE2_PARTIAL_SOFT WITH pcre2_match()"
.rs
.sp
If PCRE2_PARTIAL_SOFT is set when \fBpcre2_match()\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,
PCRE2_ERROR_PARTIAL is returned instead of PCRE2_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 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.)
.
.
.SS "PCRE2_PARTIAL_HARD WITH pcre2_match()"
.rs
.sp
If PCRE2_PARTIAL_HARD is set for \fBpcre2_match()\fP, 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 \ez, \eZ, \eb, \eB, 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.
.
.
.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
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:
.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 pcre2_dfa_match()"
.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 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.
.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 PCRE2_PARTIAL_HARD is set. Consider
the string "dog" matched against the ungreedy pattern shown above:
.sp
  /dog(sbody)??/
.sp
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.
.
.
.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 PCRE2_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 PCRE2_ERROR_PARTIAL. Using PCRE2_PARTIAL_HARD in this case does yield
PCRE2_ERROR_PARTIAL, because then the partial match takes precedence.
.
.
.SH "EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST"
.rs
.sp
If the \fBpartial_soft\fP (or \fBps\fP) modifier is present on a
\fBpcre2test\fP data line, the PCRE2_PARTIAL_SOFT option is used for the match.
Here is a run of \fBpcre2test\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$/
  data> 25jun04\e=ps
   0: 25jun04
   1: jun
  data> 25dec3\e=ps
  Partial match: 23dec3
  data> 3ju\e=ps
  Partial match: 3ju
  data> 3juj\e=ps
  No match
  data> j\e=ps
  No match
.sp
The first data string is matched completely, so \fBpcre2test\fP shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. Similar output is obtained
if DFA matching is used.
.P
If the \fBpartial_hard\fP (or \fBph\fP) modifier is present on a
\fBpcre2test\fP data line, the PCRE2_PARTIAL_HARD option is set for the match.
.
.
.SH "MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()"
.rs
.sp
When a partial match has been found using a DFA matching function, it is
possible to continue the match by providing additional subject data and calling
the function again with the same compiled regular expression, this time setting
the PCRE2_DFA_RESTART option. You must pass the same working space as before,
because this is where details of the previous partial match are stored. Here is
an example using \fBpcre2test\fP:
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 23ja\e=dfa,ps
  Partial match: 23ja
  data> n05\e=dfa,dfa_restart
   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; PCRE2 does
not retain the previously partially-matched string. It is up to the calling
program to do that if it needs to.
.P
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.
.P
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.
.
.
.SH "MULTI-SEGMENT MATCHING WITH pcre2_match()"
.rs
.sp
Unlike the DFA function, it is not possible to restart the previous match with
a new segment of data when using \fBpcre2_match()\fP. 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 PCRE2_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\e=ph
  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 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.
.
.
.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 PCRE2_NOTBOL option when the subject string for any call does start at the
beginning of a line. There is also a PCRE2_NOTEOL option, but in practice when
doing multi-segment matching you should be using PCRE2_PARTIAL_HARD, which
includes the effect of PCRE2_NOTEOL.
.P
2. If a pattern contains a lookbehind assertion, characters that precede the
start of the partial match may have been inspected during the matching process.
When using \fBpcre2_match()\fP, sufficient characters must be retained for the
next match attempt. You can ensure that enough characters are retained by doing
the following:
.P
Before doing any matching, find the length of the longest lookbehind in the
pattern by calling \fBpcre2_pattern_info()\fP 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.
.P
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 \fBstartoffset\fP argument set so that the match begins at the
same point as before.
.P
For example, if the pattern "(?<=123)abc" is partially matched against the
string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maximum
lookbehind count is 3, so all characters before offset 2 can be discarded. The
value of \fBstartoffset\fP for the next match should be 3. When \fBpcre2test\fP
displays a partial match, it indicates the lookbehind characters with '<'
characters:
.sp
    re> "(?<=123)abc"
  data> xx123ab\e=ph
  Partial match: 123ab
                 <<<
.P
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:
.sp
    re> /c(?<=abc)x/
  data> ab\e=ps
  No match
.sp
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.
.P
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
\eb or \eB. 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 possible. Consider this \fBpcre2test\fP example:
.sp
    re> /dog(sbody)?/
  data> dogsb\e=ps
   0: dog
  data> do\e=ps,dfa
  Partial match: do
  data> gsb\e=ps,dfa,dfa_restart
   0: g
  data> dogsbody\e=dfa
   0: dogsbody
   1: dog
.sp
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.
.P
Because of these problems, it is best to use PCRE2_PARTIAL_HARD when matching
multi-segment data. The example above then behaves differently:
.sp
    re> /dog(sbody)?/
  data> dogsb\e=ph
  Partial match: dogsb
  data> do\e=ps,dfa
  Partial match: do
  data> gsb\e=ph,dfa,dfa_restart
  Partial match: gsb
.sp
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:
.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\e=ph
  Partial match: 123
  data> 1237890
   0: 3789
.sp
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 \fIn\fP
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 \fIn+1\fP in
the first buffer.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 22 December 2014
Copyright (c) 1997-2014 University of Cambridge.
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