406 lines
18 KiB
HTML
406 lines
18 KiB
HTML
<html>
|
|
<head>
|
|
<title>pcre2partial specification</title>
|
|
</head>
|
|
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
|
<h1>pcre2partial man page</h1>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|
|
<p>
|
|
This page is part of the PCRE2 HTML documentation. It was generated
|
|
automatically from the original man page. If there is any nonsense in it,
|
|
please consult the man page, in case the conversion went wrong.
|
|
<br>
|
|
<ul>
|
|
<li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE2</a>
|
|
<li><a name="TOC2" href="#SEC2">REQUIREMENTS FOR A PARTIAL MATCH</a>
|
|
<li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre2_match()</a>
|
|
<li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre2_match()</a>
|
|
<li><a name="TOC5" href="#SEC5">PARTIAL MATCHING USING pcre2_dfa_match()</a>
|
|
<li><a name="TOC6" href="#SEC6">MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()</a>
|
|
<li><a name="TOC7" href="#SEC7">AUTHOR</a>
|
|
<li><a name="TOC8" href="#SEC8">REVISION</a>
|
|
</ul>
|
|
<br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE2</a><br>
|
|
<P>
|
|
In normal use of PCRE2, if there is a match up to the end of a subject string,
|
|
but more characters are needed to match the entire pattern, PCRE2_ERROR_NOMATCH
|
|
is returned, just like any other failing match. There are circumstances where
|
|
it might be helpful to distinguish this "partial match" case.
|
|
</P>
|
|
<P>
|
|
One example is an application where the subject string is very long, and not
|
|
all available at once. The requirement here is to be able to do the matching
|
|
segment by segment, but special action is needed when a matched substring spans
|
|
the boundary between two segments.
|
|
</P>
|
|
<P>
|
|
Another example is checking a user input string as it is typed, to ensure that
|
|
it conforms to a required format. Invalid characters can be immediately
|
|
diagnosed and rejected, giving instant feedback.
|
|
</P>
|
|
<P>
|
|
Partial matching is a PCRE2-specific feature; it is not Perl-compatible. It is
|
|
requested by setting one of the PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT
|
|
options 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>
|
|
<P>
|
|
If you want to use partial matching with just-in-time optimized code, as well
|
|
as setting a partial match option for the matching function, you must also call
|
|
<b>pcre2_jit_compile()</b> with one or both of these options:
|
|
<pre>
|
|
PCRE2_JIT_PARTIAL_HARD
|
|
PCRE2_JIT_PARTIAL_SOFT
|
|
</pre>
|
|
PCRE2_JIT_COMPLETE should also be set if you are going to run non-partial
|
|
matches on the same pattern. Separate code is compiled for each mode. If the
|
|
appropriate JIT mode has not been compiled, interpretive matching code is used.
|
|
</P>
|
|
<P>
|
|
Setting a partial matching option disables two of PCRE2's standard
|
|
optimization hints. 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 remembers a 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.
|
|
</P>
|
|
<br><a name="SEC2" href="#TOC1">REQUIREMENTS FOR A PARTIAL MATCH</a><br>
|
|
<P>
|
|
A possible partial match occurs during matching when the end of the subject
|
|
string is reached successfully, but either more characters are needed to
|
|
complete the match, or the addition of more characters might change what is
|
|
matched.
|
|
</P>
|
|
<P>
|
|
Example 1: if the pattern is /abc/ and the subject is "ab", more characters are
|
|
definitely needed to complete a match. In this case both hard and soft matching
|
|
options yield a partial match.
|
|
</P>
|
|
<P>
|
|
Example 2: if the pattern is /ab+/ and the subject is "ab", a complete match
|
|
can be found, but the addition of more characters might change what is
|
|
matched. In this case, only PCRE2_PARTIAL_HARD returns a partial match;
|
|
PCRE2_PARTIAL_SOFT returns the complete match.
|
|
</P>
|
|
<P>
|
|
On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if the next
|
|
pattern item is \z, \Z, \b, \B, or $ there is always a partial match.
|
|
Otherwise, for both options, the next pattern item must be one that inspects a
|
|
character, and at least one of the following must be true:
|
|
</P>
|
|
<P>
|
|
(1) At least one character has already been inspected. An inspected 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.
|
|
</P>
|
|
<P>
|
|
(2) The pattern contains one or more lookbehind assertions. This condition
|
|
exists in case there is a lookbehind that inspects characters before the start
|
|
of the match.
|
|
</P>
|
|
<P>
|
|
(3) There is a special case when the whole pattern can match an empty string.
|
|
When the starting point is at the end of the subject, the empty string match is
|
|
a possibility, and if PCRE2_PARTIAL_SOFT is set and neither of the above
|
|
conditions is true, it is returned. However, because adding more characters
|
|
might result in a non-empty match, PCRE2_PARTIAL_HARD returns a partial match,
|
|
which in this case means "there is going to be a match at this point, but until
|
|
some more characters are added, we do not know if it will be an empty string or
|
|
something longer".
|
|
</P>
|
|
<br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre2_match()</a><br>
|
|
<P>
|
|
When a partial matching option is set, the result of calling
|
|
<b>pcre2_match()</b> can be one of the following:
|
|
</P>
|
|
<P>
|
|
<b>A successful match</b>
|
|
A complete match has been found, starting and ending within this subject.
|
|
</P>
|
|
<P>
|
|
<b>PCRE2_ERROR_NOMATCH</b>
|
|
No match can start anywhere in this subject.
|
|
</P>
|
|
<P>
|
|
<b>PCRE2_ERROR_PARTIAL</b>
|
|
Adding more characters may result in a complete match that uses one or more
|
|
characters from the end of this subject.
|
|
</P>
|
|
<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 \K in the pattern has no effect
|
|
for a partial match. Consider this pattern:
|
|
<pre>
|
|
/abc\K123/
|
|
</pre>
|
|
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 subsequent re-match with additional
|
|
characters.
|
|
</P>
|
|
<P>
|
|
If there is more than one partial match, the first one that was found provides
|
|
the data that is returned. Consider this pattern:
|
|
<pre>
|
|
/123\w+X|dogY/
|
|
</pre>
|
|
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. (In this example, there are two partial
|
|
matches, because "dog" on its own partially matches the second alternative.)
|
|
</P>
|
|
<br><b>
|
|
How a partial match is processed by pcre2_match()
|
|
</b><br>
|
|
<P>
|
|
What happens when a partial match is identified depends on which of the two
|
|
partial matching options is set.
|
|
</P>
|
|
<P>
|
|
If PCRE2_PARTIAL_HARD is set, 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 is not the true end of the available data, which is
|
|
why \z, \Z, \b, \B, and $ always give a partial match.
|
|
</P>
|
|
<P>
|
|
If PCRE2_PARTIAL_SOFT is set, 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. 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; \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.
|
|
</P>
|
|
<P>
|
|
The difference between the two partial matching options can be illustrated by a
|
|
pattern such as:
|
|
<pre>
|
|
/dog(sbody)?/
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
/dog(sbody)??/
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
/dog(sbody)?/ is the same as /dogsbody|dog/
|
|
/dog(sbody)??/ is the same as /dog|dogsbody/
|
|
</pre>
|
|
The second pattern will never match "dogsbody", because it will always find the
|
|
shorter match first.
|
|
</P>
|
|
<br><b>
|
|
Example of partial matching using pcre2test
|
|
</b><br>
|
|
<P>
|
|
The <b>pcre2test</b> data modifiers <b>partial_hard</b> (or <b>ph</b>) and
|
|
<b>partial_soft</b> (or <b>ps</b>) set PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT,
|
|
respectively, when calling <b>pcre2_match()</b>. Here is a run of
|
|
<b>pcre2test</b> using a pattern that matches the whole subject in the form of a
|
|
date:
|
|
<pre>
|
|
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
|
|
data> 25dec3\=ph
|
|
Partial match: 23dec3
|
|
data> 3ju\=ph
|
|
Partial match: 3ju
|
|
data> 3juj\=ph
|
|
No match
|
|
</pre>
|
|
This example gives the same results for both hard and soft partial matching
|
|
options. Here is an example where there is a difference:
|
|
<pre>
|
|
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
|
|
data> 25jun04\=ps
|
|
0: 25jun04
|
|
1: jun
|
|
data> 25jun04\=ph
|
|
Partial match: 25jun04
|
|
</pre>
|
|
With PCRE2_PARTIAL_SOFT, the subject is matched completely. For
|
|
PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete, so
|
|
there is only a partial match.
|
|
</P>
|
|
<br><a name="SEC4" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre2_match()</a><br>
|
|
<P>
|
|
PCRE was not originally designed with multi-segment matching in mind. However,
|
|
over time, features (including partial matching) that make multi-segment
|
|
matching possible have been added. The string is searched segment by segment by
|
|
calling <b>pcre2_match()</b> repeatedly, with the aim of achieving the same
|
|
results that would happen if the entire string was available for searching.
|
|
</P>
|
|
<P>
|
|
Special logic must be implemented to handle a matched substring that spans a
|
|
segment boundary. PCRE2_PARTIAL_HARD should be used, because it returns a
|
|
partial match at the end of a segment whenever there is the possibility of
|
|
changing the match by adding more characters. The PCRE2_NOTBOL option should
|
|
also be set for all but the first segment.
|
|
</P>
|
|
<P>
|
|
When a partial match occurs, the next segment must be added to the current
|
|
subject and the match re-run, using the <i>startoffset</i> argument of
|
|
<b>pcre2_match()</b> to begin at the point where the partial match started.
|
|
Multi-segment matching is usually used to search for substrings in the middle
|
|
of very long sequences, so the patterns are normally not anchored. For example:
|
|
<pre>
|
|
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
|
|
data> ...the date is 23jan19 and on that day...\=offset=15
|
|
0: 23jan19
|
|
1: jan
|
|
</pre>
|
|
Note the use of the <b>offset</b> modifier to start the new match where the
|
|
partial match was found.
|
|
</P>
|
|
<P>
|
|
In this simple example, the next segment was just added to the one in which the
|
|
partial match was found. However, if there are memory constraints, it may be
|
|
necessary to discard text that precedes the partial match before adding the
|
|
next segment. In cases such as the above, where the pattern does not contain
|
|
any lookbehinds, it is sufficient to retain only the partially matched
|
|
substring. However, if a pattern contains a lookbehind assertion, characters
|
|
that precede the start of the partial match may have been inspected during the
|
|
matching process.
|
|
</P>
|
|
<P>
|
|
The only lookbehind information that is available is the length of the longest
|
|
lookbehind in a pattern. This may not, of course, be at the start of the
|
|
pattern, but retaining that many characters before the partial match is
|
|
sufficient, if not always strictly necessary. The way to do this is as follows:
|
|
</P>
|
|
<P>
|
|
Before doing any matching, find the length of the longest lookbehind in the
|
|
pattern by calling <b>pcre2_pattern_info()</b> 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.
|
|
</P>
|
|
<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 <b>startoffset</b> for the next match should be 3. When <b>pcre2test</b>
|
|
displays a partial match, it indicates the lookbehind characters with '<'
|
|
characters if the <b>allusedtext</b> modifier is set:
|
|
<pre>
|
|
re> "(?<=123)abc"
|
|
data> xx123ab\=ph,allusedtext
|
|
Partial match: 123ab
|
|
<<<
|
|
</pre>
|
|
Note that the \fPallusedtext\fP modifier is not available for JIT matching,
|
|
because JIT matching does not maintain the first and last consulted characters.
|
|
</P>
|
|
<br><a name="SEC5" href="#TOC1">PARTIAL MATCHING USING pcre2_dfa_match()</a><br>
|
|
<P>
|
|
The DFA function moves 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.
|
|
</P>
|
|
<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.
|
|
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>
|
|
<P>
|
|
Because the DFA function always searches for all possible matches, and there is
|
|
no difference between greedy and ungreedy repetition, its behaviour is
|
|
different from the <b>pcre2_match()</b>. Consider the string "dog" matched
|
|
against this ungreedy pattern:
|
|
<pre>
|
|
/dog(sbody)??/
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<br><a name="SEC6" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()</a><br>
|
|
<P>
|
|
When a partial match has been found using the 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. You can
|
|
set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART
|
|
to continue partial matching over multiple segments. Here is an example using
|
|
<b>pcre2test</b>:
|
|
<pre>
|
|
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
|
|
</pre>
|
|
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. This 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. For example, consider this pattern:
|
|
<pre>
|
|
1234|3789
|
|
</pre>
|
|
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. Depending on the application, this may or may not be what you
|
|
want.
|
|
</P>
|
|
<P>
|
|
If you do want to allow for starting again at the next character, one way of
|
|
doing it is to retain the matched part of the segment and try a new complete
|
|
match, as described for <b>pcre2_match()</b> above. Another possibility is to
|
|
work with two buffers. If a partial match at offset <i>n</i> 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 <i>n+1</i> in the first buffer.
|
|
</P>
|
|
<br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge, England.
|
|
<br>
|
|
</P>
|
|
<br><a name="SEC8" href="#TOC1">REVISION</a><br>
|
|
<P>
|
|
Last updated: 07 August 2019
|
|
<br>
|
|
Copyright © 1997-2019 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|