doc/html/pcrepartial.html

<html>
<head>
<title>pcrepartial specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcrepartial man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE 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 PCRE</a>
<li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec()</a>
<li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec()</a>
<li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a>
<li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
<li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
<li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>
<li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec()</a>
<li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a>
<li><a name="TOC10" href="#SEC10">AUTHOR</a>
<li><a name="TOC11" href="#SEC11">REVISION</a>
</ul>
<br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
<P>
In normal use of PCRE, if the subject string that is passed to
<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> 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>
<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 <i>ddmmmyy</i>, defined by this pattern:
<pre>
  ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
</pre>
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 sometimes be useful when the subject string
is very long and is not all available at once.
</P>
<P>
PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
PCRE_PARTIAL_HARD options, which can be set when calling <b>pcre_exec()</b> or
<b>pcre_dfa_exec()</b>. 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 matching functions. If both options
are set, PCRE_PARTIAL_HARD takes precedence.
</P>
<P>
Setting a partial matching option disables two of PCRE's optimizations. PCRE
remembers the last literal byte in a pattern, and abandons matching immediately
if such a byte 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.
</P>
<br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec()</a><br>
<P>
A partial match occurs during a call to <b>pcre_exec()</b> whenever the end of
the subject string is reached successfully, but matching cannot continue
because more characters are needed. However, at least one character must have
been matched. (In other words, a partial match can never be an empty string.)
</P>
<P>
If PCRE_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, <b>pcre_exec()</b> returns PCRE_ERROR_PARTIAL
instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
vector, the first of them is set to the offset of the earliest character that
was inspected when the partial match was found. For convenience, the second
offset points to the end of the string so that a substring can easily be
identified.
</P>
<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 \K, or begin with \b or \B, earlier characters have been
inspected while carrying out the match. For example:
<pre>
  /(?&#60;=abc)123/
</pre>
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.
</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 PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. 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.)
</P>
<P>
If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b>, it returns
PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
search for possible complete matches. The difference between the two 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
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:
<pre>
  /dog(sbody)??/
</pre>
In this case the result is always a complete match because <b>pcre_exec()</b>
finds that first, and it never continues after finding a 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" when <b>pcre_exec()</b> is
used, because it will always find the shorter match first.
</P>
<br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec()</a><br>
<P>
The <b>pcre_dfa_exec()</b> 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, again provided that at
least one character has matched.
</P>
<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>
<P>
Because <b>pcre_dfa_exec()</b> always searches for all possible matches, and
there is no difference between greedy and ungreedy repetition, its behaviour is
different from <b>pcre_exec</b> when PCRE_PARTIAL_HARD is set. Consider the
string "dog" matched against the ungreedy pattern shown above:
<pre>
  /dog(sbody)??/
</pre>
Whereas <b>pcre_exec()</b> stops as soon as it finds the complete match for
"dog", <b>pcre_dfa_exec()</b> also finds the partial match for "dogsbody", and
so returns that when PCRE_PARTIAL_HARD is set.
</P>
<br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
<P>
If a pattern ends with one of sequences \w or \W, which test for word
boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
results. Consider this pattern:
<pre>
  /\bcat\b/
</pre>
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,
<b>pcre_exec()</b> carries on with normal matching, which matches \b at the end
of the subject when the last character is a letter, thus finding a complete
match. The result, therefore, is <i>not</i> PCRE_ERROR_PARTIAL. The same thing
happens with <b>pcre_dfa_exec()</b>, because it also finds the complete match.
</P>
<P>
Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
then the partial match takes precedence.
</P>
<br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
<P>
For releases of PCRE prior to 8.00, because of the way certain internal
optimizations were implemented in the <b>pcre_exec()</b> 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 <b>pcre_exec()</b> can be requested for any pattern.
</P>
<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, <b>pcre_exec()</b> returned the error code
PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
pattern can be used for partial matching now always returns 1.
</P>
<br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
<P>
If the escape sequence \P is present in a <b>pcretest</b> data line, the
PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
that uses the date example quoted above:
<pre>
    re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
  data&#62; 25jun04\P
   0: 25jun04
   1: jun
  data&#62; 25dec3\P
  Partial match: 23dec3
  data&#62; 3ju\P
  Partial match: 3ju
  data&#62; 3juj\P
  No match
  data&#62; j\P
  No match
</pre>
The first data string is matched completely, so <b>pcretest</b> shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. Similar output is obtained
when <b>pcre_dfa_exec()</b> is used.
</P>
<P>
If the escape sequence \P is present more than once in a <b>pcretest</b> data
line, the PCRE_PARTIAL_HARD option is set for the match.
</P>
<br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>
<P>
When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible
to continue the match by providing additional subject data and calling
<b>pcre_dfa_exec()</b> again with the same compiled regular expression, this
time setting the PCRE_DFA_RESTART option. You must pass the same working
space as before, because this is where details of the previous partial match
are stored. Here is an example using <b>pcretest</b>, using the \R escape
sequence to set the PCRE_DFA_RESTART option (\D specifies the use of
<b>pcre_dfa_exec()</b>):
<pre>
    re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
  data&#62; 23ja\P\D
  Partial match: 23ja
  data&#62; n05\R\D
   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; PCRE does
not retain the previously partially-matched string. It is up to the calling
program to do that if it needs to.
</P>
<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
<b>pcre_dfa_exec()</b>.
</P>
<br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec()</a><br>
<P>
From release 8.00, <b>pcre_exec()</b> can also be used to do multi-segment
matching. Unlike <b>pcre_dfa_exec()</b>, 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.
Consider an unanchored pattern that matches dates:
<pre>
    re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
  data&#62; The date is 23ja\P
  Partial match: 23ja
</pre>
The this stage, an application could discard the text preceding "23ja", add on
text from the next segment, and call <b>pcre_exec()</b> again. Unlike
<b>pcre_dfa_exec()</b>, 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>
<P>
<b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
with \b or \B, the string that is returned for a partial match will include
characters that precede the partially matched string itself, because these must
be retained when adding on more characters for a subsequent matching attempt.
</P>
<br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
<P>
Certain types of pattern may give problems with multi-segment matching,
whichever matching function is used.
</P>
<P>
1. If the pattern contains tests for the beginning or end of a line, you need
to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
subject string for any call does not contain the beginning or end of a line.
</P>
<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>
<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
\b or \B. Another kind of difference may occur when there are multiple
matching possibilities, because 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 <b>pcretest</b> example:
<pre>
    re&#62; /dog(sbody)?/
  data&#62; dogsb\P
   0: dog
  data&#62; do\P\D
  Partial match: do
  data&#62; gsb\R\P\D
   0: g
  data&#62; dogsbody\D
   0: dogsbody
   1: dog
</pre>
The first data line passes the string "dogsb" to <b>pcre_exec()</b>, 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
<b>pcre_dfa_exec()</b> 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,
<b>pcre_dfa_exec()</b> finds both matches.
</P>
<P>
Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
matching multi-segment data. The example above then behaves differently:
<pre>
    re&#62; /dog(sbody)?/
  data&#62; dogsb\P\P
  Partial match: dogsb
  data&#62; do\P\D
  Partial match: do
  data&#62; gsb\R\P\P\D
  Partial match: gsb

</PRE>
</P>
<P>
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
<b>pcre_dfa_exec()</b> is used. 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. 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:
<pre>
  1234|ABCD
</pre>
where no string can be a partial match for both alternatives. This is not a
problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
each time:
<pre>
    re&#62; /1234|3789/
  data&#62; ABC123\P
  Partial match: 123
  data&#62; 1237890
   0: 3789

</PRE>
</P>
<br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><a name="SEC11" href="#TOC1">REVISION</a><br>
<P>
Last updated: 29 September 2009
<br>
Copyright &copy; 1997-2009 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	75	<html>
2			<head>
3			<title>pcrepartial specification</title>
4			</head>
5			<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6			<h1>pcrepartial man page</h1>
7			<p>
8			Return to the <a href="index.html">PCRE index page</a>.
9			</p>
10	ph10	111	<p>
11	nigel	75	This page is part of the PCRE HTML documentation. It was generated automatically
12			from the original man page. If there is any nonsense in it, please consult the
13			man page, in case the conversion went wrong.
14	ph10	111	<br>
15	nigel	75	<ul>
16			<li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
17	ph10	429	<li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec()</a>
18			<li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec()</a>
19			<li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a>
20			<li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
21			<li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
22			<li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>
23			<li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec()</a>
24			<li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a>
25			<li><a name="TOC10" href="#SEC10">AUTHOR</a>
26			<li><a name="TOC11" href="#SEC11">REVISION</a>
27	nigel	75	</ul>
28			<br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
29			<P>
30			In normal use of PCRE, if the subject string that is passed to
31	nigel	77	<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> matches as far as it goes, but is
32			too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
33			are circumstances where it might be helpful to distinguish this case from other
34			cases in which there is no match.
35	nigel	75	</P>
36			<P>
37			Consider, for example, an application where a human is required to type in data
38			for a field with specific formatting requirements. An example might be a date
39			in the form <i>ddmmmyy</i>, defined by this pattern:
40			<pre>
41			^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$
42			</pre>
43			If the application sees the user's keystrokes one by one, and can check that
44			what has been typed so far is potentially valid, it is able to raise an error
45	ph10	429	as soon as a mistake is made, by beeping and not reflecting the character that
46			has been typed, for example. This immediate feedback is likely to be a better
47	nigel	75	user interface than a check that is delayed until the entire string has been
48	ph10	429	entered. Partial matching can also sometimes be useful when the subject string
49			is very long and is not all available at once.
50	nigel	75	</P>
51			<P>
52	ph10	429	PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
53			PCRE_PARTIAL_HARD options, which can be set when calling <b>pcre_exec()</b> or
54	ph10	453	<b>pcre_dfa_exec()</b>. For backwards compatibility, PCRE_PARTIAL is a synonym
55			for PCRE_PARTIAL_SOFT. The essential difference between the two options is
56			whether or not a partial match is preferred to an alternative complete match,
57			though the details differ between the two matching functions. If both options
58	ph10	429	are set, PCRE_PARTIAL_HARD takes precedence.
59	nigel	75	</P>
60			<P>
61	ph10	461	Setting a partial matching option disables two of PCRE's optimizations. PCRE
62	ph10	429	remembers the last literal byte in a pattern, and abandons matching immediately
63			if such a byte is not present in the subject string. This optimization cannot
64	ph10	461	be used for a subject string that might match only partially. If the pattern
65			was studied, PCRE knows the minimum length of a matching string, and does not
66			bother to run the matching function on shorter strings. This optimization is
67			also disabled for partial matching.
68	nigel	77	</P>
69	ph10	429	<br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec()</a><br>
70	nigel	77	<P>
71	ph10	429	A partial match occurs during a call to <b>pcre_exec()</b> whenever the end of
72			the subject string is reached successfully, but matching cannot continue
73			because more characters are needed. However, at least one character must have
74			been matched. (In other words, a partial match can never be an empty string.)
75	nigel	75	</P>
76			<P>
77	ph10	429	If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
78			continues as normal, and other alternatives in the pattern are tried. If no
79			complete match can be found, <b>pcre_exec()</b> returns PCRE_ERROR_PARTIAL
80	ph10	453	instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
81			vector, the first of them is set to the offset of the earliest character that
82			was inspected when the partial match was found. For convenience, the second
83			offset points to the end of the string so that a substring can easily be
84	ph10	461	identified.
85	ph10	453	</P>
86			<P>
87			For the majority of patterns, the first offset identifies the start of the
88			partially matched string. However, for patterns that contain lookbehind
89			assertions, or \K, or begin with \b or \B, earlier characters have been
90			inspected while carrying out the match. For example:
91	nigel	75	<pre>
92	ph10	453	/(?<=abc)123/
93			</pre>
94			This pattern matches "123", but only if it is preceded by "abc". If the subject
95			string is "xyzabc12", the offsets after a partial match are for the substring
96			"abc12", because all these characters are needed if another match is tried
97			with extra characters added.
98			</P>
99			<P>
100			If there is more than one partial match, the first one that was found provides
101			the data that is returned. Consider this pattern:
102			<pre>
103	ph10	429	/123\w+X\|dogY/
104	nigel	75	</pre>
105	ph10	429	If this is matched against the subject string "abc123dog", both
106	ph10	453	alternatives fail to match, but the end of the subject is reached during
107	ph10	429	matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
108	ph10	453	offsets are set to 3 and 9, identifying "123dog" as the first partial match
109	ph10	429	that was found. (In this example, there are two partial matches, because "dog"
110			on its own partially matches the second alternative.)
111			</P>
112			<P>
113	ph10	453	If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b>, it returns
114	ph10	429	PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
115			search for possible complete matches. The difference between the two options
116			can be illustrated by a pattern such as:
117	nigel	75	<pre>
118	ph10	429	/dog(sbody)?/
119	nigel	75	</pre>
120	ph10	453	This matches either "dog" or "dogsbody", greedily (that is, it prefers the
121	ph10	429	longer string if possible). If it is matched against the string "dog" with
122	ph10	453	PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
123			PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
124	ph10	429	if the pattern is made ungreedy the result is different:
125	nigel	75	<pre>
126	ph10	429	/dog(sbody)??/
127	nigel	75	</pre>
128	ph10	453	In this case the result is always a complete match because <b>pcre_exec()</b>
129			finds that first, and it never continues after finding a match. It might be
130	ph10	429	easier to follow this explanation by thinking of the two patterns like this:
131			<pre>
132			/dog(sbody)?/ is the same as /dogsbody\|dog/
133			/dog(sbody)??/ is the same as /dog\|dogsbody/
134			</pre>
135	ph10	453	The second pattern will never match "dogsbody" when <b>pcre_exec()</b> is
136	ph10	429	used, because it will always find the shorter match first.
137	nigel	75	</P>
138	ph10	429	<br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec()</a><br>
139	nigel	75	<P>
140	ph10	453	The <b>pcre_dfa_exec()</b> function moves along the subject string character by
141			character, without backtracking, searching for all possible matches
142			simultaneously. If the end of the subject is reached before the end of the
143	ph10	429	pattern, there is the possibility of a partial match, again provided that at
144			least one character has matched.
145	nigel	75	</P>
146			<P>
147	ph10	429	When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
148			have been no complete matches. Otherwise, the complete matches are returned.
149			However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
150	ph10	453	complete matches. The portion of the string that was inspected when the longest
151			partial match was found is set as the first matching string, provided there are
152			at least two slots in the offsets vector.
153	ph10	429	</P>
154			<P>
155	ph10	453	Because <b>pcre_dfa_exec()</b> always searches for all possible matches, and
156	ph10	429	there is no difference between greedy and ungreedy repetition, its behaviour is
157	ph10	453	different from <b>pcre_exec</b> when PCRE_PARTIAL_HARD is set. Consider the
158	ph10	429	string "dog" matched against the ungreedy pattern shown above:
159			<pre>
160			/dog(sbody)??/
161			</pre>
162	ph10	453	Whereas <b>pcre_exec()</b> stops as soon as it finds the complete match for
163	ph10	429	"dog", <b>pcre_dfa_exec()</b> also finds the partial match for "dogsbody", and
164			so returns that when PCRE_PARTIAL_HARD is set.
165			</P>
166			<br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
167			<P>
168	ph10	453	If a pattern ends with one of sequences \w or \W, which test for word
169			boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
170	ph10	429	results. Consider this pattern:
171			<pre>
172			/\bcat\b/
173			</pre>
174			This matches "cat", provided there is a word boundary at either end. If the
175			subject string is "the cat", the comparison of the final "t" with a following
176	ph10	453	character cannot take place, so a partial match is found. However,
177			<b>pcre_exec()</b> carries on with normal matching, which matches \b at the end
178			of the subject when the last character is a letter, thus finding a complete
179			match. The result, therefore, is <i>not</i> PCRE_ERROR_PARTIAL. The same thing
180	ph10	429	happens with <b>pcre_dfa_exec()</b>, because it also finds the complete match.
181			</P>
182			<P>
183	ph10	453	Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
184	ph10	429	then the partial match takes precedence.
185			</P>
186			<br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
187			<P>
188			For releases of PCRE prior to 8.00, because of the way certain internal
189			optimizations were implemented in the <b>pcre_exec()</b> function, the
190			PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
191			all patterns. From release 8.00 onwards, the restrictions no longer apply, and
192			partial matching with <b>pcre_exec()</b> can be requested for any pattern.
193			</P>
194			<P>
195			Items that were formerly restricted were repeated single characters and
196			repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
197			conform to the restrictions, <b>pcre_exec()</b> returned the error code
198			PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
199			PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
200			pattern can be used for partial matching now always returns 1.
201			</P>
202			<br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
203			<P>
204	nigel	75	If the escape sequence \P is present in a <b>pcretest</b> data line, the
205	ph10	429	PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
206			that uses the date example quoted above:
207	nigel	75	<pre>
208			re> /^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$/
209			data> 25jun04\P
210			0: 25jun04
211			1: jun
212			data> 25dec3\P
213	ph10	429	Partial match: 23dec3
214	nigel	75	data> 3ju\P
215	ph10	429	Partial match: 3ju
216	nigel	75	data> 3juj\P
217			No match
218			data> j\P
219			No match
220			</pre>
221			The first data string is matched completely, so <b>pcretest</b> shows the
222			matched substrings. The remaining four strings do not match the complete
223	ph10	429	pattern, but the first two are partial matches. Similar output is obtained
224			when <b>pcre_dfa_exec()</b> is used.
225	nigel	75	</P>
226			<P>
227	ph10	429	If the escape sequence \P is present more than once in a <b>pcretest</b> data
228			line, the PCRE_PARTIAL_HARD option is set for the match.
229			</P>
230			<br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>
231			<P>
232	nigel	77	When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible
233			to continue the match by providing additional subject data and calling
234	nigel	93	<b>pcre_dfa_exec()</b> again with the same compiled regular expression, this
235	ph10	429	time setting the PCRE_DFA_RESTART option. You must pass the same working
236	nigel	93	space as before, because this is where details of the previous partial match
237			are stored. Here is an example using <b>pcretest</b>, using the \R escape
238	ph10	429	sequence to set the PCRE_DFA_RESTART option (\D specifies the use of
239			<b>pcre_dfa_exec()</b>):
240	nigel	77	<pre>
241			re> /^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$/
242			data> 23ja\P\D
243			Partial match: 23ja
244			data> n05\R\D
245			0: n05
246			</pre>
247			The first call has "23ja" as the subject, and requests partial matching; the
248			second call has "n05" as the subject for the continued (restarted) match.
249			Notice that when the match is complete, only the last part is shown; PCRE does
250			not retain the previously partially-matched string. It is up to the calling
251			program to do that if it needs to.
252			</P>
253			<P>
254	ph10	429	You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
255			PCRE_DFA_RESTART to continue partial matching over multiple segments. This
256			facility can be used to pass very long subject strings to
257			<b>pcre_dfa_exec()</b>.
258	nigel	77	</P>
259	ph10	429	<br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec()</a><br>
260	nigel	77	<P>
261	ph10	453	From release 8.00, <b>pcre_exec()</b> can also be used to do multi-segment
262			matching. Unlike <b>pcre_dfa_exec()</b>, it is not possible to restart the
263			previous match with a new segment of data. Instead, new data must be added to
264			the previous subject string, and the entire match re-run, starting from the
265	ph10	429	point where the partial match occurred. Earlier data can be discarded.
266			Consider an unanchored pattern that matches dates:
267			<pre>
268			re> /\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d/
269			data> The date is 23ja\P
270			Partial match: 23ja
271			</pre>
272	ph10	453	The this stage, an application could discard the text preceding "23ja", add on
273			text from the next segment, and call <b>pcre_exec()</b> again. Unlike
274			<b>pcre_dfa_exec()</b>, the entire matching string must always be available, and
275			the complete matching process occurs for each call, so more memory and more
276	ph10	429	processing time is needed.
277			</P>
278	ph10	453	<P>
279			<b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
280			with \b or \B, the string that is returned for a partial match will include
281			characters that precede the partially matched string itself, because these must
282			be retained when adding on more characters for a subsequent matching attempt.
283			</P>
284	ph10	429	<br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
285			<P>
286	ph10	453	Certain types of pattern may give problems with multi-segment matching,
287	ph10	429	whichever matching function is used.
288			</P>
289			<P>
290	nigel	77	1. If the pattern contains tests for the beginning or end of a line, you need
291			to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
292			subject string for any call does not contain the beginning or end of a line.
293			</P>
294			<P>
295	ph10	453	2. Lookbehind assertions at the start of a pattern are catered for in the
296			offsets that are returned for a partial match. However, in theory, a lookbehind
297			assertion later in the pattern could require even earlier characters to be
298			inspected, and it might not have been reached when a partial match occurs. This
299			is probably an extremely unlikely case; you could guard against it to a certain
300			extent by always including extra characters at the start.
301	nigel	77	</P>
302			<P>
303	ph10	429	3. Matching a subject string that is split into multiple segments may not
304			always produce exactly the same result as matching over one single long string,
305	ph10	453	especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
306			Word Boundaries" above describes an issue that arises if the pattern ends with
307	ph10	429	\b or \B. Another kind of difference may occur when there are multiple
308			matching possibilities, because a partial match result is given only when there
309			are no completed matches. This means that as soon as the shortest match has
310	nigel	77	been found, continuation to a new subject segment is no longer possible.
311	ph10	429	Consider again this <b>pcretest</b> example:
312	nigel	77	<pre>
313			re> /dog(sbody)?/
314	ph10	429	data> dogsb\P
315	ph10	453	0: dog
316	nigel	77	data> do\P\D
317			Partial match: do
318			data> gsb\R\P\D
319			0: g
320			data> dogsbody\D
321			0: dogsbody
322			1: dog
323			</pre>
324	ph10	429	The first data line passes the string "dogsb" to <b>pcre_exec()</b>, setting the
325			PCRE_PARTIAL_SOFT option. Although the string is a partial match for
326			"dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
327			"dog" is a complete match. Similarly, when the subject is presented to
328			<b>pcre_dfa_exec()</b> in several parts ("do" and "gsb" being the first two) the
329			match stops when "dog" has been found, and it is not possible to continue. On
330			the other hand, if "dogsbody" is presented as a single string,
331			<b>pcre_dfa_exec()</b> finds both matches.
332	nigel	77	</P>
333			<P>
334	ph10	429	Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
335			matching multi-segment data. The example above then behaves differently:
336			<pre>
337			re> /dog(sbody)?/
338			data> dogsb\P\P
339	ph10	453	Partial match: dogsb
340	ph10	429	data> do\P\D
341			Partial match: do
342			data> gsb\R\P\P\D
343	ph10	453	Partial match: gsb
344	ph10	429
345			</PRE>
346	nigel	77	</P>
347			<P>
348	nigel	87	4. Patterns that contain alternatives at the top level which do not all
349	ph10	453	start with the same pattern item may not work as expected when
350	ph10	429	<b>pcre_dfa_exec()</b> is used. For example, consider this pattern:
351	nigel	87	<pre>
352			1234\|3789
353			</pre>
354			If the first part of the subject is "ABC123", a partial match of the first
355			alternative is found at offset 3. There is no partial match for the second
356			alternative, because such a match does not start at the same point in the
357	ph10	429	subject string. Attempting to continue with the string "7890" does not yield a
358	nigel	87	match because only those alternatives that match at one point in the subject
359			are remembered. The problem arises because the start of the second alternative
360			matches within the first alternative. There is no problem with anchored
361			patterns or patterns such as:
362			<pre>
363			1234\|ABCD
364			</pre>
365	ph10	429	where no string can be a partial match for both alternatives. This is not a
366	ph10	453	problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
367	ph10	429	each time:
368			<pre>
369			re> /1234\|3789/
370			data> ABC123\P
371			Partial match: 123
372			data> 1237890
373			0: 3789
374
375			</PRE>
376	nigel	87	</P>
377	ph10	429	<br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
378	nigel	87	<P>
379	ph10	99	Philip Hazel
380	nigel	75	<br>
381	ph10	99	University Computing Service
382			<br>
383			Cambridge CB2 3QH, England.
384			<br>
385			</P>
386	ph10	429	<br><a name="SEC11" href="#TOC1">REVISION</a><br>
387	ph10	99	<P>
388	ph10	461	Last updated: 29 September 2009
389	ph10	99	<br>
390	ph10	429	Copyright © 1997-2009 University of Cambridge.
391	ph10	99	<br>
392	nigel	75	<p>
393			Return to the <a href="index.html">PCRE index page</a>.
394			</p>
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"