doc/html/pcrepartial.html

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<head>
<title>pcrepartial specification</title>
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<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">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a>
<li><a name="TOC3" href="#SEC3">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
<li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</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, possibly beeping and not reflecting the
character that has been typed. This immediate feedback is likely to be a better
user interface than a check that is delayed until the entire string has been
entered.
</P>
<P>
PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
option, which can be set when calling <b>pcre_exec()</b> or
<b>pcre_dfa_exec()</b>. When this flag is set for <b>pcre_exec()</b>, the return
code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
during the matching process the last part of the subject string matched part of
the pattern. Unfortunately, for non-anchored matching, it is not possible to
obtain the position of the start of the partial match. No captured data is set
when PCRE_ERROR_PARTIAL is returned.
</P>
<P>
When PCRE_PARTIAL is set for <b>pcre_dfa_exec()</b>, the return code
PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the
subject is reached, there have been no complete matches, but there is still at
least one matching possibility. The portion of the string that provided the
partial match is set as the first matching string.
</P>
<P>
Using PCRE_PARTIAL disables one 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.
</P>
<br><a name="SEC2" href="#TOC1">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a><br>
<P>
Because of the way certain internal optimizations are implemented in the
<b>pcre_exec()</b> function, the PCRE_PARTIAL option cannot be used with all
patterns. These restrictions do not apply when <b>pcre_dfa_exec()</b> is used.
For <b>pcre_exec()</b>, repeated single characters such as
<pre>
  a{2,4}
</pre>
and repeated single metasequences such as
<pre>
  \d+
</pre>
are not permitted if the maximum number of occurrences is greater than one.
Optional items such as \d? (where the maximum is one) are permitted.
Quantifiers with any values are permitted after parentheses, so the invalid
examples above can be coded thus:
<pre>
  (a){2,4}
  (\d)+
</pre>
These constructions run more slowly, but for the kinds of application that are
envisaged for this facility, this is not felt to be a major restriction.
</P>
<P>
If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
<b>pcre_exec()</b> returns the error code PCRE_ERROR_BADPARTIAL (-13).
</P>
<br><a name="SEC3" 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 flag 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$/
  data&#62; 25jun04\P
   0: 25jun04
   1: jun
  data&#62; 25dec3\P
  Partial match
  data&#62; 3ju\P
  Partial match
  data&#62; 3juj\P
  No match
  data&#62; j\P
  No match
</pre>
The first data string is matched completely, so <b>pcretest</b> shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. The same test, using
<b>pcre_dfa_exec()</b> matching (by means of the \D escape sequence), produces
the following output:
<pre>
    re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
  data&#62; 25jun04\P\D
   0: 25jun04
  data&#62; 23dec3\P\D
  Partial match: 23dec3
  data&#62; 3ju\P\D
  Partial match: 3ju
  data&#62; 3juj\P\D
  No match
  data&#62; j\P\D
  No match
</pre>
Notice that in this case the portion of the string that was matched is made
available.
</P>
<br><a name="SEC4" 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 also 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 (\P and \D are as above):
<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 PCRE_PARTIAL 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>. However, some care is needed for certain
types of pattern.
</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. If the pattern contains backward assertions (including \b or \B), you need
to arrange for some overlap in the subject strings to allow for this. For
example, you could pass the subject in chunks that are 500 bytes long, but in
a buffer of 700 bytes, with the starting offset set to 200 and the previous 200
bytes at the start of the buffer.
</P>
<P>
3. Matching a subject string that is split into multiple segments does not
always produce exactly the same result as matching over one single long string.
The difference arises when there are multiple matching possibilities, because a
partial match result is given only when there are no completed matches in a
call to fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has
been found, continuation to a new subject segment is no longer possible.
Consider this <b>pcretest</b> example:
<pre>
    re&#62; /dog(sbody)?/
  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 pattern matches the words "dog" or "dogsbody". When the subject is
presented 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, both matches are found.
</P>
<P>
Because of this phenomenon, it does not usually make sense to end a pattern
that is going to be matched in this way with a variable repeat.
</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. 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 "789" 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.
</P>
<P>
Last updated: 30 November 2006
<br>
Copyright &copy; 1997-2006 University of Cambridge.
<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			<p>
11			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			<br>
15			<ul>
16			<li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
17			<li><a name="TOC2" href="#SEC2">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a>
18			<li><a name="TOC3" href="#SEC3">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
19	nigel	77	<li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>
20	nigel	75	</ul>
21			<br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
22			<P>
23			In normal use of PCRE, if the subject string that is passed to
24	nigel	77	<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> matches as far as it goes, but is
25			too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
26			are circumstances where it might be helpful to distinguish this case from other
27			cases in which there is no match.
28	nigel	75	</P>
29			<P>
30			Consider, for example, an application where a human is required to type in data
31			for a field with specific formatting requirements. An example might be a date
32			in the form <i>ddmmmyy</i>, defined by this pattern:
33			<pre>
34			^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$
35			</pre>
36			If the application sees the user's keystrokes one by one, and can check that
37			what has been typed so far is potentially valid, it is able to raise an error
38			as soon as a mistake is made, possibly beeping and not reflecting the
39			character that has been typed. This immediate feedback is likely to be a better
40			user interface than a check that is delayed until the entire string has been
41			entered.
42			</P>
43			<P>
44			PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
45	nigel	77	option, which can be set when calling <b>pcre_exec()</b> or
46			<b>pcre_dfa_exec()</b>. When this flag is set for <b>pcre_exec()</b>, the return
47			code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
48			during the matching process the last part of the subject string matched part of
49			the pattern. Unfortunately, for non-anchored matching, it is not possible to
50			obtain the position of the start of the partial match. No captured data is set
51			when PCRE_ERROR_PARTIAL is returned.
52	nigel	75	</P>
53			<P>
54	nigel	77	When PCRE_PARTIAL is set for <b>pcre_dfa_exec()</b>, the return code
55			PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the
56			subject is reached, there have been no complete matches, but there is still at
57			least one matching possibility. The portion of the string that provided the
58			partial match is set as the first matching string.
59			</P>
60			<P>
61	nigel	75	Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
62			last literal byte in a pattern, and abandons matching immediately if such a
63			byte is not present in the subject string. This optimization cannot be used
64			for a subject string that might match only partially.
65			</P>
66			<br><a name="SEC2" href="#TOC1">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a><br>
67			<P>
68	nigel	77	Because of the way certain internal optimizations are implemented in the
69			<b>pcre_exec()</b> function, the PCRE_PARTIAL option cannot be used with all
70			patterns. These restrictions do not apply when <b>pcre_dfa_exec()</b> is used.
71			For <b>pcre_exec()</b>, repeated single characters such as
72	nigel	75	<pre>
73			a{2,4}
74			</pre>
75			and repeated single metasequences such as
76			<pre>
77			\d+
78			</pre>
79			are not permitted if the maximum number of occurrences is greater than one.
80			Optional items such as \d? (where the maximum is one) are permitted.
81			Quantifiers with any values are permitted after parentheses, so the invalid
82			examples above can be coded thus:
83			<pre>
84			(a){2,4}
85			(\d)+
86			</pre>
87			These constructions run more slowly, but for the kinds of application that are
88			envisaged for this facility, this is not felt to be a major restriction.
89			</P>
90			<P>
91			If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
92			<b>pcre_exec()</b> returns the error code PCRE_ERROR_BADPARTIAL (-13).
93			</P>
94			<br><a name="SEC3" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
95			<P>
96			If the escape sequence \P is present in a <b>pcretest</b> data line, the
97			PCRE_PARTIAL flag is used for the match. Here is a run of <b>pcretest</b> that
98			uses the date example quoted above:
99			<pre>
100			re> /^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$/
101			data> 25jun04\P
102			0: 25jun04
103			1: jun
104			data> 25dec3\P
105			Partial match
106			data> 3ju\P
107			Partial match
108			data> 3juj\P
109			No match
110			data> j\P
111			No match
112			</pre>
113			The first data string is matched completely, so <b>pcretest</b> shows the
114			matched substrings. The remaining four strings do not match the complete
115	nigel	93	pattern, but the first two are partial matches. The same test, using
116			<b>pcre_dfa_exec()</b> matching (by means of the \D escape sequence), produces
117			the following output:
118	nigel	77	<pre>
119			re> /^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$/
120			data> 25jun04\P\D
121			0: 25jun04
122			data> 23dec3\P\D
123			Partial match: 23dec3
124			data> 3ju\P\D
125			Partial match: 3ju
126			data> 3juj\P\D
127			No match
128			data> j\P\D
129			No match
130			</pre>
131			Notice that in this case the portion of the string that was matched is made
132			available.
133	nigel	75	</P>
134	nigel	77	<br><a name="SEC4" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>
135	nigel	75	<P>
136	nigel	77	When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible
137			to continue the match by providing additional subject data and calling
138	nigel	93	<b>pcre_dfa_exec()</b> again with the same compiled regular expression, this
139			time setting the PCRE_DFA_RESTART option. You must also pass the same working
140			space as before, because this is where details of the previous partial match
141			are stored. Here is an example using <b>pcretest</b>, using the \R escape
142			sequence to set the PCRE_DFA_RESTART option (\P and \D are as above):
143	nigel	77	<pre>
144			re> /^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$/
145			data> 23ja\P\D
146			Partial match: 23ja
147			data> n05\R\D
148			0: n05
149			</pre>
150			The first call has "23ja" as the subject, and requests partial matching; the
151			second call has "n05" as the subject for the continued (restarted) match.
152			Notice that when the match is complete, only the last part is shown; PCRE does
153			not retain the previously partially-matched string. It is up to the calling
154			program to do that if it needs to.
155			</P>
156			<P>
157	nigel	93	You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching
158			over multiple segments. This facility can be used to pass very long subject
159			strings to <b>pcre_dfa_exec()</b>. However, some care is needed for certain
160			types of pattern.
161	nigel	77	</P>
162			<P>
163			1. If the pattern contains tests for the beginning or end of a line, you need
164			to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
165			subject string for any call does not contain the beginning or end of a line.
166			</P>
167			<P>
168			2. If the pattern contains backward assertions (including \b or \B), you need
169			to arrange for some overlap in the subject strings to allow for this. For
170	nigel	93	example, you could pass the subject in chunks that are 500 bytes long, but in
171	nigel	77	a buffer of 700 bytes, with the starting offset set to 200 and the previous 200
172			bytes at the start of the buffer.
173			</P>
174			<P>
175			3. Matching a subject string that is split into multiple segments does not
176			always produce exactly the same result as matching over one single long string.
177			The difference arises when there are multiple matching possibilities, because a
178			partial match result is given only when there are no completed matches in a
179			call to fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has
180			been found, continuation to a new subject segment is no longer possible.
181			Consider this <b>pcretest</b> example:
182			<pre>
183			re> /dog(sbody)?/
184			data> do\P\D
185			Partial match: do
186			data> gsb\R\P\D
187			0: g
188			data> dogsbody\D
189			0: dogsbody
190			1: dog
191			</pre>
192			The pattern matches the words "dog" or "dogsbody". When the subject is
193			presented in several parts ("do" and "gsb" being the first two) the match stops
194			when "dog" has been found, and it is not possible to continue. On the other
195			hand, if "dogsbody" is presented as a single string, both matches are found.
196			</P>
197			<P>
198			Because of this phenomenon, it does not usually make sense to end a pattern
199			that is going to be matched in this way with a variable repeat.
200			</P>
201			<P>
202	nigel	87	4. Patterns that contain alternatives at the top level which do not all
203			start with the same pattern item may not work as expected. For example,
204			consider this pattern:
205			<pre>
206			1234\|3789
207			</pre>
208			If the first part of the subject is "ABC123", a partial match of the first
209			alternative is found at offset 3. There is no partial match for the second
210			alternative, because such a match does not start at the same point in the
211			subject string. Attempting to continue with the string "789" does not yield a
212			match because only those alternatives that match at one point in the subject
213			are remembered. The problem arises because the start of the second alternative
214			matches within the first alternative. There is no problem with anchored
215			patterns or patterns such as:
216			<pre>
217			1234\|ABCD
218			</pre>
219			where no string can be a partial match for both alternatives.
220			</P>
221			<P>
222	nigel	93	Last updated: 30 November 2006
223	nigel	75	<br>
224	nigel	87	Copyright © 1997-2006 University of Cambridge.
225	nigel	75	<p>
226			Return to the <a href="index.html">PCRE index page</a>.
227			</p>
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