doc/html/pcrepartial.html

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<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	<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	<li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>
20	</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	<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	</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	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	</P>
53	<P>
54	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	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	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	<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$/
101	data> 25jun04\P
102	0: 25jun04
103	1: jun
104	data> 25dec3\P
105	Partial match
106	data> 3ju\P
107	Partial match
108	data> 3juj\P
109	No match
110	data> j\P
111	No match
112	</pre>
113	The first data string is matched completely, so <b>pcretest</b> shows the
114	matched substrings. The remaining four strings do not match the complete
115	pattern, but the first two are partial matches. The same test, using
116	<b>pcre_dfa_exec()</b> matching (by means of the \D escape sequence), produces
117	the following output:
118	<pre>
119	re> /^\d?\d(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\d\d$/
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	</P>
134	<br><a name="SEC4" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>
135	<P>
136	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	<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	<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	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	</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	example, you could pass the subject in chunks that are 500 bytes long, but in
171	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	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	Last updated: 30 November 2006
223	<br>
224	Copyright © 1997-2006 University of Cambridge.
225	<p>
226	Return to the <a href="index.html">PCRE index page</a>.
227	</p>
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