trunk/doc/pcrepartial.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"
.rs
.sp
In normal use of PCRE, if the subject string that is passed to
\fBpcre_exec()\fP 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
Consider, for example, an application where a human is required to type in data
for a field with specific formatting requirements. An example might be a date
in the form \fIddmmmyy\fP, defined by this pattern:
.sp
  ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
.sp
If the application sees the user's keystrokes one by one, and can check that
what has been typed so far is potentially valid, it is able to raise an error
as soon as a mistake is made, 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
PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
option, which can be set when calling \fBpcre_exec()\fP. When this is done, the
return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any
time during the matching process the entire subject string matched part of the
pattern. No captured data is set when this occurs.
.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.
.
.
.SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
.rs
.sp
Because of the way certain internal optimizations are implemented in PCRE, the
PCRE_PARTIAL option cannot be used with all patterns. Repeated single
characters such as
.sp
  a{2,4}
.sp
and repeated single metasequences such as
.sp
  \ed+
.sp
are not permitted if the maximum number of occurrences is greater than one.
Optional items such as \ed? (where the maximum is one) are permitted.
Quantifiers with any values are permitted after parentheses, so the invalid
examples above can be coded thus:
.sp
  (a){2,4}
  (\ed)+
.sp
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
If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
\fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
.
.
.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
.rs
.sp
If the escape sequence \eP is present in a \fBpcretest\fP data line, the
PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that
uses the date example quoted above:
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 25jun04\P
   0: 25jun04
   1: jun
  data> 25dec3\P
  Partial match
  data> 3ju\P
  Partial match
  data> 3juj\P
  No match
  data> j\P
  No match
.sp
The first data string is matched completely, so \fBpcretest\fP shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches.
.
.
.P
.in 0
Last updated: 08 September 2004
.br
Copyright (c) 1997-2004 University of Cambridge.
1	nigel	75	.TH PCRE 3
2			.SH NAME
3			PCRE - Perl-compatible regular expressions
4			.SH "PARTIAL MATCHING IN PCRE"
5			.rs
6			.sp
7			In normal use of PCRE, if the subject string that is passed to
8			\fBpcre_exec()\fP matches as far as it goes, but is too short to match the
9			entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where
10			it might be helpful to distinguish this case from other cases in which there is
11			no match.
12			.P
13			Consider, for example, an application where a human is required to type in data
14			for a field with specific formatting requirements. An example might be a date
15			in the form \fIddmmmyy\fP, defined by this pattern:
16			.sp
17			^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$
18			.sp
19			If the application sees the user's keystrokes one by one, and can check that
20			what has been typed so far is potentially valid, it is able to raise an error
21			as soon as a mistake is made, possibly beeping and not reflecting the
22			character that has been typed. This immediate feedback is likely to be a better
23			user interface than a check that is delayed until the entire string has been
24			entered.
25			.P
26			PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
27			option, which can be set when calling \fBpcre_exec()\fP. When this is done, the
28			return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any
29			time during the matching process the entire subject string matched part of the
30			pattern. No captured data is set when this occurs.
31			.P
32			Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
33			last literal byte in a pattern, and abandons matching immediately if such a
34			byte is not present in the subject string. This optimization cannot be used
35			for a subject string that might match only partially.
36			.
37			.
38			.SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
39			.rs
40			.sp
41			Because of the way certain internal optimizations are implemented in PCRE, the
42			PCRE_PARTIAL option cannot be used with all patterns. Repeated single
43			characters such as
44			.sp
45			a{2,4}
46			.sp
47			and repeated single metasequences such as
48			.sp
49			\ed+
50			.sp
51			are not permitted if the maximum number of occurrences is greater than one.
52			Optional items such as \ed? (where the maximum is one) are permitted.
53			Quantifiers with any values are permitted after parentheses, so the invalid
54			examples above can be coded thus:
55			.sp
56			(a){2,4}
57			(\ed)+
58			.sp
59			These constructions run more slowly, but for the kinds of application that are
60			envisaged for this facility, this is not felt to be a major restriction.
61			.P
62			If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
63			\fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
64			.
65			.
66			.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
67			.rs
68			.sp
69			If the escape sequence \eP is present in a \fBpcretest\fP data line, the
70			PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that
71			uses the date example quoted above:
72			.sp
73			re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/
74			data> 25jun04\P
75			0: 25jun04
76			1: jun
77			data> 25dec3\P
78			Partial match
79			data> 3ju\P
80			Partial match
81			data> 3juj\P
82			No match
83			data> j\P
84			No match
85			.sp
86			The first data string is matched completely, so \fBpcretest\fP shows the
87			matched substrings. The remaining four strings do not match the complete
88			pattern, but the first two are partial matches.
89			.
90			.
91			.P
92			.in 0
93			Last updated: 08 September 2004
94			.br
95			Copyright (c) 1997-2004 University of Cambridge.