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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>
19 </ul>
20 <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
21 <P>
22 In normal use of PCRE, if the subject string that is passed to
23 <b>pcre_exec()</b> matches as far as it goes, but is too short to match the
24 entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where
25 it might be helpful to distinguish this case from other cases in which there is
26 no match.
27 </P>
28 <P>
29 Consider, for example, an application where a human is required to type in data
30 for a field with specific formatting requirements. An example might be a date
31 in the form <i>ddmmmyy</i>, defined by this pattern:
32 <pre>
33 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
34 </pre>
35 If the application sees the user's keystrokes one by one, and can check that
36 what has been typed so far is potentially valid, it is able to raise an error
37 as soon as a mistake is made, possibly beeping and not reflecting the
38 character that has been typed. This immediate feedback is likely to be a better
39 user interface than a check that is delayed until the entire string has been
40 entered.
41 </P>
42 <P>
43 PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
44 option, which can be set when calling <b>pcre_exec()</b>. When this is done, the
45 return code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any
46 time during the matching process the entire subject string matched part of the
47 pattern. No captured data is set when this occurs.
48 </P>
49 <P>
50 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
51 last literal byte in a pattern, and abandons matching immediately if such a
52 byte is not present in the subject string. This optimization cannot be used
53 for a subject string that might match only partially.
54 </P>
55 <br><a name="SEC2" href="#TOC1">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a><br>
56 <P>
57 Because of the way certain internal optimizations are implemented in PCRE, the
58 PCRE_PARTIAL option cannot be used with all patterns. Repeated single
59 characters such as
60 <pre>
61 a{2,4}
62 </pre>
63 and repeated single metasequences such as
64 <pre>
65 \d+
66 </pre>
67 are not permitted if the maximum number of occurrences is greater than one.
68 Optional items such as \d? (where the maximum is one) are permitted.
69 Quantifiers with any values are permitted after parentheses, so the invalid
70 examples above can be coded thus:
71 <pre>
72 (a){2,4}
73 (\d)+
74 </pre>
75 These constructions run more slowly, but for the kinds of application that are
76 envisaged for this facility, this is not felt to be a major restriction.
77 </P>
78 <P>
79 If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
80 <b>pcre_exec()</b> returns the error code PCRE_ERROR_BADPARTIAL (-13).
81 </P>
83 <P>
84 If the escape sequence \P is present in a <b>pcretest</b> data line, the
85 PCRE_PARTIAL flag is used for the match. Here is a run of <b>pcretest</b> that
86 uses the date example quoted above:
87 <pre>
88 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
89 data&#62; 25jun04\P
90 0: 25jun04
91 1: jun
92 data&#62; 25dec3\P
93 Partial match
94 data&#62; 3ju\P
95 Partial match
96 data&#62; 3juj\P
97 No match
98 data&#62; j\P
99 No match
100 </pre>
101 The first data string is matched completely, so <b>pcretest</b> shows the
102 matched substrings. The remaining four strings do not match the complete
103 pattern, but the first two are partial matches.
104 </P>
105 <P>
106 Last updated: 08 September 2004
107 <br>
108 Copyright &copy; 1997-2004 University of Cambridge.
109 <p>
110 Return to the <a href="index.html">PCRE index page</a>.
111 </p>

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