Annotation of embedaddon/pcre/doc/pcrematching.3, revision 1.1.1.2
1.1 misho 1: .TH PCREMATCHING 3
2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "PCRE MATCHING ALGORITHMS"
5: .rs
6: .sp
7: This document describes the two different algorithms that are available in PCRE
8: for matching a compiled regular expression against a given subject string. The
1.1.1.2 ! misho 9: "standard" algorithm is the one provided by the \fBpcre_exec()\fP and
! 10: \fBpcre16_exec()\fP functions. These work in the same was as Perl's matching
! 11: function, and provide a Perl-compatible matching operation. The just-in-time
! 12: (JIT) optimization that is described in the
! 13: .\" HREF
! 14: \fBpcrejit\fP
! 15: .\"
! 16: documentation is compatible with these functions.
! 17: .P
! 18: An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP and
! 19: \fBpcre16_dfa_exec()\fP functions; they operate in a different way, and are not
! 20: Perl-compatible. This alternative has advantages and disadvantages compared
! 21: with the standard algorithm, and these are described below.
1.1 misho 22: .P
23: When there is only one possible way in which a given subject string can match a
24: pattern, the two algorithms give the same answer. A difference arises, however,
25: when there are multiple possibilities. For example, if the pattern
26: .sp
27: ^<.*>
28: .sp
29: is matched against the string
30: .sp
31: <something> <something else> <something further>
32: .sp
33: there are three possible answers. The standard algorithm finds only one of
34: them, whereas the alternative algorithm finds all three.
35: .
1.1.1.2 ! misho 36: .
1.1 misho 37: .SH "REGULAR EXPRESSIONS AS TREES"
38: .rs
39: .sp
40: The set of strings that are matched by a regular expression can be represented
41: as a tree structure. An unlimited repetition in the pattern makes the tree of
42: infinite size, but it is still a tree. Matching the pattern to a given subject
43: string (from a given starting point) can be thought of as a search of the tree.
44: There are two ways to search a tree: depth-first and breadth-first, and these
45: correspond to the two matching algorithms provided by PCRE.
46: .
1.1.1.2 ! misho 47: .
1.1 misho 48: .SH "THE STANDARD MATCHING ALGORITHM"
49: .rs
50: .sp
51: In the terminology of Jeffrey Friedl's book "Mastering Regular
52: Expressions", the standard algorithm is an "NFA algorithm". It conducts a
53: depth-first search of the pattern tree. That is, it proceeds along a single
54: path through the tree, checking that the subject matches what is required. When
55: there is a mismatch, the algorithm tries any alternatives at the current point,
56: and if they all fail, it backs up to the previous branch point in the tree, and
57: tries the next alternative branch at that level. This often involves backing up
58: (moving to the left) in the subject string as well. The order in which
59: repetition branches are tried is controlled by the greedy or ungreedy nature of
60: the quantifier.
61: .P
62: If a leaf node is reached, a matching string has been found, and at that point
63: the algorithm stops. Thus, if there is more than one possible match, this
64: algorithm returns the first one that it finds. Whether this is the shortest,
65: the longest, or some intermediate length depends on the way the greedy and
66: ungreedy repetition quantifiers are specified in the pattern.
67: .P
68: Because it ends up with a single path through the tree, it is relatively
69: straightforward for this algorithm to keep track of the substrings that are
70: matched by portions of the pattern in parentheses. This provides support for
71: capturing parentheses and back references.
72: .
1.1.1.2 ! misho 73: .
1.1 misho 74: .SH "THE ALTERNATIVE MATCHING ALGORITHM"
75: .rs
76: .sp
77: This algorithm conducts a breadth-first search of the tree. Starting from the
78: first matching point in the subject, it scans the subject string from left to
79: right, once, character by character, and as it does this, it remembers all the
80: paths through the tree that represent valid matches. In Friedl's terminology,
81: this is a kind of "DFA algorithm", though it is not implemented as a
82: traditional finite state machine (it keeps multiple states active
83: simultaneously).
84: .P
85: Although the general principle of this matching algorithm is that it scans the
86: subject string only once, without backtracking, there is one exception: when a
87: lookaround assertion is encountered, the characters following or preceding the
88: current point have to be independently inspected.
89: .P
90: The scan continues until either the end of the subject is reached, or there are
91: no more unterminated paths. At this point, terminated paths represent the
92: different matching possibilities (if there are none, the match has failed).
93: Thus, if there is more than one possible match, this algorithm finds all of
94: them, and in particular, it finds the longest. The matches are returned in
95: decreasing order of length. There is an option to stop the algorithm after the
96: first match (which is necessarily the shortest) is found.
97: .P
98: Note that all the matches that are found start at the same point in the
99: subject. If the pattern
100: .sp
101: cat(er(pillar)?)?
102: .sp
103: is matched against the string "the caterpillar catchment", the result will be
104: the three strings "caterpillar", "cater", and "cat" that start at the fifth
105: character of the subject. The algorithm does not automatically move on to find
106: matches that start at later positions.
107: .P
108: There are a number of features of PCRE regular expressions that are not
109: supported by the alternative matching algorithm. They are as follows:
110: .P
111: 1. Because the algorithm finds all possible matches, the greedy or ungreedy
112: nature of repetition quantifiers is not relevant. Greedy and ungreedy
113: quantifiers are treated in exactly the same way. However, possessive
114: quantifiers can make a difference when what follows could also match what is
115: quantified, for example in a pattern like this:
116: .sp
117: ^a++\ew!
118: .sp
119: This pattern matches "aaab!" but not "aaa!", which would be matched by a
120: non-possessive quantifier. Similarly, if an atomic group is present, it is
121: matched as if it were a standalone pattern at the current point, and the
122: longest match is then "locked in" for the rest of the overall pattern.
123: .P
124: 2. When dealing with multiple paths through the tree simultaneously, it is not
125: straightforward to keep track of captured substrings for the different matching
126: possibilities, and PCRE's implementation of this algorithm does not attempt to
127: do this. This means that no captured substrings are available.
128: .P
129: 3. Because no substrings are captured, back references within the pattern are
130: not supported, and cause errors if encountered.
131: .P
132: 4. For the same reason, conditional expressions that use a backreference as the
133: condition or test for a specific group recursion are not supported.
134: .P
135: 5. Because many paths through the tree may be active, the \eK escape sequence,
136: which resets the start of the match when encountered (but may be on some paths
137: and not on others), is not supported. It causes an error if encountered.
138: .P
139: 6. Callouts are supported, but the value of the \fIcapture_top\fP field is
140: always 1, and the value of the \fIcapture_last\fP field is always -1.
141: .P
1.1.1.2 ! misho 142: 7. The \eC escape sequence, which (in the standard algorithm) always matches a
! 143: single data unit, even in UTF-8 or UTF-16 modes, is not supported in these
! 144: modes, because the alternative algorithm moves through the subject string one
! 145: character (not data unit) at a time, for all active paths through the tree.
1.1 misho 146: .P
147: 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
148: supported. (*FAIL) is supported, and behaves like a failing negative assertion.
149: .
1.1.1.2 ! misho 150: .
1.1 misho 151: .SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM"
152: .rs
153: .sp
154: Using the alternative matching algorithm provides the following advantages:
155: .P
156: 1. All possible matches (at a single point in the subject) are automatically
157: found, and in particular, the longest match is found. To find more than one
158: match using the standard algorithm, you have to do kludgy things with
159: callouts.
160: .P
161: 2. Because the alternative algorithm scans the subject string just once, and
1.1.1.2 ! misho 162: never needs to backtrack (except for lookbehinds), it is possible to pass very
! 163: long subject strings to the matching function in several pieces, checking for
! 164: partial matching each time. Although it is possible to do multi-segment
! 165: matching using the standard algorithm by retaining partially matched
! 166: substrings, it is more complicated. The
1.1 misho 167: .\" HREF
168: \fBpcrepartial\fP
169: .\"
170: documentation gives details of partial matching and discusses multi-segment
171: matching.
172: .
173: .
174: .SH "DISADVANTAGES OF THE ALTERNATIVE ALGORITHM"
175: .rs
176: .sp
177: The alternative algorithm suffers from a number of disadvantages:
178: .P
179: 1. It is substantially slower than the standard algorithm. This is partly
180: because it has to search for all possible matches, but is also because it is
181: less susceptible to optimization.
182: .P
183: 2. Capturing parentheses and back references are not supported.
184: .P
185: 3. Although atomic groups are supported, their use does not provide the
186: performance advantage that it does for the standard algorithm.
187: .
188: .
189: .SH AUTHOR
190: .rs
191: .sp
192: .nf
193: Philip Hazel
194: University Computing Service
195: Cambridge CB2 3QH, England.
196: .fi
197: .
198: .
199: .SH REVISION
200: .rs
201: .sp
202: .nf
1.1.1.2 ! misho 203: Last updated: 08 January 2012
! 204: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 205: .fi
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