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3: <title>pcrematching specification</title>
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6: <h1>pcrematching 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">PCRE MATCHING ALGORITHMS</a>
17: <li><a name="TOC2" href="#SEC2">REGULAR EXPRESSIONS AS TREES</a>
18: <li><a name="TOC3" href="#SEC3">THE STANDARD MATCHING ALGORITHM</a>
19: <li><a name="TOC4" href="#SEC4">THE ALTERNATIVE MATCHING ALGORITHM</a>
20: <li><a name="TOC5" href="#SEC5">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
21: <li><a name="TOC6" href="#SEC6">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a>
22: <li><a name="TOC7" href="#SEC7">AUTHOR</a>
23: <li><a name="TOC8" href="#SEC8">REVISION</a>
24: </ul>
25: <br><a name="SEC1" href="#TOC1">PCRE MATCHING ALGORITHMS</a><br>
26: <P>
27: This document describes the two different algorithms that are available in PCRE
28: for matching a compiled regular expression against a given subject string. The
29: "standard" algorithm is the one provided by the <b>pcre_exec()</b> function.
30: This works in the same was as Perl's matching function, and provides a
31: Perl-compatible matching operation.
32: </P>
33: <P>
34: An alternative algorithm is provided by the <b>pcre_dfa_exec()</b> function;
35: this operates in a different way, and is not Perl-compatible. It has advantages
36: and disadvantages compared with the standard algorithm, and these are described
37: below.
38: </P>
39: <P>
40: When there is only one possible way in which a given subject string can match a
41: pattern, the two algorithms give the same answer. A difference arises, however,
42: when there are multiple possibilities. For example, if the pattern
43: <pre>
44: ^<.*>
45: </pre>
46: is matched against the string
47: <pre>
48: <something> <something else> <something further>
49: </pre>
50: there are three possible answers. The standard algorithm finds only one of
51: them, whereas the alternative algorithm finds all three.
52: </P>
53: <br><a name="SEC2" href="#TOC1">REGULAR EXPRESSIONS AS TREES</a><br>
54: <P>
55: The set of strings that are matched by a regular expression can be represented
56: as a tree structure. An unlimited repetition in the pattern makes the tree of
57: infinite size, but it is still a tree. Matching the pattern to a given subject
58: string (from a given starting point) can be thought of as a search of the tree.
59: There are two ways to search a tree: depth-first and breadth-first, and these
60: correspond to the two matching algorithms provided by PCRE.
61: </P>
62: <br><a name="SEC3" href="#TOC1">THE STANDARD MATCHING ALGORITHM</a><br>
63: <P>
64: In the terminology of Jeffrey Friedl's book "Mastering Regular
65: Expressions", the standard algorithm is an "NFA algorithm". It conducts a
66: depth-first search of the pattern tree. That is, it proceeds along a single
67: path through the tree, checking that the subject matches what is required. When
68: there is a mismatch, the algorithm tries any alternatives at the current point,
69: and if they all fail, it backs up to the previous branch point in the tree, and
70: tries the next alternative branch at that level. This often involves backing up
71: (moving to the left) in the subject string as well. The order in which
72: repetition branches are tried is controlled by the greedy or ungreedy nature of
73: the quantifier.
74: </P>
75: <P>
76: If a leaf node is reached, a matching string has been found, and at that point
77: the algorithm stops. Thus, if there is more than one possible match, this
78: algorithm returns the first one that it finds. Whether this is the shortest,
79: the longest, or some intermediate length depends on the way the greedy and
80: ungreedy repetition quantifiers are specified in the pattern.
81: </P>
82: <P>
83: Because it ends up with a single path through the tree, it is relatively
84: straightforward for this algorithm to keep track of the substrings that are
85: matched by portions of the pattern in parentheses. This provides support for
86: capturing parentheses and back references.
87: </P>
88: <br><a name="SEC4" href="#TOC1">THE ALTERNATIVE MATCHING ALGORITHM</a><br>
89: <P>
90: This algorithm conducts a breadth-first search of the tree. Starting from the
91: first matching point in the subject, it scans the subject string from left to
92: right, once, character by character, and as it does this, it remembers all the
93: paths through the tree that represent valid matches. In Friedl's terminology,
94: this is a kind of "DFA algorithm", though it is not implemented as a
95: traditional finite state machine (it keeps multiple states active
96: simultaneously).
97: </P>
98: <P>
99: Although the general principle of this matching algorithm is that it scans the
100: subject string only once, without backtracking, there is one exception: when a
101: lookaround assertion is encountered, the characters following or preceding the
102: current point have to be independently inspected.
103: </P>
104: <P>
105: The scan continues until either the end of the subject is reached, or there are
106: no more unterminated paths. At this point, terminated paths represent the
107: different matching possibilities (if there are none, the match has failed).
108: Thus, if there is more than one possible match, this algorithm finds all of
109: them, and in particular, it finds the longest. The matches are returned in
110: decreasing order of length. There is an option to stop the algorithm after the
111: first match (which is necessarily the shortest) is found.
112: </P>
113: <P>
114: Note that all the matches that are found start at the same point in the
115: subject. If the pattern
116: <pre>
117: cat(er(pillar)?)?
118: </pre>
119: is matched against the string "the caterpillar catchment", the result will be
120: the three strings "caterpillar", "cater", and "cat" that start at the fifth
121: character of the subject. The algorithm does not automatically move on to find
122: matches that start at later positions.
123: </P>
124: <P>
125: There are a number of features of PCRE regular expressions that are not
126: supported by the alternative matching algorithm. They are as follows:
127: </P>
128: <P>
129: 1. Because the algorithm finds all possible matches, the greedy or ungreedy
130: nature of repetition quantifiers is not relevant. Greedy and ungreedy
131: quantifiers are treated in exactly the same way. However, possessive
132: quantifiers can make a difference when what follows could also match what is
133: quantified, for example in a pattern like this:
134: <pre>
135: ^a++\w!
136: </pre>
137: This pattern matches "aaab!" but not "aaa!", which would be matched by a
138: non-possessive quantifier. Similarly, if an atomic group is present, it is
139: matched as if it were a standalone pattern at the current point, and the
140: longest match is then "locked in" for the rest of the overall pattern.
141: </P>
142: <P>
143: 2. When dealing with multiple paths through the tree simultaneously, it is not
144: straightforward to keep track of captured substrings for the different matching
145: possibilities, and PCRE's implementation of this algorithm does not attempt to
146: do this. This means that no captured substrings are available.
147: </P>
148: <P>
149: 3. Because no substrings are captured, back references within the pattern are
150: not supported, and cause errors if encountered.
151: </P>
152: <P>
153: 4. For the same reason, conditional expressions that use a backreference as the
154: condition or test for a specific group recursion are not supported.
155: </P>
156: <P>
157: 5. Because many paths through the tree may be active, the \K escape sequence,
158: which resets the start of the match when encountered (but may be on some paths
159: and not on others), is not supported. It causes an error if encountered.
160: </P>
161: <P>
162: 6. Callouts are supported, but the value of the <i>capture_top</i> field is
163: always 1, and the value of the <i>capture_last</i> field is always -1.
164: </P>
165: <P>
166: 7. The \C escape sequence, which (in the standard algorithm) matches a single
167: byte, even in UTF-8 mode, is not supported in UTF-8 mode, because the
168: alternative algorithm moves through the subject string one character at a time,
169: for all active paths through the tree.
170: </P>
171: <P>
172: 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
173: supported. (*FAIL) is supported, and behaves like a failing negative assertion.
174: </P>
175: <br><a name="SEC5" href="#TOC1">ADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
176: <P>
177: Using the alternative matching algorithm provides the following advantages:
178: </P>
179: <P>
180: 1. All possible matches (at a single point in the subject) are automatically
181: found, and in particular, the longest match is found. To find more than one
182: match using the standard algorithm, you have to do kludgy things with
183: callouts.
184: </P>
185: <P>
186: 2. Because the alternative algorithm scans the subject string just once, and
187: never needs to backtrack, it is possible to pass very long subject strings to
188: the matching function in several pieces, checking for partial matching each
189: time. Although it is possible to do multi-segment matching using the standard
190: algorithm (<b>pcre_exec()</b>), by retaining partially matched substrings, it is
191: more complicated. The
192: <a href="pcrepartial.html"><b>pcrepartial</b></a>
193: documentation gives details of partial matching and discusses multi-segment
194: matching.
195: </P>
196: <br><a name="SEC6" href="#TOC1">DISADVANTAGES OF THE ALTERNATIVE ALGORITHM</a><br>
197: <P>
198: The alternative algorithm suffers from a number of disadvantages:
199: </P>
200: <P>
201: 1. It is substantially slower than the standard algorithm. This is partly
202: because it has to search for all possible matches, but is also because it is
203: less susceptible to optimization.
204: </P>
205: <P>
206: 2. Capturing parentheses and back references are not supported.
207: </P>
208: <P>
209: 3. Although atomic groups are supported, their use does not provide the
210: performance advantage that it does for the standard algorithm.
211: </P>
212: <br><a name="SEC7" href="#TOC1">AUTHOR</a><br>
213: <P>
214: Philip Hazel
215: <br>
216: University Computing Service
217: <br>
218: Cambridge CB2 3QH, England.
219: <br>
220: </P>
221: <br><a name="SEC8" href="#TOC1">REVISION</a><br>
222: <P>
223: Last updated: 19 November 2011
224: <br>
225: Copyright © 1997-2010 University of Cambridge.
226: <br>
227: <p>
228: Return to the <a href="index.html">PCRE index page</a>.
229: </p>
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