PCRE - Perl-compatible regular expressions
This document describes the two different algorithms that are available in PCRE for matching a compiled regular expression against a given subject string. The "standard" algorithm is the one provided by the pcre_exec() function. This works in the same was as Perls matching function, and provides a Perl-compatible matching operation.
An alternative algorithm is provided by the pcre_dfa_exec() function; this operates in a different way, and is not Perl-compatible. It has advantages and disadvantages compared with the standard algorithm, and these are described below.
When there is only one possible way in which a given subject string can match a pattern, the two algorithms give the same answer. A difference arises, however, when there are multiple possibilities. For example, if the pattern
is matched against the string
<something> <something else> <something further>
there are three possible answers. The standard algorithm finds only one of them, whereas the DFA algorithm finds all three.
The set of strings that are matched by a regular expression can be represented as a tree structure. An unlimited repetition in the pattern makes the tree of infinite size, but it is still a tree. Matching the pattern to a given subject string (from a given starting point) can be thought of as a search of the tree. There are two standard ways to search a tree: depth-first and breadth-first, and these correspond to the two matching algorithms provided by PCRE.
In the terminology of Jeffrey Friedls book Mastering Regular Expressions, the standard algorithm is an "NFA algorithm". It conducts a depth-first search of the pattern tree. That is, it proceeds along a single path through the tree, checking that the subject matches what is required. When there is a mismatch, the algorithm tries any alternatives at the current point, and if they all fail, it backs up to the previous branch point in the tree, and tries the next alternative branch at that level. This often involves backing up (moving to the left) in the subject string as well. The order in which repetition branches are tried is controlled by the greedy or ungreedy nature of the quantifier.
If a leaf node is reached, a matching string has been found, and at that point the algorithm stops. Thus, if there is more than one possible match, this algorithm returns the first one that it finds. Whether this is the shortest, the longest, or some intermediate length depends on the way the greedy and ungreedy repetition quantifiers are specified in the pattern.
Because it ends up with a single path through the tree, it is relatively straightforward for this algorithm to keep track of the substrings that are matched by portions of the pattern in parentheses. This provides support for capturing parentheses and back references.
DFA stands for "deterministic finite automaton", but you do not need to understand the origins of that name. This algorithm conducts a breadth-first search of the tree. Starting from the first matching point in the subject, it scans the subject string from left to right, once, character by character, and as it does this, it remembers all the paths through the tree that represent valid matches.
The scan continues until either the end of the subject is reached, or there are no more unterminated paths. At this point, terminated paths represent the different matching possibilities (if there are none, the match has failed). Thus, if there is more than one possible match, this algorithm finds all of them, and in particular, it finds the longest. In PCRE, there is an option to stop the algorithm after the first match (which is necessarily the shortest) has been found.
Note that all the matches that are found start at the same point in the subject. If the pattern
is matched against the string "the caterpillar catchment", the result will be the three strings "cat", "cater", and "caterpillar" that start at the fourth character of the subject. The algorithm does not automatically move on to find matches that start at later positions.
There are a number of features of PCRE regular expressions that are not supported by the DFA matching algorithm. They are as follows:
1. Because the algorithm finds all possible matches, the greedy or ungreedy nature of repetition quantifiers is not relevant. Greedy and ungreedy quantifiers are treated in exactly the same way.
2. When dealing with multiple paths through the tree simultaneously, it is not straightforward to keep track of captured substrings for the different matching possibilities, and PCREs implementation of this algorithm does not attempt to do this. This means that no captured substrings are available.
3. Because no substrings are captured, back references within the pattern are not supported, and cause errors if encountered.
4. For the same reason, conditional expressions that use a backreference as the condition are not supported.
5. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always -1.
6. The \C escape sequence, which (in the standard algorithm) matches a single byte, even in UTF-8 mode, is not supported because the DFA algorithm moves through the subject string one character at a time, for all active paths through the tree.
Using the DFA matching algorithm provides the following advantages:
1. All possible matches (at a single point in the subject) are automatically found, and in particular, the longest match is found. To find more than one match using the standard algorithm, you have to do kludgy things with callouts.
2. There is much better support for partial matching. The restrictions on the content of the pattern that apply when using the standard algorithm for partial matching do not apply to the DFA algorithm. For non-anchored patterns, the starting position of a partial match is available.
3. Because the DFA algorithm scans the subject string just once, and never needs to backtrack, it is possible to pass very long subject strings to the matching function in several pieces, checking for partial matching each time.
The DFA algorithm suffers from a number of disadvantages:
1. It is substantially slower than the standard algorithm. This is partly because it has to search for all possible matches, but is also because it is less susceptible to optimization.
2. Capturing parentheses and back references are not supported.
3. The "atomic group" feature of PCRE regular expressions is supported, but does not provide the advantage that it does for the standard algorithm.
Last updated: 28 February 2005
Copyright (c) 1997-2005 University of Cambridge.