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<something> <something else> <something further> 
<something> <something else> <something further> 
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.sp 
.sp 
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there are three possible answers. The standard algorithm finds only one of 
there are three possible answers. The standard algorithm finds only one of 
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them, whereas the DFA algorithm finds all three. 
them, whereas the alternative algorithm finds all three. 
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. 
. 
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.SH "REGULAR EXPRESSIONS AS TREES" 
.SH "REGULAR EXPRESSIONS AS TREES" 
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.rs 
.rs 
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matched by portions of the pattern in parentheses. This provides support for 
matched by portions of the pattern in parentheses. This provides support for 
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capturing parentheses and back references. 
capturing parentheses and back references. 
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. 
. 
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.SH "THE DFA MATCHING ALGORITHM" 
.SH "THE ALTERNATIVE MATCHING ALGORITHM" 
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.rs 
.rs 
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.sp 
.sp 
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DFA stands for "deterministic finite automaton", but you do not need to 
This algorithm conducts a breadthfirst search of the tree. Starting from the 
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understand the origins of that name. This algorithm conducts a breadthfirst 
first matching point in the subject, it scans the subject string from left to 
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search of the tree. Starting from the first matching point in the subject, it 
right, once, character by character, and as it does this, it remembers all the 
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scans the subject string from left to right, once, character by character, and 
paths through the tree that represent valid matches. In Friedl's terminology, 
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as it does this, it remembers all the paths through the tree that represent 
this is a kind of "DFA algorithm", though it is not implemented as a 
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valid matches. 
traditional finite state machine (it keeps multiple states active 
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simultaneously). 
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.P 
.P 
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The scan continues until either the end of the subject is reached, or there are 
The scan continues until either the end of the subject is reached, or there are 
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no more unterminated paths. At this point, terminated paths represent the 
no more unterminated paths. At this point, terminated paths represent the 
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matches that start at later positions. 
matches that start at later positions. 
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.P 
.P 
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There are a number of features of PCRE regular expressions that are not 
There are a number of features of PCRE regular expressions that are not 
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supported by the DFA matching algorithm. They are as follows: 
supported by the alternative matching algorithm. They are as follows: 
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.P 
.P 
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1. Because the algorithm finds all possible matches, the greedy or ungreedy 
1. Because the algorithm finds all possible matches, the greedy or ungreedy 
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nature of repetition quantifiers is not relevant. Greedy and ungreedy 
nature of repetition quantifiers is not relevant. Greedy and ungreedy 
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quantifiers are treated in exactly the same way. 
quantifiers are treated in exactly the same way. However, possessive 
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quantifiers can make a difference when what follows could also match what is 
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quantified, for example in a pattern like this: 
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.sp 
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^a++\ew! 
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.sp 
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This pattern matches "aaab!" but not "aaa!", which would be matched by a 
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nonpossessive quantifier. Similarly, if an atomic group is present, it is 
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matched as if it were a standalone pattern at the current point, and the 
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longest match is then "locked in" for the rest of the overall pattern. 
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.P 
.P 
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2. When dealing with multiple paths through the tree simultaneously, it is not 
2. When dealing with multiple paths through the tree simultaneously, it is not 
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straightforward to keep track of captured substrings for the different matching 
straightforward to keep track of captured substrings for the different matching 
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not supported, and cause errors if encountered. 
not supported, and cause errors if encountered. 
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.P 
.P 
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4. For the same reason, conditional expressions that use a backreference as the 
4. For the same reason, conditional expressions that use a backreference as the 
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condition are not supported. 
condition or test for a specific group recursion are not supported. 
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.P 
.P 
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5. Callouts are supported, but the value of the \fIcapture_top\fP field is 
5. Callouts are supported, but the value of the \fIcapture_top\fP field is 
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always 1, and the value of the \fIcapture_last\fP field is always 1. 
always 1, and the value of the \fIcapture_last\fP field is always 1. 
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.P 
.P 
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6. 
6. 
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The \eC escape sequence, which (in the standard algorithm) matches a single 
The \eC escape sequence, which (in the standard algorithm) matches a single 
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byte, even in UTF8 mode, is not supported because the DFA algorithm moves 
byte, even in UTF8 mode, is not supported because the alternative algorithm 
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through the subject string one character at a time, for all active paths 
moves through the subject string one character at a time, for all active paths 
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through the tree. 
through the tree. 
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. 
. 
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.SH "ADVANTAGES OF THE DFA ALGORITHM" 
.SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM" 
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.rs 
.rs 
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.sp 
.sp 
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Using the DFA matching algorithm provides the following advantages: 
Using the alternative matching algorithm provides the following advantages: 
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.P 
.P 
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1. All possible matches (at a single point in the subject) are automatically 
1. All possible matches (at a single point in the subject) are automatically 
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found, and in particular, the longest match is found. To find more than one 
found, and in particular, the longest match is found. To find more than one 
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.P 
.P 
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2. There is much better support for partial matching. The restrictions on the 
2. There is much better support for partial matching. The restrictions on the 
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content of the pattern that apply when using the standard algorithm for partial 
content of the pattern that apply when using the standard algorithm for partial 
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matching do not apply to the DFA algorithm. For nonanchored patterns, the 
matching do not apply to the alternative algorithm. For nonanchored patterns, 
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starting position of a partial match is available. 
the starting position of a partial match is available. 
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.P 
.P 
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3. Because the DFA algorithm scans the subject string just once, and never 
3. Because the alternative algorithm scans the subject string just once, and 
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needs to backtrack, it is possible to pass very long subject strings to the 
never needs to backtrack, it is possible to pass very long subject strings to 
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matching function in several pieces, checking for partial matching each time. 
the matching function in several pieces, checking for partial matching each 
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time. 
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. 
. 
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.SH "DISADVANTAGES OF THE DFA ALGORITHM" 
.SH "DISADVANTAGES OF THE ALTERNATIVE ALGORITHM" 
152 
.rs 
.rs 
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.sp 
.sp 
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The DFA algorithm suffers from a number of disadvantages: 
The alternative algorithm suffers from a number of disadvantages: 
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.P 
.P 
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1. It is substantially slower than the standard algorithm. This is partly 
1. It is substantially slower than the standard algorithm. This is partly 
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because it has to search for all possible matches, but is also because it is 
because it has to search for all possible matches, but is also because it is 
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.P 
.P 
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2. Capturing parentheses and back references are not supported. 
2. Capturing parentheses and back references are not supported. 
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.P 
.P 
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3. The "atomic group" feature of PCRE regular expressions is supported, but 
3. Although atomic groups are supported, their use does not provide the 
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does not provide the advantage that it does for the standard algorithm. 
performance advantage that it does for the standard algorithm. 
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.P 
.P 
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.in 0 
.in 0 
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Last updated: 06 June 2006 
Last updated: 24 November 2006 
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.br 
.br 
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Copyright (c) 19972006 University of Cambridge. 
Copyright (c) 19972006 University of Cambridge. 