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revision 99 by ph10, Tue Mar 6 12:27:42 2007 UTC revision 426 by ph10, Wed Aug 26 15:38:32 2009 UTC
# Line 41  correspond to the two matching algorithm Line 41  correspond to the two matching algorithm
41  .SH "THE STANDARD MATCHING ALGORITHM"  .SH "THE STANDARD MATCHING ALGORITHM"
42  .rs  .rs
43  .sp  .sp
44  In the terminology of Jeffrey Friedl's book \fIMastering Regular  In the terminology of Jeffrey Friedl's book "Mastering Regular
45  Expressions\fP, the standard algorithm is an "NFA algorithm". It conducts a  Expressions", the standard algorithm is an "NFA algorithm". It conducts a
46  depth-first search of the pattern tree. That is, it proceeds along a single  depth-first search of the pattern tree. That is, it proceeds along a single
47  path through the tree, checking that the subject matches what is required. When  path through the tree, checking that the subject matches what is required. When
48  there is a mismatch, the algorithm tries any alternatives at the current point,  there is a mismatch, the algorithm tries any alternatives at the current point,
# Line 119  not supported, and cause errors if encou Line 119  not supported, and cause errors if encou
119  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
120  condition or test for a specific group recursion are not supported.  condition or test for a specific group recursion are not supported.
121  .P  .P
122  5. Callouts are supported, but the value of the \fIcapture_top\fP field is  5. Because many paths through the tree may be active, the \eK escape sequence,
123    which resets the start of the match when encountered (but may be on some paths
124    and not on others), is not supported. It causes an error if encountered.
125    .P
126    6. Callouts are supported, but the value of the \fIcapture_top\fP field is
127  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.
128  .P  .P
129  6.  7. The \eC escape sequence, which (in the standard algorithm) matches a single
 The \eC escape sequence, which (in the standard algorithm) matches a single  
130  byte, even in UTF-8 mode, is not supported because the alternative algorithm  byte, even in UTF-8 mode, is not supported because the alternative algorithm
131  moves 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
132  through the tree.  through the tree.
133    .P
134    8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
135    supported. (*FAIL) is supported, and behaves like a failing negative assertion.
136  .  .
137  .SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM"  .SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM"
138  .rs  .rs
# Line 138  found, and in particular, the longest ma Line 144  found, and in particular, the longest ma
144  match using the standard algorithm, you have to do kludgy things with  match using the standard algorithm, you have to do kludgy things with
145  callouts.  callouts.
146  .P  .P
147  2. There is much better support for partial matching. The restrictions on the  2. Because the alternative algorithm scans the subject string just once, and
 content of the pattern that apply when using the standard algorithm for partial  
 matching do not apply to the alternative algorithm. For non-anchored patterns,  
 the starting position of a partial match is available.  
 .P  
 3. Because the alternative algorithm scans the subject string just once, and  
148  never needs to backtrack, it is possible to pass very long subject strings to  never needs to backtrack, it is possible to pass very long subject strings to
149  the matching function in several pieces, checking for partial matching each  the matching function in several pieces, checking for partial matching each
150  time.  time.
# Line 177  Cambridge CB2 3QH, England. Line 178  Cambridge CB2 3QH, England.
178  .rs  .rs
179  .sp  .sp
180  .nf  .nf
181  Last updated: 06 March 2007  Last updated: 25 August 2009
182  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
183  .fi  .fi

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