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revision 25 by nigel, Sat Feb 24 21:38:45 2007 UTC revision 27 by nigel, Sat Feb 24 21:38:49 2007 UTC
# Line 1090  same length of string. An assertion such Line 1090  same length of string. An assertion such
1090    
1091    (?<=ab(c|de))    (?<=ab(c|de))
1092    
1093  is not permitted, because its single branch can match two different lengths,  is not permitted, because its single top-level branch can match two different
1094  but it is acceptable if rewritten to use two branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
1095    
1096    (?<=abc|abde)    (?<=abc|abde)
1097    
1098  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1099  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed width and then try to
1100  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1101  match is deemed to fail.  match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1102    can be particularly useful for matching at the ends of strings; an example is
1103    given at the end of the section on once-only subpatterns.
1104    
1105  Assertions can be nested in any combination. For example,  Several assertions (of any sort) may occur in succession. For example,
1106    
1107      (?<=\\d{3})(?<!999)foo
1108    
1109    matches "foo" preceded by three digits that are not "999". Furthermore,
1110    assertions can be nested in any combination. For example,
1111    
1112    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1113    
# Line 1157  number of digits they match in order to Line 1164  number of digits they match in order to
1164  This construction can of course contain arbitrarily complicated subpatterns,  This construction can of course contain arbitrarily complicated subpatterns,
1165  and it can be nested.  and it can be nested.
1166    
1167    Once-only subpatterns can be used in conjunction with lookbehind assertions to
1168    specify efficient matching at the end of the subject string. Consider a simple
1169    pattern such as
1170    
1171      abcd$
1172    
1173    when applied to a long string which does not match it. Because matching
1174    proceeds from left to right, PCRE will look for each "a" in the subject and
1175    then see if what follows matches the rest of the pattern. If the pattern is
1176    specified as
1177    
1178      .*abcd$
1179    
1180    then the initial .* matches the entire string at first, but when this fails, it
1181    backtracks to match all but the last character, then all but the last two
1182    characters, and so on. Once again the search for "a" covers the entire string,
1183    from right to left, so we are no better off. However, if the pattern is written
1184    as
1185    
1186      (?>.*)(?<=abcd)
1187    
1188    then there can be no backtracking for the .* item; it can match only the entire
1189    string. The subsequent lookbehind assertion does a single test on the last four
1190    characters. If it fails, the match fails immediately. For long strings, this
1191    approach makes a significant difference to the processing time.
1192    
1193    
1194  .SH CONDITIONAL SUBPATTERNS  .SH CONDITIONAL SUBPATTERNS
1195  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
# Line 1236  Cambridge CB2 3QG, England. Line 1269  Cambridge CB2 3QG, England.
1269  .br  .br
1270  Phone: +44 1223 334714  Phone: +44 1223 334714
1271    
1272  Copyright (c) 1998 University of Cambridge.  Copyright (c) 1997-1999 University of Cambridge.
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