On Thu, 25 Mar 2004, Robert Allerstorfer wrote:
On Thu, 25 Mar 2004, 09:50 GMT-01 Klaus Johannes Rusch wrote:
Regular expressions generally define matching patterns and have no
concept of negation
this is not a general rule for regexes. Perl's regex machine knows
constructs that match the presence and the absence of a pattern, in
both directions. They are known as positive look-aheads, negative
look-aheads, positive look-behinds and negative look-behinds.
Perl's negative look-aheads and look-behinds are not equivalent to negated
regular expressions. Among other restrictions documented in the "perlre"
manual, look-ahead/behind must be fixed-length subpatterns (no "?", "+",
"*", or "{,}" operators). And even if they were fully general patterns,
the statement "true for a string that matches foo without bar" is not
equivalent to "true for a string that does not match bar".
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