[Linux-disciples] Negative Perl Regexp

[Chung-chieh Shan]

On 2004-07-22T14:26:11-0400, Adam Rosi-Kessel wrote:
> This should be easy, but I've looked at perlre and am stuck.

Really?  Search for "negat" in perlre perhaps?

> Does anyone know how to make a regexp that only returns true if there is
> no match in perl, assuming you don't have access to the surrounding code,
> just the regexp?

You can always do it, even without the negation stuff in perlre, for
"true" regular expressions (i.e., those for languages recognizable by a
finite automaton).  For example,

    not /ab/

is like

    /\A[^a]*((a+[^ab])+[^a]*)*a*\z/

but it's easier to do it with

    /\A(?:(?!ab).)*\z/

(Untested.)

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