From: Eric Mahurin <eric_mahurin@...>
Date: 2005-04-29T21:21:19+09:00
Subject: Re: Inverting a regular expression?

The problem with trying to do this is the length to consider a
mismatch.  If you wanted the mismatch to be considered up to
the next match, you could do something like this:

/(?!#{re}).*?(?=#{re}|$)/

or

/((?!#{re}).)*/

I don't see any practical application for doing something like
this.  Is this just a academic question?

 
--- Harry Ohlsen <Harry_Ohlsen@hyperion.com> wrote:
> A colleague of mine just asked me whether it was possible to
> invert an
> arbitrary regular expression.  Ie, create a new regular
> expression that
> matches whatever the original *didn't*.
> 
> It seems like quite a difficult problem to me, but maybe I'm
> just not
> looking at it the right way.
> 
> Obviously simple cases are easy. Eg
> 
>    re = /[a-z]/
>    inverse = /[^a-z]/
> 
> However, I think it would be much harder for arbitrary cases.
>  Eg, how
> would one automatically invert something like the following
> 
>    /<[^<>]*id="!?(region8-).*?>/
> 
> Note that I'm not talking about finding lines that don't
> contain the
> pattern (a la "grep -v"); I'm talking about finding all the
> occurrences
> within a single line that don't match it.
> 
> Inquisitively yours,
> 
> Harry O.
> 
> 
>
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