> From: Dan Doel [mailto:dolio / case.edu] 
>
> 1) Convert the regex to a DFA
> 2) Invert the accept states of the DFA, causing it to accept the
complement
>    of the initial language
> 3) Convert the modified DFA into a regex

Good point!  I assume steps 1 and 2 are left as exercises for the reader
:-).

> However, it should be noted that regular expressions aren't really
regular
> these days. For example, Ruby allows the following regex:
>
>   /(a*)b(\1)/

> Which accepts the language { (a^n)b(a^n) }, which is context free, not
> regular (that is, it cannot be encoded as the language of a finite
> automaton---it requires something like a pushdown automaton (FA with a
> stack)---or the language of a true regular expression---it requires a
> context free grammar). So, if you want to invert an arbitrary
> Ruby/Perl/Python "regular expression," you've got a tougher time on
your
> hands.

Another good point.  Yes, REs as used in computer languages have
certainly moved on.

Thanks for the insight.

H.


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