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From: pereira@research.att.com (Fernando Pereira)
Subject: Re: Regexp notation
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References: <qijg20caw6l.fsf@lambda.ai.mit.edu> <qijenfubntr.fsf@lambda.ai.mit.edu> <qijd8vebe9x.fsf@lambda.ai.mit.edu> <5b4kgn$b8h@news.jf.intel.com> <qijbuaxbn7l.fsf@lambda.ai.mit.edu>
Date: Tue, 14 Jan 1997 01:36:05 GMT
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Xref: glinda.oz.cs.cmu.edu comp.lang.scheme.scsh:373 comp.lang.scheme:17929

In article <qijbuaxbn7l.fsf@lambda.ai.mit.edu>, shivers@ai.mit.edu wrote:
> Thank you for your input, Mike. I was hoping an implementor like you would
> comment on the issues.
> 
> I came to Kelvin's conclusions on my own last night, namely
> 
>     - Complement means just complementing accept/non-accept states
>       in the NDFA. No blowup in NDFA space at all. How this affects
>       state count in the result DFA compared to the original DFA, 
>       I don't know.
Not quite. Exchanging accepting with non-accepting states only works for
DFAs, because it assumes implicitly a single path for any string. More
detail: In a NFA, there could be two paths for string s, one accepting,
the other nonaccpting. According to the standard defs, s is accepted by
the NFA. If you now exchange accept and non-accept, s is still accepted in
the new NFA, that is, s is in the language and in its complement, a
contradiction.
