Which brings to mind a good point, for which I thank you. That is--the challenge "compare two algorithms, and decide if they do the same thing" is provably incomputable in the general case (c.f. "the Halting Problem")--and yet, compilers replace code with more efficient but guaranteed-identical code every day. Thus, while it might not be possible to determine for all regexps whether one is a subset of another, it may well be possible for most regexps to determine if one is a subset of another.

And this is all that is needed in our case, because we're trying to derive as much information as possible, not all information. If we can only prove nine of ten steps in a process, we can't say that we've proven the process, but we're a lot better off than if we couldn't prove any of them.

So. If we restate the problem as "For most simple regexps, determine whether regexp A matches a subset of the strings that regexp B matches", does that make it seem more tractable?

Hopefully,

Mickey.

In reply to Re^3: Comparative satisfiability of regexps. by Meowse
in thread Comparative satisfiability of regexps. by Meowse

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