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No, I meant solved by an NFA.
Today NFAs can solve all problems that DFAs can solve, but
it is possible to by accident write an NFA that will take
several years to finish. My suggestion allows you to
convert an NFA into an equivalent NFA which backtracks at
fewer points. However it remains an NFA, and where an
NFA would find a different match than a DFA, it will find
the match that an NFA would find, and not the one that a
DFA would. (Thereby allowing it to be used in an NFA
engine without changing the behaviour.)
Now if the RE is one that you could use a DFA to solve,
then you can convert the NFA into an NFA with no
backtracking anywhere. However it still retains NFA
characteristics. For instance with alternation it will
prefer to match the first alternative in the sequence, and
won't match the second if that allows a longer overall
match.
Unfortunately optimizing the NFA that far may result in
a combinatorial explosion from n states to n!, but since
the optimization goes in steps, you can just optimize until
you hit some threshold, and then stop. (Or you could run,
profile, incrementally optimize, wash, rinse, and repeat.)
| [reply] [Watch: Dir/Any] |
Ok's, I've gotchas now tilly.
I understand the difference between NFA's and DFA's, in fact I think
it would be nice if you could have a DFA-preference modifier in Perl on
your regex's so that if they are DFA-compatible, they would use the DFA
instead of an NFA. (or even as egrep does, -pretend- they are DFA
compatible, then check your answer with an NFA).
I'm no wiz with regex's though, but what you propose seems very,
very difficult! At least, it seems that if you told it to do even
something simple like: /vr(o*)*m/ (which if I'm thinking
correctly would cause a normal NFA to go ga-ga), you would be trying to
get the regex engine to realize first that perhaps we should either ignore
the double *, or (as it would be in more complicate situations) reverse
the processing order of the *'s, or even something more 'odd'.
And so my point is, even if you can get the regex engine to realize it
can do something to avert catastrophe, couldn't many of the 'solutions'
that the regex decides upon change which match the NFA chooses? That is,
changed from what the original badly written form asked for? The whole
intent on using an NFA is so you can choose optimized or taylored regex's
that either give you speed or choice in your matches (and sometimes both
:) ). But wouldn't this sort of optimization make which match the NFA
picks sort of, non-obvious? (in a more non-obvious than normal for regex's
way)?
Again, I'm not an expert regex'er, but it seems using a DFA in these
sorts of situations (even if a DFA can't be used directly), would be
similar -- you would avoid the pitfalls of killing the performance, but
you suffer in that maybe you're not getting the match you expected.
Well, I'm probably brain-fried right now, so don't pay any attention
to me :) .
Ciao,
Gryn
p.s. for those that are in the dark the main difference between a NFA
and a DFA is one of performance and control.
A DFA takes almost always the same amount of time to make a match, and
it will always make the same one for equivalent regex's (the longest of
the leftmost (er, or the otherway around, sorry)).
A NFA is the opposite, it may (often) be much fater than a DFA, but it
also may get stuck in a rut and finish sometime after the Sun burns out.
Besides the speed improvement, you often get a control advantage. That
is, for equivalent regex's (ones that would match the same thing, but
don't look exactly the same because of reordering or what-not) you can
control which match you want by rearranging your regex (which can't be
done for DFA's).
| [reply] [Watch: Dir/Any] [d/l] |
Are you sure you know the theory? | [reply] [Watch: Dir/Any] |
I'm positive that I know the theory, and that my suggestion will work. Figuring things like this out is what I studied math for. :-)
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