Re: [Maxima-discuss] regex packages, was: Reading data from header

[Volker v. N. <vol...@gm...>]

Thank you, Michel.

These are very helpful informations.

Volker van Nek

2015-10-19 10:31 GMT+02:00 Michel Talon <ta...@lp...>:

> Le 18/10/2015 23:10, Robert Dodier a écrit :
> >> In share/stringproc there is an alternative regex parser (portable regex
> >> >parser by Dorai Sitaram) with an interface at Maxima level.
> >> >
> >> >It works nicely but it appears to be quite slow.
> > Thanks for the reminder, I had forgotten about that. I suspect that
> > sregex is strictly more powerful than nregex; I'd be surprised if nregex
> > were any faster, but that is not a key point. Also it's helpful that
> > there is a Maxima interface for sregex. Given all that, I'm in favor of
> > moving sregex into src and nuking nregex. There are a few calls to
> > nregex in src, but those would be easily replaced by sregex, I believe.
> >
>
> Well, i have looked a little bit at the programes nregex, pregexp, and
> the way nregex is used in maxima. Obviously pregexp is a complete regex
> parser, covering more or less the perl regexp syntax, while nregex is
> a very basic regex parser covering the standard new regex syntax as in
> grep -E except that modern stuff [:alnum:] etc. is not supported. Of
> course only 256 characters alphabets are supported. There are some
> extensions as in emacs regexps, like \w matching words, \b matching
> boundaries, etc. (and their opposites \W, \B etc.). The repeating
> patterns {n,m} are not supported, i am not even sure the alternative
> patterns patt1|patt2 are. But thanks to all these shortcomings nregex
> is very small and can use a clever trick to speed up character classes
> matching. Basically when encountering [...] it builds a bitstring of
> length 256 having ones at each matching position (with special cases for
> \w etc.) or inverted if one has [^...] and this can be very fast to
> match on something. On the contrary pregexp does straightforward
> comparisons. Hence one may expect considerable speed difference between
> them. In the way in which regexp is used in maxima (in cl-info.lisp for
> finding stuff and in in commac.lisp for stripping a string of trailing
> zeroes) the speed difference could cause problems.
>
>
> --
> Michel Talon
>
>
>
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