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

[Leo B. <l_b...@us...>]

One further observation: sregex is written in scheme and uses recursion
extensively. When I looked at sregex a few years ago as a possible
replacement for nregex, I recall that the lisp version ran very slow and
had numerous stack overflows when run against Maxima's info files.

Leo

Volker van Nek <vol...@gm...> writes:

> 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
>>
>>
>>
>> ------------------------------------------------------------------------------
>> _______________________________________________
>> Maxima-discuss mailing list
>> Max...@li...
>> https://lists.sourceforge.net/lists/listinfo/maxima-discuss
>>
> ------------------------------------------------------------------------------
> _______________________________________________
> Maxima-discuss mailing list
> Max...@li...
> https://lists.sourceforge.net/lists/listinfo/maxima-discuss