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

[Michel T. <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