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Re: Make peg.el a built-in library?

From: Stefan Monnier
Subject: Re: Make peg.el a built-in library?
Date: Sat, 09 Oct 2021 23:43:28 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

>> (4) How hard would it be to parse regexps (or translate  `rx' forms) into
>> an equivalent peg?
> I had this idea as well -- we've already got "regexps that look like
> forms", it seems like it would be a natural to integrate this with rx.
> One thing we're not short of here is new ideas for code, but I do think
> this would make a lot of sense.

I think turning a regexp into a PEG should be easy, but at one condition:
you shouldn't expect that PEG to be *equivalent* to the regexp.

E.g. when matching

        (string-match "\\(ab\\|a)bc" "abc")

the "natural" PEG for that regexp will fail to match (because it will
see a success to match "ab" and will hence just skip the "a"

Correctly matching regexps requires a deeper form of backtracking than
provided by PEGs.


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