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Re: regular expressions that match nothing

From: Stefan Monnier
Subject: Re: regular expressions that match nothing
Date: Tue, 14 May 2019 15:41:31 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> Thank you, and sorry about my bad initial attempt. I tried a few more,
> like [z-a], \c* and \sq, but these were no better. The distribution is
> decidedly bimodal; there seems to be no significant difference between
> the 'fast' ones,

Not surprised: the "fast" ones are the ones that the regexp engine
recognizes as "anchored" so the search is reduced to a looking-at.

> so I went with \`a\` in the attached patch.

Sounds good.

>> Of course this may be dependent on the internals of the specific
>> regexp library at hand. I do not know enough to judge. In fact I
>> believe that a solid regular expression library should provide a
>> specific regular expression that matches nothing with special but
>> easy treatment that guarantees best response time.
> We could add a standard constant for it, like unmatchable-regexp, so

Yes, please.  I'd recommend a `regexp-` prefix for it.
[ And I'll carefully avoid having an opinion on the rest of the name. ]

> that at least people don't keep reinventing it.

Not only that, but I'm pretty sure casual users will find it much easier
to understand what's going on when they see `regexp-<foo>` than when
they bump into "\\`a\\`".  I.e. the constant's name should work as
a good comment.


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