tis 2019-05-14 klockan 09:25 +0200 skrev philippe schnoebelen:
> I was wondering why (regexp-opt nil) uses a\` and not \'a or another
> option like \=a\= so I did some profiling (see attached code).

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

> 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
that at least people don't keep reinventing it.
We could also make (rx (or)) work. (It does in my complete rx rewrite.)