I was very happy to see that in v27.0.50 (regexp-opt nil) now properly
returns a regular expression that matches nothing, namely a\`. Thanks to
whoever fixed that old bug.

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).

The different options that I tried have more or less the same response time
when one checks, via looking-at, whether the regexp matches at point. But
when one searches for a match across a whole buffer, some options behave
notably faster than the others. And a\` is not the best, e.g., \=a\= is way
faster. Maybe some other solutions would be even faster.

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.

--phs