I believe you. Please ignore this babbling baboon.
That's what I get for typing on the bus.
On Apr 13, 2015 4:34 PM, "David Kastrup" <dak@gnu.org> wrote:

> Artur Malabarba <bruce.connor.am@gmail.com> writes:
>
> > 2015-04-12 10:50 GMT+01:00 David Kastrup
> >> Both
> >>
> >> M-: (regexp-opt nil) RET
> >>
> >> and
> >>
> >> M-: (regexp-opt nil t) RET
> >>
> >> return "".  However, they should return a regexp matching nothing
> >> rather than everything, and the second invocation should also count as
> >> one () pairing.
> >
> > I agree there should be () on the second one, but I strongly disagree
> > they should match nothing.
> >
> > regexp-opt is NOT meant to match only the given strings. It is meant
> > to match anything containing the given strings.
>
> Well, and no string to match has been given.  This is not
> (regexp-opt '(""))
> but rather
> (regexp-opt '())
>
> > There is a very fundamental difference in that. The less strings you
> > pass to regexp-opt, the MORE things the regexp will match.
>
> Come again?
>
> > Why would we suddently flip that on its head when going from 1 to 0
> > strings?
>
> (regexp-opt '("a" "b" "c")) -> "[abc]"
> (regexp-opt '("a" "b")) -> "[ab]"
>
> Quite literally (execute C-x C-e after the expressions above if you
> don't believe me).  So how does "[ab]" match more than "[abc]" ?
>
> --
> David Kastrup
>