Per the replace-regexp-in-string docstring: "Replace all matches for REGEXP
with REP in STRING."

My email was a comment to an existing open bug from 2013-08-15:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15107

On Wed, Aug 31, 2016 at 9:25 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Erik Anderson <erikbpanderson@gmail.com>
> > Date: Tue, 30 Aug 2016 23:57:35 +0000
> >
> > I can confirm the buggy behavior on emacs 24.5.1 and 25.1.50.1 for
> Kevin's example as well as:
> >
> > (replace-regexp-in-string "^.\\| ." #'upcase "foo bar")
> > > "Foo bar"  (should be "Foo Bar")
>
> Maybe I'm missing something, but I don't see why this is a bug.  The
> input string "foo bar" matches the "^." alternative in its entirety,
> so there's no reason to expect Emacs to apply 'upcase' twice.
>