Tested in the latest emacs-25 build as of today.

emacs -Q
C-u C-s

The echo area will show "I-search:".
Earlier it used to show "Regexp I-search:" and that made it clear that we
are doing regexp search.

This change happened after this revert:
http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-25&id=b417c5a3b35bf31e66a170529c5aeb34cb3318d0

So it looks like that isearch-message-prefix was bound to
search-default-mode being non-nil.

I verified that "C-u C-s" shows "Regexp I-search:" fine in emacs 24.5. But
that version did not have the search-default-mode.

I believe that the clarity of regexp isearch being on/off is missing,
especially when C-u C-s DOES do regexp search and still displays "I-search:
" in the echo.

If I set search-default-mode back to "#'character-fold-to-regexp", this
issue is gone. So displaying of "Regexp I-search:" should be decoupled from
the character-fold.el library.

--
Kaushal Modi