efficient regular expression that never matches

[cinsk <cinsky@gmail.com>]

hi,

Is there any special regular expression string that never matches
anything?

I'm thinking of "$a", but it should scan all the string until end
of the string, so it may not be efficient.

Is there any short, efficient regular expression that never
matches anything? (it's okay with Emacs-specific regular
expression, since I'm going to use it with `directory-files' or
`string-match' function)
 
Thank you.