what means "\\s-" used as a regexp?

[stefano . sabatini-lala]

Hello everybody.

I recently found in some code the use of "\\s-". Documentation
appearantly said nothing on it.

(string-match "\\s-" "hello world")
==>5

(string-match "\\s-" "hello\\tworld")
==>nil

(string-match "\\s-" "hello\\nworld")
==>nil

So it seems "\\s-" match only the space.
But I still doesn't understand the  meaning of the hypen at the end of
the
regexp "\\s-".

(string-match "\\s" "hello world")
returns the error: (invalid-regexp "Premature end of regular
expression")

Thank you. 
--stefano