REGEX - PROBLEM

Quoting Newb Newb <rails-mailing-list@andreas-s.net>:

Helloo..
  how to make this below regular expression not to accept _ anywhere in
the string
  /^[a-zA-Z0-9.]+[\w\s]*$/
  how can i modify it..
  Any Advices or Suggesions...
  Thanks

if s =~ /_/
  reject string
end