module WEBrick::HTTPAuth::Authenticator

def check_scheme(req)
  unless credentials = req[@request_field]
    error("no credentials in the request.")
    return nil
  end
  unless match = /^#{@auth_scheme}\s+/i.match(credentials)
    error("invalid scheme in %s.", credentials)
    info("%s: %s", @request_field, credentials) if $DEBUG
    return nil
  end
  return match.post_match
end