# File formvalidator.rb, line 542
    def apply_string_constraint(key, constraint)
      # FIXME: multiple elements
      res = self.send("match_#{constraint}".intern, @form[key].to_s)
      if res
        if untaint?(key)
          @form[key] = res 
          @form[key].untaint
        end
      else
        @form.delete(key)
        @invalid_fields[key] ||= []
        unless @invalid_fields[key].include?(constraint)
          @invalid_fields[key].push(constraint) 
        end
        nil
      end
    end