# File lib/active_record/validations.rb, line 789
      def validates_format_of(*attr_names)
        configuration = { :on => :save, :with => nil }
        configuration.update(attr_names.extract_options!)

        raise(ArgumentError, "A regular expression must be supplied as the :with option of the configuration hash") unless configuration[:with].is_a?(Regexp)

        validates_each(attr_names, configuration) do |record, attr_name, value|
          unless value.to_s =~ configuration[:with]
            record.errors.add(attr_name, :invalid, :default => configuration[:message], :value => value) 
          end
        end
      end