    # 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