private function validateAgainstExpression( $field, $value, $rule, $message = null )
    {
        if (preg_match($this->expressions[ $rule ], $value)) {
            return true;
        }

        if (!$message) {
            $message = $this->error_messages[ $rule ];
        }

        static::$errors[ $field ] = $message;

        return false;
    }