public static function validate_fields($field, $value) {
        switch ($field) {
            case 'minute' :
            case 'hour' :
                $regex = "/\A\*\z|\A[0-5]?[0-9]\z|\A\*\/[0-5]?[0-9]\z|\A[0-5]?[0-9](,[0-5]?[0-9])*\z|\A[0-5]?[0-9]-[0-5]?[0-9]\z/";
                break;
            case 'day':
                $regex = "/\A\*\z|\A([1-2]?[0-9]|3[0-1])\z|\A\*\/([1-2]?[0-9]|3[0-1])\z|";
                $regex .= "\A([1-2]?[0-9]|3[0-1])(,([1-2]?[0-9]|3[0-1]))*\z|\A([1-2]?[0-9]|3[0-1])-([1-2]?[0-9]|3[0-1])\z/";
                break;
            case 'month':
                $regex = "/\A\*\z|\A([0-9]|1[0-2])\z|\A\*\/([0-9]|1[0-2])\z|\A([0-9]|1[0-2])(,([0-9]|1[0-2]))*\z|";
                $regex .= "\A([0-9]|1[0-2])-([0-9]|1[0-2])\z/";
                break;
            case 'dayofweek':
                $regex = "/\A\*\z|\A[0-6]\z|\A\*\/[0-6]\z|\A[0-6](,[0-6])*\z|\A[0-6]-[0-6]\z/";
                break;
            default:
                return false;
        }
        return (bool)preg_match($regex, $value);
    }