ManaPHP\Http\Filter::_filter_alpha PHP Метод

    protected function _filter_alpha($value)
    {
        /** @noinspection NotOptimalRegularExpressionsInspection */
        if (preg_match('#^[a-zA-Z]+$#', $value) === 1) {
            return $value;
        } else {
            return null;
        }
    }