public function validateUsername($username = null)
    {
        if ($username === null) {
            $username = $this->_username;
        }

        // If the username is empty, then it is considered valid
        if (strlen($username) === 0) {
            return true;
        }

        // Check the username against the allowed values
        $status = @preg_match('/^(?:' . $this->_regex['escaped'] . '|[' .
            self::CHAR_ALNUM . self::CHAR_MARK . ';:&=+$,' . '])+$/', $username);

        if ($status === false) {
            throw new Zend_Uri_Exception('Internal error: username validation failed');
        }

        return $status === 1;
    }