protected function validateUrlDownload()
    {
        $invalid = array();

        foreach ($this->getSubmitted('sources') as $line => $url) {
            $line++;
            if (filter_var($url, FILTER_VALIDATE_URL) === false) {
                $invalid[] = $line;
            }
        }

        if (!empty($invalid)) {
            $this->setError('sources', $this->text('Error on line @num', array('@num' => implode(',', $invalid))));
        }
    }