Add nofollow to links

So i try to write my firs function for wordpress and have a problem.

function add_nofollow_to_links($link) { 

	$domain = str_replace("www.",'', parse_url(site_url(), PHP_URL_HOST));
	preg_match_all("/<a\s[^>]*href=(\"??)([^\" >]*?)\\1[^>]*>(.*)<\/a>/siU", $link, $matches);

	for ($i=0; $i < count($matches[2]); $i++) {
		if(strpos($matches[2][$i], $domain) === false){
			$matches[0][$i] = str_replace('<a', '<a rel="nofollow"', $matches[0][$i]);
		}
		$new_link.= $matches[0][$i]."\n";
	}

	return $new_link;

}

add_filter('the_content','add_nofollow_to_links', 0);

The problem is in the end of lines… In the $link (via var_dump) i couldn’t found any transfers to a new line, but it transfers links automatically as they are in post editor, but when i return $new_link string, all goes in one line…