如何给wordpress外部链接自动添加nofollow

add_filter('the_content', 'wpjam_auto_nofollow'); 
function wpjam_auto_nofollow($content) {
	return preg_replace_callback('/<a>]+/', 'wpjam_auto_nofollow_callback', $content);
}
function wpjam_auto_nofollow_callback($matches) {
	$link = $matches[0];
	$site_link = get_bloginfo('url');

	if (strpos($link, 'rel') === false) {
		$link = preg_replace("%(href=S(?!$site_link))%i", 'rel="nofollow" $1', $link);
	} elseif (preg_match("%href=S(?!$site_link)%i", $link)) {
		$link = preg_replace('/rel=S(?!nofollow)S*/i', 'rel="nofollow"', $link);
	}
	return $link;
}