To replace
nofollow
withnofollow external _blank
Put this code in your theme’s functions.php
file to replace text as required.
function wpse_60668_relnofollow($text) {
$return = str_replace('nofollow', 'external nofollow _blank', $text);
return $return;
}
add_filter('get_comment_author_link', 'wpse_60668_relnofollow'); //this'll change author link
add_filter('comment_text', 'wpse_60668_relnofollow'); //This'll change the content