Modify links in user comments

To replace nofollow with nofollow 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