//attaches in-reply-to classes to the first link in the post

function the_content_in_reply_to_callback($matches){
    if(preg_match('`href=.mailto`i',$matches[0])){
        return $matches[0];
    }else{
        return '<a class="u-in-reply-to" rel="in-reply-to" '.$matches[1].'>'.$matches[2].'@'.$matches[3].'</a>';
    }
}
function the_content_in_reply_to_filter($content) {
  return preg_replace_callback('`\<[aA] (.*?)>(.*?)\@(.*?)</[aA]>`','the_content_in_reply_to_callback',$content,1);
}

add_filter( 'the_content', 'the_content_in_reply_to_filter' );