//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{
		$class = "u-in-reply-to";
		if(preg_match('`(\<|&lt;)3`i',$matches[3])){
			$class = $class ." u-like";
		}
		return '<a class="'.$class.'" rel="in-reply-to" '.$matches[1].'>'.$matches[2].$matches[3].$matches[4].'</a>';
	}
}
function the_content_in_reply_to_filter($content) {
  return preg_replace_callback('`\<[aA] (.*?)>(.*?)(\<3|&lt;3|@)(.*?)</[aA]>`','the_content_in_reply_to_callback',$content,1);
}

add_filter( 'the_content', 'the_content_in_reply_to_filter' );