youtube-rel.php

<?php
/*
Plugin Name: YouTube PrettyPhoto links
Description: This will filter YoutTube links in posts to add rel="PrettyPhoto" to each link.
Author: Jan Dembowski
Version: 0.1
Author URI: http://blog.dembowski.net/

*/

add_filter( 'the_content' , 'mh_youtube_rel' , 50 );

function mh_youtube_rel( $content ) {

// Regex to put all YouTube links into an array
$mh_url_regex = "/\<a\ href\=\"http\:\/\/.*youtube\.com\/.*\"/";
preg_match_all( $mh_url_regex , $content, $mh_matches );

// Go through that array and add rel="PrettyPhoto" to all selected links
for ( $mh_count = 0; $mh_count < count( $mh_matches[0] ); $mh_count++ )
    {
    $mh_old_url = $mh_matches[0][$mh_count];
    $mh_new_url = substr_replace( $mh_matches[0][$mh_count] , '" rel="PrettyPhoto"' , -1 );
    $content = str_replace( $mh_old_url  , $mh_new_url , $content );
    }

return $content;
}