external-shadowbox-automation.php

<?php
/*
Plugin Name: External Shadowbox Automation
Plugin URI: http://sivel.net/wordpress/
Description: Automatically adds Shadowbox the activator to external links in your post content
Author: Matt Martz
Author URI: http://sivel.net
Version: 11
*/

function external_shadowbox_automation($content) {
        global $post;
        $pattern = "/<a(.*?)href=('|\")([^'\"]*)('|\")(.*?)>/i";
        preg_match_all($pattern,$content,$matches,PREG_SET_ORDER);
        foreach ($matches as $match) {
                if (!stristr($match[3], $_SERVER['HTTP_HOST']) && 
                   !preg_match('/\ rel=(\'|")(.*?)(shadow|light|no)box(.*?)(\'|")/i', $match[0]) &&   
                    !preg_match('/\ target=(\'|")_blank(\'|")/i', $match[0])) {
                        $replacement = '<a$1href=$2$3$4 rel=$2shadowbox[post-' . $post->ID . ']$4$5>';
                        $link = preg_replace($pattern, $replacement, $match[0]);
                        $content = str_replace($match[0], $link, $content);
                }
        }
        return $content;
}

add_filter('the_content', 'external_shadowbox_automation', 100);
?>