or_target_blank.php

<?
/*
  Plugin Name: Target Blank
  Plugin Author: Olivier Ruffin
  Description: A Wordpress plugin to add a target='_blank' to all post and comments links which are external to the blog
  Version: 1.3
  Author: Olivier Ruffin
  Author URI: http://veilleperso.com
*/

define('OR_INTERNAL_LINK', "/^".preg_replace("/:[\/]{2,2}/", ":\/\/[^\/]*", preg_quote(preg_replace("/\/\/www./", "//", get_option("siteurl"))))."/i");

function or_convert_external_link($matches) {
  if (preg_match(OR_INTERNAL_LINK, $matches[2])) return $matches[0];
  else {
    if (preg_match("/(class=['\"])/im", $matches[3], $cls)) $matches[3] = str_replace($cls[1], $cls[1]."external ", $matches[3]);
    else $matches[3].= " class=\"external\"";
    return "<a$matches[1]href=\"$matches[2]\" ".trim($matches[3])." target=\"_blank\">";
  }
}
	
function or_external_link($text) {
  // use a target blank on external links
	$pattern = '/<a([^>]*?)href=[\"\'](https*:\/\/.*?)[\"\']([^>]*?)>/i';
	return preg_replace_callback($pattern, 'or_convert_external_link', $text);
}

if (!is_feed()) {
  add_filter('comment_text', 'or_external_link', 999);
  add_filter('the_content', 'or_external_link', 999);
  add_filter('the_excerpt', 'or_external_link', 999);
  add_filter('get_comment_author_link', 'or_external_link', 999);
}