Making parsed links open a new tab/window

The background to this is a small site where the site owner is adding links in the form www.sitename.com to filtered HTML text. Drupal does a great job of recognizing these and converting them into "a" tags.

However, these links take the visitor away from his site, so I'd like them to open a tab / window. I've dug down into the code and found the drupal/modules/filter/filter.module file and within it the _filter_url_parse_full_links and _filter_url_parse_partial_links functions.

I changed them to add target attributes as follows:

/**
 * Make links out of absolute URLs.
 */
function _filter_url_parse_full_links($match) {
  $match[2] = decode_entities($match[2]);
  $caption = check_plain(_filter_url_trim($match[2]));
  $match[2] = check_url($match[2]);
  return $match[1] . '<ahref="'. $match[2] .'" title="'. $match[2] .'" target=\"_blank\">'. $caption .'</a>'. $match[5];
}

/**
 * Make links out of domain names starting with "www."
 */
function _filter_url_parse_partial_links($match) {
  $match[2] = decode_entities($match[2]);
  $caption = check_plain(_filter_url_trim($match[2]));
  $match[2] = check_plain($match[2]);
  return $match[1] . '<ahref="http://'. $match[2] .'" title="'. $match[2] .'" target=\"_blank\">'. $caption .'</a>'. $match[3];
}

However, these changes seem to be completely ignored. I've double checked the code on the server and the changes are definitely there, but they aren't reflected in the HTML being generated.

So, am I in completely the wrong place? or am I missing something silly and obvious?

All help and ideas appreciated.