/**
 * Turns all urls into clickable links.
 */
function _auto_link_urls($text, $href_options = array(), $truncate = false, $truncate_len = 40, $pad = '...')
{
  $href_options = _tag_options($href_options);

  $callback_function = '
    if (preg_match("/<a\s/i", $matches[1]))
    {
      return $matches[0];
    }
    ';

  if ($truncate)
  {
    $callback_function .= '
      else if (strlen($matches[2].$matches[3]) > '.$truncate_len.')
      {
        return $matches[1].\'<a href="\'.($matches[2] == "www." ? "http://www." : $matches[2]).$matches[3].\'"'.$href_options.'>\'.substr($matches[2].$matches[3], 0, '.$truncate_len.').\''.$pad.'</a>\'.$matches[4];
      }
      ';
  }

  $callback_function .= '
    else
    {
      return $matches[1].\'<a href="\'.($matches[2] == "www." ? "http://www." : $matches[2]).$matches[3].\'"'.$href_options.'>\'.$matches[2].$matches[3].\'</a>\'.$matches[4];
    }
    ';

  return preg_replace_callback(
    SF_AUTO_LINK_RE,
    create_function('$matches', $callback_function),
    $text
    );
}