preg_replace <pre ที่มี class หรือไม่มี class ก็ได้. ทำยังไงครับ?

class Modifier
{


        /**
         * automatically add line numbers class to pre and code content.
         * 
         * @link http://www.devnetwork.net/viewtopic.php?f=38&t=102670 long pattern from here.
         * @param string $content
         * @return string
         */
        public function automaticAddLineNumbersClass($content = '')
        {
            $pattern = '/<pre?(([^>]*)class=["|\'](?<class>[^"]*)["|\'])?.*\s*><code/iu';
            $content = preg_replace_callback($pattern, [&$this, 'replaceAddClassToPre'], $content);
            return $content;
        }// automaticAddLineNumbersClass


        /**
         * replace or add class to pre tag.
         * 
         * @param array $matches
         * @return string
         */
        private function replaceAddClassToPre($matches)
        {
            if (is_array($matches) && array_key_exists(0, $matches)) {
                if (strpos($matches[0], 'class=') !== false) {
                    $matches[0] = str_replace('\'', '"', $matches[0]);
                    return str_replace('class="', 'class="line-numbers ', $matches[0]);
                } elseif (strpos($matches[0], '<pre>') !== false) {
                    return str_replace('<pre', '<pre class="line-numbers"', $matches[0]);
                } else {
                    return $matches[0];
                }
            }
        }// replaceAddClassToPre


}