How to edit ajax output?

Question

Drupal

I receive data from the parser and write it to the node using Ajax. What is the problem? I'm trying to replace links in the callback function, but nothing happens. What did you do wrong?

function ajax_callback($page_callback_result) {
   // Only render content and validates return
  $content = is_string($page_callback_result) ? $page_callback_result :   drupal_render($page_callback_result);
  $html = '' . drupal_get_css() . drupal_get_js() . '' . $content . '';
 if (!user_is_logged_in()) { // действия, касающиеся авторизованного пользователя
    $html = preg_replace("!<a.*\/?a>!U","Link",$html); 
  }
    print $html;
    drupal_page_footer(); 
}