How to sort database query result by expression?

Hello everybody!

I've a database query with expression that use regexp.
Everything is working perfectly, but I can't sort the result by expression:

  $query = db_select('node', 'n');
  $query->addField('n', 'title', 'node_title');
  $query->addExpression("title REGEXP '^(.*(compact|watts|different|bulb).*){2}$'", 'r');

  $query->orderBy('r', 'DESC'); // this is not working!

  $result = $query->execute();

When I comment the string with 'orderBy' - everything is working: expression field has value 0 or 1.
When I enable sorting: expression field always shows 0.

Any ideas?

Thanks, Yuriy.