rewrite rules question

Hello,
Could somebody help me with rewrite rules ?

Let’s say I have a link:
http://example.com/city/?city_id=99

How I can make it like this:
http://example.com/city/99/

I tried this:
add_rewrite_rule('^city/([^/]+)/?$','index.php?pagename=city&city_id=$matches[1]', 'top');

but it does not work.