help out adding rewrite rule

Hi.
I want to test wordpress add rewrite rule function.
I added below code to my theme’s functions.php in order to rewrite tha category permalinks.

add_action( 'init', 'add_cat_rules' );
function add_cat_rules() {
    add_rewrite_rule(
        "/cid=([0-9]+)/?",
        "index.php?cat=$matches[1]",
        "top");
}

but it does not do anything.
can anyone give me a working example on how to rewrite my permalinks?
Thanks a lot.