Query_String not matching

[Roland RoLaNd]

Hello,

I am obviously a new nginx user, so please bear with me.

I have setup nginx as a content caching service, running on port 80 and directing traffic to backend servers.

all works okay, though one specific scenario where i want to cache an object matching a specific query string.

Here's a sample:

.../ getimage.view&id=19823&class=5617&size=80 

I want to cache objecting matching : class=5617&size=80 
since obviously the ID will change depending on user.


i googled  nginx config,etc... everything i found simply explains that i could match $uri and $args but not how to bypass args 

i tried the following:

 if ($uri ~* "^getimage.view") {
 	 set $args $2$3;
 	set $cache_key $scheme$host$uri$is_args$args;
    }


But it's not working....
any advice? or better yet any place i can learn more about manipulating query strings ?

Thanks in advance.