How can I chab=nge the format of a URL?

This looks right, but it is still passing the original URI. I'm stumped.

 

 

when HTTP_REQUEST {

 

 

if { [HTTP::uri] contains "adserver/impression" }{

 

 

set found [scan [HTTP::uri] "/adserver/impression/pid=%d/oid=%d/rand=%d" pidnum oidnum randnum]

 

 

if { $found == 3 } {

 

 

log local0. "This is the URI = [HTTP::uri]"

 

 

log local0. "Pid = $pidnum, Oid = $oidnum, rand = $randnum"

 

 

set newuri [string map [subst {"adserver/impression/pid=$pidnum/oid=$oidnum/rand=$randnum/?click" "ad.imp?pid=$pidnum&oid=$oidnum&rand=$randnum/?pclk"}] [HTTP::uri]]

 

 

log local0. "New URI: $newuri";

 

 

HTTP::uri $newuri;

 

 

}

 

 

}

 

 

}