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

String map won't work with variables wrapped in curly braces. So you can either use subst like Joe listed, or remove the curly braces. Also, Citizen's suggestion was to add a second HTTP request event with a priority of 501 so you could see the effect of the change once it is made. And you'll need to include a leading forward slash in the scan pattern or it won't match any URI.

Can you try this and reply with the log output from /var/log/ltm if it doesn't work expected?

 
 when HTTP_REQUEST { 
  
    log local0. "[IP::client_addr]:[TCP::client_port]: New HTTP request to [HTTP::uri]" 
  
    if { [HTTP::uri] contains "adserver/impression" }{ 
       log local0. "[IP::client_addr]:[TCP::client_port]: Matched URI check" 
  
        Scan the URI looking for the pid, oid and rand values 
       if { [scan [HTTP::uri] "/adserver/impression/pid=%d/oid=%d/rand=%d" pid oid rand] == 3 } { 
  
          log local0. "[IP::client_addr]:[TCP::client_port]: Scanned three values: pid = $pid, oid = $oid, rand = $rand" 
          HTTP::uri [string map "adserver/impression/pid=$pid/oid=$oid/rand=$rand/?click ad.imp?pid=$pid&oid=$oid&rand=$rand/?pclk" [HTTP::uri]] 
       } 
    } 
 } 
 when HTTP_REQUEST priority 501 { 
    log local0. "[IP::client_addr]:[TCP::client_port]: 501: Updated URI: [HTTP::uri]" 
 } 
 

Aaron