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

Yeah, that one burned me a long time ago and forgot about it. the "string map" command performs a literal replacement (ie. it doesn't do variable substitution). So in your example it was looking for the literal string "adserver/impression/pid=$pidnum/oid=$oidnum/rand=$randnum/?click" instead of the correct "adserver/impression/pid=123/oid=860/rand=12345/?click" Same goes for the replacement string.

Also, your second log statement as a typo with the $oidnumm variable which may throw an error.

The good news about the variable substitution is that you can fix that by wrapping the map strings with a "subst" command. This should work for you:

 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; 
     } 
   } 
 }

I threw in a sanity check for the scan to verify that it did indeed find the three values and I also put in the newly mapped URI into a variable and logged it so you can verify in the BIG-IP logs whether the "string map" worked. Oh, and I removed the $uri variable for optimization. You can remove the $newuri variable after you've tested and it works.

A fully optimized iRule would be this:

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

Hope this helps!

-Joe