Persist based on a portion of SID

Assuming the sid=JSNd.fstR2gz5Y. is a static length before your Identifier of 6, and that the & is always the terminator, you can pull out the identifier and set it to a variable. Here's a start:

when HTTP_REQUEST { 
   if { [HTTP::cookie exists "persist_me"] } { 
     persist uie [HTTP::cookie "persist_me"] 
     } 
 } 
        
 when HTTP_RESPONSE { 
   set server_id [ findstr [HTTP::uri] "sid" 19 "&"] 
   You'll need logic here to compare the value of server_id 
   to the real server, and create new variable to insert into 
   cookie so when the HTTP_REQUEST is performed, no additional 
   processing will be required. 
   HTTP::cookie insert name persist_me value $server_id 
 }

If you just want to make sure you are persisting to the server responding, it might be easier to persist by extracting the responding server's information:

when HTTP_REQUEST {  
   if { [HTTP::cookie exists "persist_me"] {  
     set cookie_contents [HTTP::cookie persist_me] 
     set poolid [lindex cookie_contents 0] 
     set serverid [lindex cookie_contents 1] 
     set portid [lindex cookie_contents 2] 
     use pool $poolid member $serverid $portid 
     } else { 
       use pool $poolid 
       }     
   }  
  
 when HTTP_RESPONSE {  
   HTTP::cookie insert name persist_me value [LB::server] 
   }