Add a prefix before sending the request to the Pool

Thank you very much for all the help.

I have finally get it working and here is the final irules that I use. I add some log command as a troubleshooting tool to log the variable in the /var/log/ltm

 Add /web prefix 
 Unless it started with /reports

when HTTP_REQUEST {

  log local0. "in HTTP_REQUEST"

  set orguri [HTTP::uri]
  log local0. "Original: $orguri."  

if {[string tolower $orguri] starts_with "/reports"}
  {
  pool pool_b
  }
else {
    set newuri "/web[HTTP::uri]"
    HTTP::uri $newuri 
    pool pool_a
    }
log local0. "After mapping: [HTTP::uri]"
log local0. "Pool: [LB::server pool]"
}