iRule Script in Ver 9 is not functioning

hello

here is my problem, i have a set of bigip for bunch of web farm on ver 4.5.10, which i have this iRule for it and it is functioning Very Well...

if (getfield(http_uri, '/', 2) matches_regex "[Ww][Ee][Bb][Aa][Pp][Pp]") {
   redirect to "https://%h/%u"
}
else {
   use pool WebApp-50
}

and now we are upgrading to version 9 and compatable iRule for version 9 is COMPILING but NOT FUNCTIONING ...

here is the script...

when HTTP_REQUEST {
   if { [string tolower [getfield [HTTP::uri] '/' 2]] == "webapp" } {
      HTTP::redirect "https://[HTTP::host][HTTP::uri]"
   } else {
      pool POOL_HTTP_Vlan70
   }
}

could you please tell me WHY?

thanks alot

Mojdeh