NEWBIE: iRule for choosing pool based on URL

I am trying to point my subdomain towards a different pool of servers then my main site. For example:

www.mywebsite.com -> goes to pool a

beta.mywebsite.com -> goes to pool b

This is what i tried to use:

when HTTP_REQUEST { 
   if { [string tolower [HTTP::host]] equals "beta.mywebsite.com"}
   {
     pool pool_A
   }
   else
   {
     pool pool_B
   }
}

However nothing was ever routed to pool_A when i went to that URL. I even tried using a hard-coded URL so I would know when the iRule is going into effect:

when HTTP_REQUEST { 
   if { [string tolower [HTTP::host]] equals "beta.mywebsite.com"}
   {
   HTTP::redirect "http://www.google.com"
   }
   else
   {
     pool pool_B
   }
}

How can I figure out why my IF statement is failing? Would I be better using HTTP::host]] starts_with "beta."?

Again, I am a total newbie so maybe there is another setting that I don't have turned on which will make this iRule go "live". Any help would be GREATLY appreciated.