regexp match HTTP::host to choose pool

Taking Joe's idea (Click here) of using catch to handle errors if the pool doesn't exist, you could try this:

when HTTP_REQUEST {
    Get the string from the host header value up to the first period.  Set the string to lower case.
   set parsed_pool pool_[string tolower [getfield [HTTP::host] . 1]]
   log local0. "[IP::client_addr]:[TCP::client_port]: \$parsed_pool: $parsed_pool"
    Try to assign the parsed pool as the pool for this request.  Use catch to handle the error if the pool doesn't exist.
   if {[catch {pool $parsed_pool}]}{
       There was an error in trying to use the pool parsed from the hostname, so use the default pool
      pool default_pool
      log local0. "[IP::client_addr]:[TCP::client_port]: \$parsed_pool didn't exist: $parsed_pool.  Using default pool."
   }
}

Or here's a shorter version which expects that there is a default pool assigned to the VIP:

when HTTP_REQUEST {
    Try to assign the parsed pool as the pool for this request.  Use catch to handle the error if the pool doesn't exist.
   if {[catch {pool pool_[string tolower [getfield [HTTP::host] . 1]]}]}{
       The pool parsed from the host header didn't exist.  Do nothing as the default pool will be used.
      log local0. "[IP::client_addr]:[TCP::client_port]: parsed pool didn't exist: pool_[string tolower [getfield [HTTP::host] . 1]].  Using default pool."
   }
}

And here's one more version that validates that the parsed pool is actually allowed (defined in a list in the iRule). This would prevent a user from being able to arbitrarily make requests to any pool defined on the BIG-IP:

when RULE_INIT {
    Define a list of the pool names that can be accessed through this iRule.
   set ::allowed_pool_names [list \
      web-dev \
      web-tst \
      web-stg \
   ]
}
when HTTP_REQUEST {
    Get the string from the host header value up to the first period.  Set the string to lower case.
   set parsed_pool [string tolower [getfield [HTTP::host] . 1]]
    Check that the parsed pool name is defined in the list.
   if {[matchclass $parsed_pool equals $::allowed_pool_names]}{
       Assign the parsed pool for this request.
      pool pool_${parsed_pool}
      log local0. "[IP::client_addr]:[TCP::client_port]: Using pool pool_${parsed_pool}"
   }
}

Aaron