Hope below example can help you:

if you match "aol" traffic is sent to aol_pool otherwise is sent to the default pool associated to the VS.

when HTTP_REQUEST {
  # case sensitive
  if { [HTTP::uri] contains "aol" } {
     pool aol_pool
  }

}

In below cases we can the default pool in the code.

when HTTP_REQUEST {
  # case sensitive
  if { [HTTP::uri] contains "aol" } {
     pool aol_pool
  } else {
     pool all_pool
  }

  # case insensitive
  if { [string tolower [HTTP::uri]] contains "aol" } {
     pool aol_pool
  } else {
     pool all_pool
  }
}

References:

https://clouddocs.f5.com/api/irules/contains.html

https://clouddocs.f5.com/api/irules/HTTP__uri.html