simple stream replace not working

Thank you both for your responses. It appears the text wasn't being replaced due to compression. I changed the irule to include:

when HTTP_REQUEST {

    Disable the stream filter for all requests
   STREAM::disable

     remove the compression offerings from the client
   HTTP::header remove "Accept-Encoding"
}
when HTTP_RESPONSE {

    Check if response type is text
   if {[HTTP::header value Content-Type] contains "text"}{

       Replace
      STREAM::expression {@Hello@12345@}

       Enable the stream filter for this response only
      STREAM::enable
   }
} 

I think the 'overload' function referred to by Michael will come in usefull when we go live.