Search and Replace with iRule

Sorry i missed this part then you can try something like this:

   
   when HTTP_REQUEST {    
       set replace_content 0   
       if {[HTTP::uri] contains "/atoz/"} {    
           set replace_content 1    
       }    
    }    
       
   when HTTP_RESPONSE {   
     if {$replace_content equals "1"} {   
          Disable the stream filter by default   
         STREAM::disable   
      
         Check if response type is text   
        if {[HTTP::header value Content-Type] contains "text" and [HTTP::header "User-Agent"] contains "***"}{   
              Replace   
             STREAM::expression "@123@xyz@ @456@xyz@"   
              Enable the stream filter for this response only   
             STREAM::enable   
         }   
     }   
   }    
   

the variable replace_content can be shared as a local variable among all the events you want. This variable will be available for this TCP connection only