iRule for replace URL characters and do Base64 Encoding

Certainly not the only option, but the following should work:

   
set hosturi [string tolower [HTTP::host][HTTP::uri]]
if { $hosturi starts_with "www.example.com/xyz/xyz.do?asar=equal?x=" } {
        
        set firstpart [string range $hosturi 0 [expr [string first "?x=" $hosturi] + 2]]
        set secondpart [string rang $hosturi [expr [string first "?x=" $hosturi] + 3] end]
        
        log local0. [string map {"?x" "&x"} $firstpart][b64encode $secondpart]        
}