6/ 45 X  7/ X3/
14  9 20 20 20
14 23 43 63 83
"6/45X7/X3/"
"6/" matches; $1="6", $2="/", $3="4"; change to "X4"
  "45" matches; $1="4", $2="5"; change to "45"
    "X" matches; $4="7/"; change to "X7/"
     "7/" matches; $1="7", $2="/", $3="X"; change to "XX"
       "X" matches; $4="3/"; change to "X3/"
        "3" matches; change to "$1$2" eq ""
         "/" matches; change to "$1$2" eq ""
(result: "X445X7/XXX3/")