I am trying to restrict 8 digit dial on my dialplan after the prefix 6859A , could someone help me with the correct syntax?

Please see below what i have:

Dpid: 1

Pr: 1

Match Op: 1

Match Exp: sip:6859A(only 8 additional digits allowed).*

Match Len: 0

Subst Exp: sip:6859A(only 8 additional digits allowed).*

Repl Exp: 3005A1883@xx.xx.xx.xx

So basically any 8 digits after the prefix 6859A is allowed otherwise there shouldn't be a match, how could i go about doing this?

Thanks in advance

Phillip