BNG Creation for Invoice to include SCAC code in XID

Hi All,

We have requirement to have SCAC code in Invoice xid which we receive through Integration.
Invoice Xid has to be like SCACYYYYMMDD-0001, eg- RBTW20151205-0001.
I tried below but no luck.

Business Rule : {r*:id=1:xml=SERVPROV}{dddddddd:id=2}-{nnnn:contexts=1:start=1}
output : 20151203-0001

System is not able to find SCAC. I understand that I am missing the obvious thing here but could anyone please point out what ? Much appreciated.

Regards,
Calvin