The following doesn't use an UDF or CLR, and seems to cover all the (4) scenarios you described above.

declare @Invoice as table( BillID varchar(20) primary key );

insert into @Invoice ( BillID )

values ('A6304158-2'),('A6304158'),('6304158-2'),('6304158');

select BillID

, substring

(

BillID

, patindex('%[0-9]%',BillID)

, len(BillID) - patindex('%[0-9]%',BillID) + 1

- case when patindex('%-%',BillID) > 0

then len(BillID) - patindex('%-%',BillID) + 1

else 0

end

) as NewBillID

from @Invoice;

BillID NewBillID

6304158 6304158

6304158-2 6304158

A6304158 6304158

A6304158-2 6304158