Function myFormat(s As String)
Dim n1 As String, n2 As String
Dim ns() As String
ns = Split(s, "-")
n1 = "000000" & ns(0)
n2 = "000000" & ns(1)
myFormat = Right(n1, 6) & "-" & Right(n2, 6)
End Function
SELECT myFormat("1234-5678")
001234-005678
newvalue: "0" & left (fieldname, 6) & "00" & right(fieldname, 4)