I know exactly what you want to do. The issue is, where are you getting the elements?
I see that of "IND/25/07", 25 and 07 probably come from the current date. Where does the IND come from? Without knowing that, here is some code:
On the Before Insert event of the form:
(Assuming field [IND] and field [BillNumber])
(Assuming field [BillNumber] is locked and disabled)
Dim sBillNo As String
Dim sIND As String
If IsNull(Me.IND) Or Me.IND = "" Then
MsgBox "Please enter that field value."
sIND = Me.IND
sBillNo = sIND & "/" & DatePart("d", Date) & "/" & DatePart("m", Date)
If IsNull(Me.BillNumber) Or Me.BillNumber = "" Then
Me.BillNumber = sBillNo
MsgBox "There is already a Bill Number for this record."
This will automatically create the Bill Number. However, there may be some issues with when the user sees it.
Did that help any? Do a search of Microsoft for "regular expressions".