First off, your String values appear to be 14 characters long, not 12
characters long as you stated. Assuming 14 characters is correct, give this
Workbook_Change event code a try...

Private Sub Worksheet_Change(ByVal Target As Range)
With Target
If .Column = 1 Then
If Len(Replace(.Value, "-", "")) = 14 Then
On Error GoTo Whoops
Application.EnableEvents = False
.Value = Format(Replace(.Value, "-", ""), "@@-@@@@@@@@@@@-@")
End If
End If
End With
Whoops:
Application.EnableEvents = True
End Sub

Rick


"franciz" wrote in message
...
Hi

I have string, eg. TH00000000789F in a range, let say in column A, how do
I
insert "-" after the first 2 characters and before the last character
resulting
TH-00000000789-F. The string are always 12 Characters.

Thanks in advance

regards, francis