Function MySubst(text As String, find As String, repl As String, Optional num)
Dim i&, j&
If IsMissing(num) Then
    MySubst = Replace(text, find, repl, , , vbBinaryCompare)
Else
    For i = 1 To num
        j = InStr(j + 1, text, find, vbBinaryCompare)
        If j = 0 Then MySubst = text: Exit Function
    Next
    MySubst = Left$(text, j - 1) & repl & Mid$(text, j + Len(find))
End If
End Function