custom data validation

Private Sub Worksheet_Change(ByVal Target As Range)

    Dim POCell As Range

    Set POCell = Range("B5")

    If IsEmpty(POCell) = False Then
        If Not (Len(POCell) = 8) Then
            MsgBox "Entry must be in format PO######"
            Exit Sub
        ElseIf Not (Left(POCell, 2) = "PO") Then
            MsgBox "Entry must be in format PO######"
            Exit Sub
        ElseIf Not IsNumeric(Right(POCell, 6)) Then
            MsgBox "Entry must be in format PO######"
            Exit Sub
        ElseIf Not Right(POCell, 6) < 999999 And Right(POCell, 6) > 100000 Then
            MsgBox "Entry must be in format PO######"
            Exit Sub
        End If
    End If

End Sub