Mine

Private Sub cmdEnter_Click()
    Dim arrayErr(1 To 5) As Boolean
    Dim i As Integer
    Dim code As String
    Dim Error As Boolean
    Dim ch1, ch2, ch3, ch4, ch5 As String * 1
    Dim Length As Integer
    Dim Index As Integer
    
    code = txtInput.Text
    Length = Len(code)
    For Index = 1 To 5: arrayErr(Index) = False: Next Index
    Error = False
    If Length = 0 Then
        MsgBox "Empty String is not allowed"
        Error = True
    Else
      If Length <> 5 Then
        MsgBox "The string should be 5 characters long"
        Error = True
      Else
        ch1 = Mid(code, 1, 1)
        ch2 = Mid(code, 2, 1)
        ch3 = Mid(code, 3, 1)
        ch4 = Mid(code, 4, 1)
        ch5 = Mid(code, 5, 1)
        If ch1 < "0" Or ch1 > "9" Or ch2 < "0" Or ch2 > "9" Then
            arrayErr(1) = True
            If ch1 < "A" Or ch1 > "Z" Or ch2 < "A" Or ch2 > "Z" Then
                arrayErr(2) = True
            End If
        Else
            arrayErr(2) = True
        End If
        
        If ch3 < "0" Or ch3 > "9" Then arrayErr(3) = True
        If ch4 < "0" Or ch4 > "9" Then arrayErr(4) = True
        If ch5 < "0" Or ch5 > "9" Then arrayErr(5) = True

        If arrayErr(1) And arrayErr(2) Then
            MsgBox "Invalid country"
            Error = True
        Else
            If arrayErr(1) Then
                MsgBox "Overseas Centre"
            Else
                MsgBox "UK Centre"
            End If
        End If
        
        For i = 3 To 5
            If arrayErr(i) Then
                MsgBox "Invalid " & i & "th character"
                Error = True
            End If
        Next i
        If Error Then
            MsgBox "Invalid code"
        Else
            MsgBox "Valid code"
        End If
      End If
   End If
End Sub