You could try using a regex (See below)
Dim regex As Regex = New Regex("^[A-Za-z]{1,2}\d[A-Za-z]{3}$")
Dim match As Match
match = regex.Match("A1XXX")
If (match.Success) Then
Console.WriteLine("Matched")
End If
match = regex.Match("AA1XXX")
If (match.Success) Then
Console.WriteLine("Matched")
End If