Validating a field with unique value does not include letters
I want to make sure that one field always have a unique value of LL#### or L####L (L is a letter and # is a number). So example MB1542 and M4352M.
When I choose validation unique value, it only takes into account the numbers. How do I make it also take into account the letters? (As I might have MS1374 and MB1374 and both should be able to enter. But as it is now I can only enter one of them...)