The next function can give you some ideas:
CLEAR
?[MBT1006],GoodString([MBT1006])
?[1006MBT],GoodString([1006MBT])
?[12345],GoodString([12345])
?[TE],GoodString([TE])
?[Only alpha but too many (not exactly 2)],[TEA],GoodString([TEA])
?[Two few alpha], [TE3210],GoodString([TE3210])
?[Two few...