Want VBA to verify length on a cell

[pholo]

Need help on VBA code.
Here are conditions:
If cell A1 = either UP or UA then cell B1 must be 12 digits in length and ERROR massage will pop up if the length is incorrect
If cell A1 = EN then cell B1 must be 13 digits in length and ERROR massage will pop up if the length is incorrect
If cell A1 = UK then cell B1 must be 14 digits in length and ERROR massage will pop up if the length is incorrect

Thanks