text: The letters I, J and Z are not used in the second position.
  The only letters to appear in the third position are A, B, C, D, E, F, G, H, J, K, P, S, T, U and W when the structure starts with A9A.
  The only letters to appear in the fourth position are A, B, E, H, M, N, P, R, V, W, X and Y when the structure starts with AA9A.
  The final two letters do not use C, I, K, M, O or V, so as not to resemble digits or each other when hand-written.
  Postcode sectors are one of ten digits: 0 to 9, with 0 only used once 9 has been used in a post town, save for Croydon (see above).
  A postcode can be validated against a table of all 1.7 million postcodes in Code-Point Open. The full delivery address including postcode can be validated against the Royal Mail Postcode Address File (PAF), which lists 29 million valid delivery addresses, constituting most (but not all) addresses in the UK.
  The following regular expression can be used for the purpose of validation:
  ^[A-Z]{1,2}[0-9][A-Z0-9]? ?[0-9][A-Z]{2}$
  The following regular expression can be used for the purpose of validation and includes postcode formats from Special Cases: