module UK
  Postcode = begin
    # The letters Q, V and X are not used in the first position...
    alpha_a = '[ABCDEFGHIJKLMNOPRTSUWYZ]'
    # The letters I, J and Z are not used in the second position...
    alpha_b = '[ABCDEFGHKLMNOPQRSTUVWXY]'
    alpha_c = '[ABCDEFGHJKSTUW]'
    alpha_d = '[ABEHMNPRVWXY]'
    part1_a = %{#{alpha_a}\\d{1,2}}
    part1_b = %{#{alpha_a}#{alpha_b}\\d{1,2}}
    part1_c = %{#{alpha_a}\\d#{alpha_c}}
    part1_d = %{#{alpha_a}#{alpha_b}\\d#{alpha_d}}
    part1 = %{(#{part1_a}|#{part1_b}|#{part1_c}|#{part1_d})}
    # the letters C, I, K, M, O and V are not used in the 2nd part...
    part2 = %{\\d[ABDEFGHJLNPQRSTUWXYZ]{2}}
    # GIR 0AA is historic and in use, but invalid according to modern rules...
    pattern = /(GIR 0AA|#{part1} #{part2})/i
  end  
end