gistfile1.rb

    def check_postcode(postcode)
      alpha1 = "[abcdefghijklmnoprstuwyz]"
      alpha2 = "[abcdefghklmnopqrstuvwxy]"
      alpha3 = "[abcdefghjkstuw]"
      alpha4 = "[abehmnprvwxy]"
      alpha5 = "[abdefghjlnpqrstuwxyz]"

      postcode_expressions = [
        %r{^(#{alpha1}{1}#{alpha2}?[0-9]{1,2})(\s*)([0-9]{1}#{alpha5}{2})$}, # AN NAA, ANN NAA, AAN NAA, and AANN NAA
        %r{^(#{alpha1}{1}[0-9]{1}#{alpha3}{1})(\s*)([0-9]{1}#{alpha5}{2})$}, # ANA NAA
        %r{^(#{alpha1}{1}#{alpha2}?[0-9]{1}#{alpha4}{1})(\s*)([0-9]{1}#{alpha5}{2})$} # AANA NAA
      ]

      not postcode_expressions.collect{ |expression| postcode.downcase =~ expression }.compact.empty?
    end