postcode_validator.rb

class PostcodeValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value =~ /^([A-PR-UWYZ]([0-9]{1,2}|([A-HK-Y][0-9]|[A-HK-Y][0-9]([0-9]|[ABEHMNPRV-Y]))|[0-9][A-HJKS-UW])\s?[0-9][ABD-HJLNP-UW-Z]{2}|(GIR\ 0AA)|(SAN\ TA1)|(BFPO\ (C\/O\ )?[0-9]{1,4})|((ASCN|BBND|[BFS]IQQ|PCRN|STHL|TDCU|TKCA)\ 1ZZ))$$/i
      record.errors[attribute] << (options[:message] || "invalid postcode")
    end
  end
end