An International Securities Identification Number (ISIN) uniquely identifies a security. Its structure is defined in ISO 6166. This Ruby code implements a validity check for a given ISIN string.

isin.rb

class Isin
  COUNTRY_CODES = %w{AF AX AL DZ AS AD AO AI AQ AG AR AM AW AU AT AZ BS BH BD BB BY BE BZ BJ BM BT BO BQ BA BW BV BR IO BN BG BF BI KH CM CA CV KY CF TD CL CN CX CC CO KM CG CD CK CR CI HR CU CW CY CZ DK DJ DM DO EC EG SV GQ ER EE ET FK FO FJ FI FR GF PF TF GA GM GE DE GH GI GR GL GD GP GU GT GG GN GW GY HT HM VA HN HK HU IS IN ID IR IQ IE IM IL IT JM JP JE JO KZ KE KI KP KR KW KG LA LV LB LS LR LY LI LT LU MO MK MG MW MY MV ML MT MH MQ MR MU YT MX FM MD MC MN ME MS MA MZ MM NA NR NP NL NC NZ NI NE NG NU NF MP NO OM PK PW PS PA PG PY PE PH PN PL PT PR QA RE RO RU RW BL SH KN LC MF PM VC WS SM ST SA SN RS SC SL SG SX SK SI SB SO ZA GS SS ES LK SD SR SJ SZ SE CH SY TW TJ TZ TH TL TG TK TO TT TN TR TM TC TV UG UA AE GB US UM UY UZ VU VE VN VG VI WF EH YE ZM ZW}

  def initialize(v)
    @content = v.upcase
  end

  def to_s
    @content
  end

  def valid?
    return false if @content.blank?
    if m = @content.match(/^(\w{2})(\w{9})(\d)$/)
      country, identifier, checksum = m.captures
      return false unless COUNTRY_CODES.include?(country)
      if c = self.class.calculate_checksum(country + identifier)
        return true if checksum.to_i == c
      end
    end
    false
  end

  def self.calculate_checksum(txt)
    return nil unless txt.ascii_only?
    digits = txt.chars.map do |c|
      case c
        when 'A'..'Z' then (c.ord - 55).to_s.chars #we're ascii-only, A has code 65
        when '0'..'9' then c
      end
    end.flatten.map(&:to_i)
    pos = 1
    sum = digits.reverse.map{|a| pos += 1; (a*(2-pos%2)).to_s}.join.chars.map(&:to_i).sum
    (10 - (sum%10)) % 10
  end

end

isin_spec.rb

require 'spec_helper'

describe Isin do
  it "can be instantiated" do
    i = Isin.new 'DE000BAY0017'
    i.to_s.should == 'DE000BAY0017'
  end

  context "validity" do

    it "validates proper ISIN" do
      %w{DE000BAY0017 AU0000XVGZA3 US0378331005 US30231G1022}.each do |isin|
        Isin.new(isin).valid?.should be_true
      end
    end

    it "detects invalid formats" do
      %w{ZZ1234567890 DE00BAY0017 DE000BAY00178}.each do |isin|
        Isin.new(isin).valid?.should be_false
      end
    end

    it "detects invalid checksum" do
      Isin.new('DE000BAY0018').valid?.should be_false
      Isin.new('DE000BAZ0017').valid?.should be_false
      Isin.new('AU0000XVGZA5').valid?.should be_false
    end

    it "accepts lowercase" do
      Isin.new('de000bay0017').valid?.should be_true
    end

  end

end

Eleven years later and we have 'EU' as a valid country code :)

@jake-007 eleven years later, and I don't even remember what I used ISINs for back in the day :-)

Seriously, if you have any use for it, please feel free to edit the code as you may see fit.