def valid_iban? iban
  len = {
    AL: 28, AD: 24, AT: 20, AZ: 28, BE: 16, BH: 22, BA: 20, BR: 29,
    BG: 22, CR: 21, HR: 21, CY: 28, CZ: 24, DK: 18, DO: 28, EE: 20,
    FO: 18, FI: 18, FR: 27, GE: 22, DE: 22, GI: 23, GR: 27, GL: 18,
    GT: 28, HU: 28, IS: 26, IE: 22, IL: 23, IT: 27, KZ: 20, KW: 30,
    LV: 21, LB: 28, LI: 21, LT: 20, LU: 20, MK: 19, MT: 31, MR: 27,
    MU: 30, MC: 27, MD: 24, ME: 22, NL: 18, NO: 15, PK: 24, PS: 29,
    PL: 28, PT: 25, RO: 24, SM: 27, SA: 24, RS: 22, SK: 24, SI: 19,
    ES: 24, SE: 24, CH: 21, TN: 24, TR: 26, AE: 23, GB: 22, VG: 24
  }

  # Ensure upper alphanumeric input.
  iban.delete! " \t"
  return false unless iban =~ /^[\dA-Z]+$/

  # Validate country code against expected length.
  cc = iban[0, 2].to_sym
  return false unless iban.size == len[cc]

  # Shift and convert.
  iban = iban[4..-1] + iban[0, 4]
  iban.gsub!(/./) { |c| c.to_i(36) }

  iban.to_i % 97 == 1
end

p valid_iban? "GB82 WEST 1234 5698 7654 32" #=> true
p valid_iban? "GB82 TEST 1234 5698 7654 32" #=> false