ISIN code validator in Ruby

isin_code_validator.rb

class IsinCodeValidator
  ISIN_FORMAT = '^[A-Z]{2}[A-Z0-9]{9}[0-9]$'.freeze
  ISIN_REGEX = Regexp.new(ISIN_FORMAT).freeze
  LETTER_TO_CODE_VALUE = 55 # Ascii code for 'A' == 65 && 'A' == 10 in isin calculation cod

  def self.valid?(str)
    return false if str.nil? || !str.match?(ISIN_REGEX)

    proc_isin = str.split(//)
    key = proc_isin.pop.to_i
    proc_isin = proc_isin.map do |e|
      e.match?(/[A-Z]/) ? (e.ord - LETTER_TO_CODE_VALUE).to_s.split(//) : e
    end.flatten!.reverse!
    proc_isin.each_index do |i|
      proc_isin[i] = (proc_isin[i].to_i * 2).to_s.split(//) if (i % 2).zero?
    end.flatten!
    ((key + proc_isin.map(&:to_i).inject(:+)) % 10).zero?
  end
end