gistfile1.rb

def check_cas_number
  # Check the CAS number for validity (the last number in a CAS number is a checksum)
  if cas_number.present?
    matches = cas_number.scan(/^(\d{1,7})-(\d{1,2})-(\d)$/)
    
    if matches.first.blank? || matches.first.size != 3
      self.errors.add(:cas_number, "format invalid")
    else
      match = matches.first

      toadd = match[0].to_s + match[1].to_s
      check = match[2].to_s

      sum = 0
      count = toadd.length

      digits = toadd.scan(/(\d)/)
      while(digit = digits.shift) do
        sum = sum + digit[0].to_i * count
        count = count - 1
      end
      self.errors.add(:cas_number, "format invalid") unless check.to_i == (sum % 10).to_i
    end
  end
end