Validating CAS Numbers

The Chemical Abstracts Service (CAS) registry number system was designed to be fault-tolerant. Built into every CAS number is a check-digit that makes it possible to detect mis-typed numbers. Validation is a mathematical and repetitive process well-suited for software.

The Ruby program below validates arbitrary CAS numbers:

module CAS
  def validate cas_number
    return false unless cas_number && cas_number.match(/[0-9]{2,7}-[0-9]{2}-[0-9]/)

    check_digit = cas_number[-1,1].to_i
    sum = 0

    cas_number.reverse.scan(/[0-9]/).each_with_index do |digit, i|
      sum = sum + digit.to_i * i

    check_digit == sum.remainder(10)

include CAS

while true do
  print "CAS Number: "

  cas_number = gets.strip

  break if cas_number.empty?

  puts CAS.validate(cas_number) ? "valid" : "invalid"

The program can be tested from the command line:

<pre class="console">
$ ruby cas.rb
CAS Number: 107-07-3
CAS Number: 107-87-3
CAS Number:

Note that a validated CAS number can still be absent from the CAS database; validation only says that a CAS number could be valid based on its format.