Cas Number Lookup, CAS RN, CAS Registry Number, CAS number is the identifying number of a chemical substance, and almost all chemical databases allow substance search by CAS number. CAS number is a numerical identifier that consists of up to 10 digits, divided into three parts by hyphens: the first part has 2 to 7 digits, second part has 2 digits, and the third part has a single digit as a check digit, which is used to verify the validity and uniqueness of the CAS number.