How to Validate CUSIP and ISIN Security Identifiers

CUSIP (9-character) and ISIN (12-character) are unique identifiers for securities traded in US/Canadian and international markets respectively, both containing a check digit validated via a modified Luhn algorithm. The article details the structure of each identifier and provides a JavaScript implementation for algorithmic validation without database lookup. Validating these identifiers at data entry prevents errors from propagating through trading platforms, portfolio trackers, and reconciliation systems.