Regex Alone Cannot Validate IBANs — Here Is What You Are Missing

A common mistake in IBAN validation is relying solely on a regex pattern that checks only the general shape of the number, such as two letters followed by two digits and alphanumerics. This approach fails to enforce country-specific fixed lengths, meaning a regex may accept a German IBAN that is the wrong number of characters for Germany. Crucially, regex cannot perform the mod-97 checksum calculation mandated by ISO 7064, which verifies the two check digits embedded in every IBAN using arithmetic over the full string. Developers must also account for per-country BBAN structure rules that dictate which positions must be digits or letters within the account number. A robust IBAN validator should apply all three checks in sequence: shape via regex, then the mod-97 checksum, then country-specific length and structure validation.