Distributors and collecting societies reject releases whose codes are malformed, and a single transposed digit in a UPC or ISWC is enough. The validator recognises ISRC, UPC-A, EAN-13 and ISWC codes in a list, checks their structure and check digits, compares ISRC prefixes with the IFPI list of valid codes, splits every code into its parts and shows the correct check digit when a code does not add up.
ISRC (International Standard Recording Code, ISO 3901) identifies a recording: 12 characters made of a prefix (a two-letter code from the IFPI list and a three-character registrant code), the last two digits of the year of reference and a five-digit designation code. It has no check digit, so the check covers the structure, the prefix list and the cases that the IFPI validation bulletin names as invalid or doubtful.
UPC-A and EAN-13 identify the release as a product. The last digit is a check digit: from the right, the digits are weighted alternately by 3 and 1, and the check digit brings the sum up to the next multiple of ten. A UPC-A becomes an EAN-13 by adding a leading zero.
ISWC (International Standard Musical Work Code, ISO 15707) identifies the composition, not the recording: T, nine digits and a check digit. The T counts as 1, each of the nine digits is multiplied by its position (1 to 9), and the check digit brings the sum up to the next multiple of ten.
Sources: ISRC Handbook (IFPI) · Validating an International Standard Recording Code (IFPI Bulletin 2015/01) · ISRC Agency Code Allocations (IFPI) · ISO 15707:2022 (ISWC) · How to calculate a check digit manually (GS1)
Everything runs in the browser. The check covers structure and check digits; whether a code has actually been issued or belongs to a particular release can only be confirmed by the issuing agency or the distributor. The MusicBrainz links open the open music database in a new tab and only then pass the code on.