ISBN supports a 2 or 5 digit add-on, example: 978-1-873671-00-9 54499  Can you improve your library to validate this? Thanks. Source: - https://github.com/bwipp/postscriptbarcode/wiki/ISBN - https://github.com/bwipp/postscriptbarcode/wiki/EAN-2 - https://github.com/bwipp/postscriptbarcode/wiki/EAN-5