ISBN-10 check digit validator
Multiply each of the first 9 digits by its position weight (10 down to 2), add the check digit (weight 1, and the only position allowed to be 'X' meaning 10), and require the total to be a multiple of 11.
Try it: a valid example · a broken example
curl 'https://check-digit.gumballtools.com/api/v1/run?input=0-306-40615-2&format=isbn10'