Originally Posted by kiwidude
My main concern would be false positives as you say from telephone numbers or similar. If you come up with something that you are confident will not suffer from that issue then I'm sure everyone would be grateful for your effort.
There is a 'check_isbn' function that is already in use in the various calibre metadata plugins that do some validation on whether a specific string of numbers is truly an ISBN vs a random string of numbers like a phone number. These get used before the metadata plugins send an ISBN to a metadata provider, but they should be good for this too.
from calibre.ebooks.metadata import check_isbn