This Ruby on Rails plugin implements an ActiveRecord validation helper called validates_as_isbn. The helper validates that the string provided is a valid ISBN. It works on both the original 10-character ISBN, or the newer 13-character version.
TLDR: Project (validates_as_isbn_old) is now hosted on Sourcehut due to GitHub's continued collaboration with ICE (U.S. Immigration and Customs Enforcement).
Hello 👋, nice to see you! If you're here you're presumably looking for the project (validates_as_isbn_old) that used to be here. The project still exists, it has just moved elsewhere. Let me tell you both why, and where you can find the project now.
Given GitHub's ongoing policy of collaborating with ICE I don't feel comfortable supporting the platform (as I've mentioned elsewhere), so I've gradually moved all of my repositories (both private and public) elsewhere.
I've hoped that GitHub would cancel their policy (as do many of the staff, including good people I know), but nothing has happened, so I've moved. You can find the project at Sourcehut.