There's the Public Suffix List https://publicsuffix.org/ https://publicsuffix.org/ but it's limited to domain names, so your github.com/rails example isn't cove

There's the Public Suffix List https://publicsuffix.org/ https://publicsuffix.org/ but it's limited to domain names, so your github.com/rails example isn't covered. I'm pretty sure HN simply has a manually coded list of URL patterns for popular domains.