Does a re-entry permit in Thailand expire if my visa or extension expires while I am outside the country?

If you have a single entry visa, or extension stamp, and you leave Thailand without getting a re-entry permit, your visa/extension expires as soon as you are stamped out through immigration. With a re-entry permit obtained before you leave, it expires when your visa/extension expires. The re-entry permit also expires when you renew your extension/visa.

A re-entry permit allows you to leave and re-enter Thailand in order to resume your current permission to stay. If you were to stay outside of Thailand and not return while you still have a current permission to stay there would be nothing to resume and you would not be able to enter Thailand on that basis.

You need to have a current permission to stay, often obtained via getting an extension of stay from Immigration in order to benefit from a re-entry permit. You should have in your passport a stamp that indicates your current permission to stay. You may purchase a re-entry permit from immigration for a single re-entry or a multiple re-entry (1,000 baht or 3,800 baht, respectively).

A re-entry permit does two things for you. Assuming it is still valid (and it is valid to the end of your current permission to stay) it allows you to enter Thailand. It also gets you stamped in with a new current permission to stay identical to your previous permission to stay.

A visa expiry date and an “allowed to stay” date are completely separate. Your visa could expire in January but you’re allowed to stay until December. If so a re-entry permit would allow you to re-enter before December and you’d get stamped in to that exact date in December, regardless that your visa expired in January.

It expires when your entry stamp expires, not your visa. They are different dates.

Yes. All a re-entry permit does is keep your existing “allowed to stay” date alive. If that expires then so does your re-entry permit stamp.