The problem with abstracting that is osTicket sends From many different emails (one per department and a few others on mine), but they are in the database, or just look for the line with our domain.
If you've seen my implementation, it works without the separator, because it runs after it, reverses the text to find it from the bottom. But there's a bit of a "deletes the whole message" problem if you're matching an email address or domain and they don't send that line.
We could also pull the thread from the $entry and locate the last sent, then we'd know what date to look for. Day level accuracy is probably enough. Only issue is number formats vary.. but only 6 ways with three numbers.. pretty doable.
Any other context would actually be relevant, because they're responding to a previous message, not the latest. So are doing it intentionally. Hmm.
It sure is frustrating to test!