This is exactly what we did to diagnose (source: I was on the call). The only tricky part was figuring out which post it was, since it wasn't in the stacktrace. To do that, we grabbed the 3000 most recent posts and ran the regex against them. By that point we already had the code fix (another dev working on it in parallel), but if we hadn't we also could have gotten back up by just deleting the post.