Websockets server: which character(s) to delimit?

I’m listening to messages on a raw websocket stream (with PHP), but there are weird characters delimiting the messages; it seems to be ▒ (dunno if this comes through in the forum, it’s Unicode 2592) followed by a tilde and then the same 2592 char again. But sometimes it’s slightly different and adds a t or u or something, and I can’t filter on 2592 for some reason so it might be some different character that’s translated in some way. Is there a specification for this?

I get the best result from delimiting on the tilde, but I don’t really want to do that as that might well a part of some other info.