Some thoughts..

Unfortunately, according to google this is the kind of thing that people like to do for graduate theses.. A number of commercial firewalls and routers seem to use Protocol Definition Language, for example PacketBoy which lets you define your own protocol. It's possible that bustools.com sells something you want too.

Much work has been done with this in packet sniffing, especially snort. Maybe not applicable to you but it does have rule writing, variables, and decomposition into separate streams and stateful analysis. Although, it just might work if you could play the data across the network as tcp packets. (or not)

That said, Expect sounds really good. One other possibility I could imagine would be to capture into a database and write SQL snippets. Or convert a processed stream to tokens and use regexes :) my favorite flavor. Good luck!