I ended up using one in MaraDNS 2.0 to parse configuration files, since the C code for parsing MaraDNS 1.0 configurations files was a lot more error-prone and hard to maintain.

The state machine’s grammar is embedded in the Deadwood (MaraDNS 2 recursor) binary.

Here is a full explanation of the finite state machine, along with the actual embedded code:

https://github.com/samboy/MaraDNS/blob/master/deadwood-githu...