Three different approaches:

• State is kept as a number in a variable. E.g. from a enum. And handled by switch-case
• State is kept as pointer to struct with function pointers
• State is kept as a index pointing to a state-transition table

SO question with interesting answers C state machine design.

An article that might contain good stuff State machine shortcuts

A (not really) related pattern that can be used for parsing is the Chain of responsibility pattern.

Ragel – Generate state machines specialized at parsing

PADS – Also for parsing protocols