Ragel compiles finite state machines from regular languages into runnable C
code. Ragel state machines can not only recognize byte sequences as regular
expression machines do, but can also execute code at arbitrary points in the
recognition of a regular language.

When you wish to write down a regular language you start with some simple
regular language and build a bigger one using the regular language operators
union, concatenation, kleene star, intersection and subtraction. This is
precisely the way you describe to Ragel how to compile your finite state
machines. Ragel also understands operators that insert function calls into
machines and operators that control any non-determinism in machines.