Short example of regular expression converted to a state machine?

Question

In the Stack Overflow podcast #36 (http://blog.stackoverflow.com/2009/01/podcast-36/), this opinion was expressed: Once you understand how easy it is to set up a state machi

Answer 1

Is the question "How do I choose the states and the transition conditions?", or "How do I implement my abstract state machine in Foo?"

How do I choose the states and the transition conditions?

I usually use FSMs for fairly simple problems and choose them intuitively. In my answer to another question about regular expressions, I just looked at the parsing problem as one of being either Inside or outside a tag pair, and wrote out the transitions from there (with a beginning and ending state to keep the implementation clean).

How do I implement my abstract state machine in Foo?

If your implementation language supports a structure like c's switch statement, then you switch on the current state and process the input to see which action and/or transition too perform next.

Without switch-like structures, or if they are deficient in some way, you if style branching. Ugh.

Written all in one place in c the example I linked would look something like this:

token_t token;
state_t state=BEGIN_STATE;
do {
   switch ( state.getValue() ) {
   case BEGIN_STATE;
      state=OUT_STATE;
      break;
   case OUT_STATE:
      switch ( token.getValue() ) {
         case CODE_TOKEN:
            state = IN_STATE;
            output(token.string());
            break;
         case NEWLINE_TOKEN;
            output("");
            output(token.string());
            break;
         ...
      }
      break;
   ...
   }
} while (state != END_STATE);

which is pretty messy, so I usually rip the state cases out to separate functions.