int
fsm_main(const char *s)
{
	const char *p;

	enum {
		S0, S1, S2
	} state;

	state = S0;

	for (p = s; *p != '\0'; p++) {
		switch (state) {
		case S0: /* start */
			switch ((unsigned char) *p) {
			case '0': break;
			case '1': state = S1; break;
			default:  return -1; /* leaf */
			}
			break;

		case S1: /* e.g. "1" */
			switch ((unsigned char) *p) {
			case '1': state = S0; break;
			case '0': state = S2; break;
			default:  return -1; /* leaf */
			}
			break;

		case S2: /* e.g. "10" */
			switch ((unsigned char) *p) {
			case '0': state = S1; break;
			case '1': break;
			default:  return -1; /* leaf */
			}
			break;

		default:
			; /* unreached */
		}
	}

	/* end states */
	switch (state) {
	case S0: return 0x1; /* "(0|(1(01*0)*1))*" */
	default: return -1; /* unexpected EOT */
	}
}