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

	enum {
		S0, S1, S2, S3, S4, S5, S6, S7, S8, S9, 
		S10, S11, S12
	} state;

	state = S0;

	for (p = s; *p != '\0'; p++) {
		switch (state) {
		case S0: /* start */
			switch ((unsigned char) *p) {
			case '1': state = S1; break;
			case '0': state = S2; break;
			default:  return -1; /* leaf */
			}
			break;

		case S1: /* e.g. "1" */
			switch ((unsigned char) *p) {
			case '0':
			case '1':
			case '2': state = S3; break;
			default:  return -1; /* leaf */
			}
			break;

		case S2: /* e.g. "0" */
			switch ((unsigned char) *p) {
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9': state = S3; break;
			default:  return -1; /* leaf */
			}
			break;

		case S3: /* e.g. "01" */
			switch ((unsigned char) *p) {
			case ':': state = S4; break;
			default:  return -1; /* leaf */
			}
			break;

		case S4: /* e.g. "01:" */
			switch ((unsigned char) *p) {
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5': state = S5; break;
			default:  return -1; /* leaf */
			}
			break;

		case S5: /* e.g. "01:0" */
			switch ((unsigned char) *p) {
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9': state = S6; break;
			default:  return -1; /* leaf */
			}
			break;

		case S6: /* e.g. "01:00" */
			switch ((unsigned char) *p) {
			case ':': state = S7; break;
			default:  return -1; /* leaf */
			}
			break;

		case S7: /* e.g. "01:00:" */
			switch ((unsigned char) *p) {
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5': state = S8; break;
			default:  return -1; /* leaf */
			}
			break;

		case S8: /* e.g. "01:00:0" */
			switch ((unsigned char) *p) {
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9': state = S9; break;
			default:  return -1; /* leaf */
			}
			break;

		case S9: /* e.g. "01:00:00" */
			switch ((unsigned char) *p) {
			case ' ': state = S10; break;
			default:  return -1; /* leaf */
			}
			break;

		case S10: /* e.g. "01:00:00 " */
			switch ((unsigned char) *p) {
			case 'A':
			case 'P': state = S11; break;
			default:  return -1; /* leaf */
			}
			break;

		case S11: /* e.g. "01:00:00 A" */
			switch ((unsigned char) *p) {
			case 'M': state = S12; break;
			default:  return -1; /* leaf */
			}
			break;

		case S12: /* e.g. "01:00:00 AM" */
			return -1; /* leaf */

		default:
			; /* unreached */
		}
	}

	/* end states */
	switch (state) {
	case S12: return 0x1; /* "(01|02|03|04|05|06|07|08|09|10|11|12):[0-5][0-9]:[0-5][0-9] (A|P)M" */
	default: return -1; /* unexpected EOT */
	}
}