code: private void parse(String expression) throws IllegalArgumentException {
  		String[] fields = Strings.splitIgnoreBlank(expression, " ");
  		if (fields.length != 6) {
  			throw new IllegalArgumentException(String.format(
  					"Cron expression must consist of 6 fields (found %d in \"%s\")", fields.length, expression));
  		}
  		setNumberHits(this.seconds, fields[0], 0, 60);
  		setNumberHits(this.minutes, fields[1], 0, 60);
  		setNumberHits(this.hours, fields[2], 0, 24);
  		setDaysOfMonth(this.daysOfMonth, fields[3]);
  		setMonths(this.months, fields[4]);
  		setDays(this.daysOfWeek, replaceOrdinals(fields[5], "SUN,MON,TUE,WED,THU,FRI,SAT"), 8);
  		if (this.daysOfWeek.get(7)) {
  			// Sunday can be represented as 0 or 7
  			this.daysOfWeek.set(0);
  			this.daysOfWeek.clear(7);
  		}
  	}
docs: Parse the given pattern expression.