code: public static boolean isValid(@Nullable final String multiDayOfTheWeeks) {
          if (multiDayOfTheWeeks == null) {
              return true;
          }
          final StringTokenizer tok = new StringTokenizer(multiDayOfTheWeeks, "/");
          if (tok.countTokens() == 0) {
              return false;
          }
          final List<DayOfTheWeek> days = new ArrayList<>();
          while (tok.hasMoreTokens()) {
              final String part = tok.nextToken();
              final int p = part.indexOf('-');
              if (p > -1) {
                  // dd-dd
                  final String part1 = part.substring(0, p);
                  final String part2 = part.substring(p + 1);
                  if (!DayOfTheWeek.isValid(part1) || !DayOfTheWeek.isValid(part2)) {
                      return false;
                  }
                  final DayOfTheWeek from = DayOfTheWeek.valueOf(part1);
                  final DayOfTheWeek to = DayOfTheWeek.valueOf(part2);
                  if (from == to || from.after(to)) {
                      return false;
                  }
                  for (final DayOfTheWeek dow : DayOfTheWeek.getPart(from, to)) {
                      if (days.contains(dow)) {
                          return false;
                      }
                      days.add(dow);                    
                  }
                  
              } else {
                  // dd
                  if (!DayOfTheWeek.isValid(part)) {
                      return false;
                  }
                  final DayOfTheWeek dow = DayOfTheWeek.valueOf(part);
                  if (days.contains(dow)) {
                      return false;
                  }
                  days.add(dow);                    
              }
          }
          return true;
      }
docs: Verifies if the string is valid and could be converted into an object.
  
  @param multiDayOfTheWeeks
  Hour multipleDayOfTheWeek string to test.
  
  @return {@literal true} if the string is a valid string, else {@literal false}.