function parseWeekString(str) {
  const matches = weekRe.exec(str);
  if (!matches) {
    return null;
  }
  const year = Number(matches[1]);
  if (year <= 0) {
    return null;
  }
  const week = Number(matches[2]);
  if (week < 1 || week > weekNumberOfLastDay(year)) {
    return null;
  }
  return { year, week };
}