function _getMemoizedRules(localeArg: ?string): Rules {
  const locale = localeArg ?? '';
  let rules = rulesPerLocale[locale];
  if (rules == null) {
    rules = rulesPerLocale[locale] = _getRules(localeArg);
  }
  return rules;
}
Regular expression snippet containing all the characters that we
count as sentence-final punctuation.