code: def _sounds_re(include_erhua=False):
      
  
      tone = '[1-5]'
      optional_final_erhua = '|r\\b' if include_erhua else ''
      pattern = '({}{}{})'.format(_joined_syllables_re(), tone, optional_final_erhua)
      return re.compile(pattern, re.IGNORECASE)
docs: Sounds are syllables + tones