docs: def _unescape_token(token):
    r"""Replaces escaped characters in the token with their unescaped versions.
  
    Applies inverse transformations as _escape_token():
      1. Replace "\u" with "_", and "\\" with "\".
      2. Replace "\###;" with the unicode character the ### refers to.
  
    Args:
      token: escaped string
  
    Returns:
      unescaped string
    """
  
    def match(m):
      r"""Returns replacement string for matched object.
  
      Matched objects contain one of the strings that matches the regex pattern:
        r"\\u|\\\\|\\([0-9]+);"
      The strings can be '\u', '\\', or '\###;' (### is any digit number).
  
      m.group(0) refers to the entire matched string ('\u', '\\', or '\###;').
      m.group(1) refers to the first parenthesized subgroup ('###').
  
      m.group(0) exists for all match objects, while m.group(1) exists only for
      the string '\###;'.
  
      This function looks to see if m.group(1) exists. If it doesn't, then the
      matched string must be '\u' or '\\' . In this case, the corresponding
      replacement ('_' and '\') are returned. Note that in python, a single
      backslash is written as '\\', and double backslash as '\\\\'.
  
      If m.goup(1) exists, then use the integer in m.group(1) to return a
      unicode character.
  
      Args:
        m: match object
  
      Returns:
        String to replace matched object with.
      """
      # Check if the matched strings are '\u' or '\\'.
      if m.group(1) is None:
        return u"_" if m.group(0) == u"\\u" else u"\\"
  
      # If m.group(1) exists, try and return unicode character.
      try:
        return six.unichr(int(m.group(1)))
      except (ValueError, OverflowError) as _:
        return _UNDEFINED_UNICODE
  
    # Use match function to replace escaped substrings in the token.
    return _UNESCAPE_REGEX.sub(match, token)