he input string must be enclosed in []'s, and the returned string
    is the expanded character set joined into a single string. The
    values enclosed in the []'s may be:

    - a single character
    - an escaped character with a leading backslash (such as ``\-``
      or ``\]``)
    - an escaped hex character with a leading ``'\x'``
      (``\x21``, which is a ``'!'`` character) (``\0x##``
      is also supported for backwards compatibility)
    - an escaped octal character with a leading ``'\0'``
      (``\041``, which is a ``'!'`` character)
    - a range of any of the above, separated by a dash (``'a-z'``,
      etc.)
    - any combination of the above (``'aeiouy'``,
      ``'a-zA-Z0-9_$'``, etc.)
    c