def register_options(regdescr):
    """
    Very reduced regular expressions for describing a group of registers.

    Only groups in square bracktes and unions with pipes (|) are supported.

    Examples:
    >>> list(register_options('PMC[0-3]'))
    ['PMC0', 'PMC1', 'PMC2', 'PMC3']
    >>> list(register_options('MBOX0C[01]'))
    ['MBOX0C0', 'MBOX0C1']
    >>> list(register_options('CBOX2C1'))
    ['CBOX2C1']
    >>> list(register_options('CBOX[0-3]C[01]'))
    ['CBOX0C0', 'CBOX0C1', 'CBOX1C0', 'CBOX1C1', 'CBOX2C0', 'CBOX2C1', 'CBOX3C0', 'CBOX3C1']
    >>> list(register_options('PMC[0-1]|PMC[23]'))
    ['PMC0', 'PMC1', 'PMC2', 'PMC3']

    """
    if not regdescr:
        yield None
    tokenizer = ('\[(?P<grp>[^]]+)\]|'
                 '(?P<chr>.)')
    for u in regdescr.split('|'):
        m = re.match(tokenizer, u)

        if m.group('grp'):
            current = group_iterator(m.group('grp'))
        else:
            current = [m.group('chr')]

        for c in current:
            if u[m.end():]:
                for r in register_options(u[m.end():]):
                    yield c + r
            else:
                yield c
Very reduced regular expressions for describing a group of registers.

    Only groups in square bracktes and unions with pipes (|) are supported.

    Examples:
    >>> list(register_options('PMC[0-3]'))
    ['PMC0', 'PMC1', 'PMC2', 'PMC3']
    >>> list(register_options('MBOX0C[01]'))
    ['MBOX0C0', 'MBOX0C1']
    >>> list(register_options('CBOX2C1'))
    ['CBOX2C1']
    >>> list(register_options('CBOX[0-3]C[01]'))
    ['CBOX0C0', 'CBOX0C1', 'CBOX1C0', 'CBOX1C1', 'CBOX2C0', 'CBOX2C1', 'CBOX3C0', 'CBOX3C1']
    >>> list(register_options('PMC[0-1]|PMC[23]'))
    ['PMC0', 'PMC1', 'PMC2', 'PMC3']