Empty/unused groups
===================

    >>> import re
    >>> import re2
    >>> re2.set_fallback_notification(re2.FALLBACK_EXCEPTION)

Unused vs. empty group:

    >>> re.search( '(foo)?((.*).)(bar)?', 'a').groups()
    (None, 'a', '', None)
    >>> re2.search('(foo)?((.*).)(bar)?', 'a').groups()
    (None, 'a', '', None)

    >>> re.search(r'((.*)?.)', 'a').groups()
    ('a', '')
    >>> re2.search(r'((.*)?.)', 'a').groups()
    ('a', '')
    >>> re.search(r'((.*)+.)', 'a').groups()
    ('a', '')
    >>> re2.search(r'((.*)+.)', 'a').groups()
    ('a', '')
    >>> re.search(r'((.*)*.)', 'a').groups()
    ('a', '')
    >>> re2.search(r'((.*)*.)', 'a').groups()
    ('a', '')

The following show different behavior for re and re2:

    >>> re.search(r'((.*)*.)', 'Hello').groups()
    ('Hello', '')
    >>> re2.search(r'((.*)*.)', 'Hello').groups()
    ('Hello', 'Hell')

    >>> re2.set_fallback_notification(re2.FALLBACK_QUIETLY)
