A grouped regular expression.
This includes both capturing and non-capturing groups. This does not
include flag-only groups like
(?is), but does contain any group that
contains a sub-expression, e.g.,
The span of this group.
The kind of this group.
The regular expression in this group.
If this group is non-capturing, then this returns the (possibly empty)
set of flags. Otherwise,
None is returned.
othervalues to be equal, and is used by