As some of us know, gnu sed has been accepting the '-E' switch (to enable ERE) for quite some time now, in addition to the '-r' switch ('-E' will also be in the next major issue of the POSIX sed spec). However, the '-E' option is completely unknown to the general public as there is no mention of this switch in the info / man pages (gnu sed 4.2.2 if it matters).
Regards,
Don