Question about NetBSDs implementation of sed.

NetBSD-help,
I am learning how to use sed and can't seem to figure out how to have
sed stop on word boundaries.  Yes, I read both man sed and man 7
re_format.  I am at a loss.  I went on line and googled for my
solution.  From there I came up with some possibilities but nothing
seems to work for NetBSDs sed.  Here they are:
I made a small file to test with:
j.txt
----
This is junk
This is the junkiest!
This is prejunk

Trial #1:
--------
$ sed -E 's/junk\b/great/' j.txt
This is junk
This is the junkiest!
This is prejunk

BUT, you say \b is in the GNU implementation.  Very true but I had to try.

Trial #2
--------
~/junk> sed -E 's/\<junk\>/great\!/' j.txt
This is junk
This is the junkiest!
This is prejunk

The \< \> is even mentioned in NetBSDs grep, and ed man pages.  I
thought this had to be it.  Still wrong answer.  What about the -E that
isn't that supposed to enable extended regular expressions?

So how does one use sed to search on word boundaries?

Eager to learn,
Glen Johnson