possible in bash?: filter stderr of a prog & send filtered-output out on stderr leaving stdout untouched

[L A Walsh]

Is there a way, in bash, to filter stderr and let the
filtered result continue on stderr with the original
stdout being output on stdout?

with prog being the program to filter, and RE_filt being the
filtering expression, conceptually, I wanted to do something
like this:

$prog >&3 2>&1 |grep -v "$RE_filt" >&2 3>&1

(which of course doesn't work)....

Is it possible/doable?

Tnx!
-l