Using GNU grep to output just the match(es) and not the entire line containing the match(es)

echo "hello there." | grep -o "ello t"
ello t

The flag ‘o’ (not Zero) does the trick.
From the man page:

-o, –only-matching
Show only the part of a matching line that matches PATTERN.

I couldn’t find that flag as an option in the BSD Grep man page. So this might only be available with GNU version of Grep.