Assuming you made a bit of a typo with your regexp, any of these should work as you want:
grep -oE '/dev/loop[0-9]+'
awk 'match($0, /\/dev\/loop[0-9]+/) { print substr($0, RSTART, RLENGTH) }'
sed -r 's%.*(/dev/loop[0-9]+).*%\1%'
AWK one is a bit cursed as you can see. Such ways of manipulating text is not exactly it’s strong suite.
I use Gentoo. I install systemd willingly. We are not the same.