pacman - list installed packages from given repo
Solution 1:
Try with paclist:
paclist - List all packages installed from a given repo
e.g. to list all packages installed from community:
paclist community
If you want only the names (w/o version number) you could use something like:
awk '{print $1}' <(paclist community)
or
cut -d' ' -f1 <(paclist community)
Solution 2:
There is no need to install pacman-contrib package to achieve that goal:
pacman -Sl reponame | grep "\[installed\]"