I work with GitHub, so this oneliner relly helps me out. It also doesn't rely on grep, since --format have all you need:

    git branch --format '%(if:equals=gone)%(upstream:track,nobracket)%(then)%(refname:short)%(end)' --omit-empty | xargs --verbose -r git branch -D

It deletes all the branches for which remotes were deleted. GitHub deletes branches after PR was merged. I alias it to delete-merged