Use compgen -W in _longopt().

bash_completion

@@ -4275,9 +4275,9 @@ _longopt()
 	fi
 
 	if [[ "$cur" == -* ]]; then
-		COMPREPLY=( $( $1 --help 2>&1 | sed -e '/--/!d' \
-				-e 's/.*\(--[-A-Za-z0-9]\+\).*/\1/' | \
-			       command grep "^$cur" | sort -u ) )
+		COMPREPLY=( $( compgen -W "$( $1 --help 2>&1 | sed -e '/--/!d' \
+				-e 's/.*\(--[-A-Za-z0-9]\+\).*/\1/' |sort -u )"\
+				-- $cur ) )
 	elif [[ "$1" == rmdir ]]; then
 		_filedir -d
 	else