добавьте пробел 's / ing / ed / g;
в третьем тесте;
#echo "walking ringer talking" | sed 's/^ing/ed/g;s/ing$/ed/g'
echo "walking ringer talking" | sed 's/ing /ed /g;s/ing$/ed/g'
например:
user@host:/tmp/test$ echo "walking ringer talking" | sed 's/ing /ed/g;s/ing$/ed/g'
walked ringer talked