Let's say you have two or more identical strings of data. In this example, the text "Hello" is found more than once.
Hello John Hello Amy Hello Sal
A simple sed statement will replace every occurrence of the world "Hello".
~]# echo "Hello John Hello Amy Hello Sal" | sed "s|Hello|Goodbye|" Goodbye John Goodbye Amy Goodbye Sal
The following sed statement can be used to only replace the last occurrence of "Hello".
~]# echo "Hello John Hello Amy Hello Sal" | sed "s|\(.*\)Hello|\1Goodbye|" Hello John Hello Amy Goodbye Sal