sed replace last line matching pattern

Question

Given a file like this:

a
b
a
b

I\'d like to be able to use sed to replace just the last line that contains an instance of \"a

Answer 1

Here's another option:

sed -e '$ a a' -e '$ d' file 

The first command appends an a and the second deletes the last line. From the sed(1) man page:

$ Match the last line.

d Delete pattern space. Start next cycle.

a text Append text, which has each embedded newline preceded by a backslash.