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

Another approach:

sed "`grep -n '^a$' a | cut -d \: -f 1 | tail -1`s/a/c/" a

The advantage of this approach is that you run sequentially on the file twice, and not read it to memory. This can be meaningful in large files.