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 one:

tr '\n' ' ' | sed 's/\(.*\)a/\1c/' | tr ' ' '\n'

in action:

$ printf "%s\n" a b a b a b | tr '\n' ' ' | sed 's/\(.*\)a/\1c/' | tr ' ' '\n'
a
b
a
b
c
b