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

It can also be done in perl:

perl -e '@a=reverse<>;END{for(@a){if(/a/){s/a/c/;last}}print reverse @a}' temp > your_new_file

Tested:

> cat temp
a
b
c
a
b
> perl -e '@a=reverse<>;END{for(@a){if(/a/){s/a/c/;last}}print reverse @a}' temp
a
b
c
c
b
>