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

A two-pass solution for when buffering the entire input is intolerable:

sed "$(sed -n /a/= file | sed -n '$s/$/ s,a,c,/p' )" file

(the earlier version of this hit a bug with history expansion encountered on a redhat bash-4.1 install, this way avoids a $!d that was being mistakenly expanded.)

A one-pass solution that buffers as little as possible:

sed '/a/!{1h;1!H};/a/{x;1!p};$!d;g;s/a/c/'

Simplest:

tac | sed '0,/a/ s/a/c/' | tac