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

Not quite sed only:

tac file | sed '/a/ {s//c/; :loop; n; b loop}' | tac

testing

% printf "%s\n" a b a b a b | tac | sed '/a/ {s//c/; :loop; n; b loop}' | tac
a
b
a
b
c
b

Reverse the file, then for the first match, make the substitution and then unconditionally slurp up the rest of the file. Then re-reverse the file.

Note, an empty regex (here as s//c/) means re-use the previous regex (/a/)

I'm not a huge sed fan, beyond very simple programs. I would use awk:

tac file | awk '/a/ && !seen {sub(/a/, "c"); seen=1} 1' | tac