regex - 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" in the file. So if I wanted to replace it with "c", then the output should look like:

a
b
c
b

Note that I need this to work irrespective of how many matches it might encounter, or the details of exactly what the desired pattern or file contents might be. Thanks in advance.

Answer 1

Not quite sed only:

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

testing

% printf "%s
" 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