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

Many good answers here; here's a conceptually simple two-pass sed solution assisted by tail that is POSIX-compliant and doesn't read the whole file into memory, similar to Eran Ben-Natan's approach:

sed "$(sed -n '/a/ =' file | tail -n 1)"' s/a/c/' file

• sed -n '/a/=' file outputs the numbers of the lines (function =) matching regex a, and tail -n 1 extracts the output's last line, i.e. the number of the line in file file containing the last occurrence of the regex.

• Placing command substitution $(sed -n '/a/=' file | tail -n 1) directly before ' s/a/c' results in an outer sed script such as 3 s/a/c/ (with the sample input), which performs the desired substitution only on the last on which the regex occurred.

If the pattern is not found in the input file, the whole command is an effective no-op.