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

Given:

$ cat file
a
b
a
b

You can use POSIX grep to count the matches:

$ grep -c '^a' file
2

Then feed that number into awk to print a replacement:

$ awk -v last=$(grep -c '^a' file) '/^a/ && ++cnt==last{ print "c"; next } 1' file
a
b
c
b