Well it taken from the “awk-exercises”.

For the input file patterns.txt, filter lines containing three or more
occurrences of “ar” and replace the last but second “ar” with “X”

par car tar far Cart

part cart mart

Expected output

par car tX far Cart

pXt cart mart

awk 'BEGIN{r = @/(.*)ar((.*ar){2})/} $0~r{print gensub(r, "\1X\2", 1)}' patterns.txt

There is one think i cannot understand. What is the “@” means in BEGIN block?