Replace the first five characters, the middle five characters, and the last five characters of each line in the file "filename.txt" with the first five characters, "kjdfm", and the last five characters, respectively.
sed -i 's/^\([a-zA-Z0-9]\{5\}\)\([a-zA-Z0-9]\{5\}\)\([a-zA-Z0-9]\{5\}\)$/\1kjdfm\3/' filename.txt