You want to match five-letter palindromes such as radar, rotor, and tenet. Which sed option should you use?
sed -E -n '/^(.)(.)\3\2\1$/p'
sed -E -n '/^(.)(.)(.)\2\1$/p'
sed -E -n '/^(.)(.)(.).\2\1$/p'
sed -E -n '/^(.)(.)(.)(.)\3\2\1$/p'