The regular expression that would match a 5 letter palindrome, (e.g. "radar"), would be
([a-z])([a-z])[a-z]\2\1
([a-z])([a-z])[a-z]\2\3
\2\1([a-z])([a-z])[a-z]
([a-z])([a-z])[0-9]\2\1