Determine if a string is a palindrome

palin.erl

-module (palin).
-export ([palindrome/1, cleanString/1]).

% "Madam, I\'m Adam." must return true
palindrome(String) ->
    palinClean(cleanString(String)).

palinClean(String) ->
    {HeadHalf, TailHalf} = lists:split(length(String) div 2, String),
    lists:prefix(HeadHalf, lists:reverse(TailHalf)).

cleanString([]) -> [];
cleanString([X|Xs]) when (X >= $a) and (X =< $z)  ->
    [X|cleanString(Xs)];
cleanString([X|Xs]) when (X >= $A) and (X =< $Z) ->
    [X+($a-$A) | cleanString(Xs)];
cleanString([_X|Xs]) ->
    cleanString(Xs).