% palindrome (strict palindrome)
palindrome(X) ->
    palindrome(X, lists:reverse(X), length(X) div 2).

palindrome([X|X1s], [X|X2s], Remaining) ->
    if Remaining == 0 ->
        true;
    true ->
        palindrome(X1s, X2s, Remaining-1)
    end;
palindrome([_X|_X1s], [_Y|_X2s], _Remaining) ->
    false.

% palindrome (relaxed palindrome, only considers letters)
relaxed_palindrome(X) ->
    CleanedX = only_letters(X),
    palindrome(CleanedX).

only_letters(X) ->
    only_letters(X, []).

only_letters([], Acc) -> Acc;
only_letters([X|Xs], Acc) ->
    case re:run([X], "^[A-Za-z]$") of
        {match, _} -> only_letters(Xs, Acc ++ string:lowercase([X]));
        nomatch    -> only_letters(Xs, Acc)
    end.