Valid Palindrome

public boolean isPalindrome(String s) {
    if (s == null) return false;

    s = s.toLowerCase().replaceAll("[^a-z0-9]", "")

    int left = 0;
    int right = s.length() - 1;

    while (left < right) {
        if (s.charAt(left) != s.charAt(right)) {
            return false;
        }
        left++;
        right--;
    }

    return true;
}