Example: PalindromeChecker in Java

// PalindromeChecker
import java.util.Scanner;

// Main class containing the entry point of the program
public class Main {

    /**
     * Checks if a given string is a palindrome using recursion.
     * Ignores non-alphanumeric characters and case.
     * @param s The input string.
     * @return true if the string is a palindrome, false otherwise.
     */
    public static boolean isPalindromeRecursive(String s) {
        // Step 1: Handle null or empty string as a palindrome
        if (s == null || s.length() == 0 || s.length() == 1) {
            return true;
        }

        // Step 2: Normalize the string: remove non-alphanumeric and convert to lowercase
        String normalizedString = s.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();

        // Step 3: Call the helper function for recursive check
        return checkPalindrome(normalizedString, 0, normalizedString.length() - 1);
    }

    /**
     * Helper recursive function to check for palindrome.
     * @param s The normalized string.
     * @param left The left pointer index.
     * @param right The right pointer index.
     * @return true if the substring from left to right is a palindrome, false otherwise.
     */
    private static boolean checkPalindrome(String s, int left, int right) {
        // Base Case 1: If left pointer crosses or meets the right pointer, it's a palindrome
        if (left >= right) {
            return true;
        }

        // Step 4: Compare characters at current left and right pointers
        if (s.charAt(left) != s.charAt(right)) {
            return false; // Characters don't match, not a palindrome
        }

        // Step 5: If characters match, recursively check the inner substring
        return checkPalindrome(s, left + 1, right - 1);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Step 1: Prompt user for input string
        System.out.print("Enter a string to check if it's a palindrome: ");
        String input = scanner.nextLine();

        // Step 2: Call the palindrome checker function
        boolean result = isPalindromeRecursive(input);

        // Step 3: Print the result
        if (result) {
            System.out.println("'" + input + "' is a palindrome.");
        } else {
            System.out.println("'" + input + "' is NOT a palindrome.");
        }

        // Test with more examples
        System.out.println("\n--- Testing with pre-defined examples ---");
        System.out.println("'Madam' is a palindrome: " + isPalindromeRecursive("Madam"));          // True
        System.out.println("'Racecar' is a palindrome: " + isPalindromeRecursive("Racecar"));      // True
        System.out.println("'A man, a plan, a canal: Panama' is a palindrome: " + isPalindromeRecursive("A man, a plan, a canal: Panama")); // True
        System.out.println("'hello' is a palindrome: " + isPalindromeRecursive("hello"));          // False
        System.out.println("'' (empty string) is a palindrome: " + isPalindromeRecursive(""));     // True
        System.out.println("'a' (single char) is a palindrome: " + isPalindromeRecursive("a"));    // True
        System.out.println("'ab' is a palindrome: " + isPalindromeRecursive("ab"));                // False

        scanner.close();
    }
}