input: Write a function to check if a given string is a palindrome. The function should return True if the string is a palindrome, and False otherwise. The function should ignore case sensitivity.
  assert is_palindrome("racecar")==True
  assert is_palindrome("A man, a plan, a canal, Panama")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("")==True
output: def is_palindrome(s):
      s = ''.join(filter(str.isalnum, s)).lower()
      return s == s[::-1]