input: Write a function to determine if a given string is a palindrome. A palindrome is a string that reads the same backward as forward, ignoring case. If the given string is a palindrome, return True; otherwise, return False. If the string is empty, return True.
  assert is_palindrome("racecar")==True
  assert is_palindrome("Racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("A man, a plan, a canal, Panama")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      s = ''.join(filter(str.isalnum, s)).lower()
      return s == s[::-1]