input: Write a function to check if a given string is a palindrome. A palindrome is a string that reads the same forwards and backwards, ignoring case. The function should return True if the string is a palindrome and False otherwise.
  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("12321")==True
output: def is_palindrome(s):
      s = ''.join(filter(str.isalnum, s)).lower()
      return s == s[::-1]