input: Write a function that takes in a string and checks if it is a palindrome. A palindrome is a word, phrase, number, or other sequences of characters that reads the same forward and backward (ignoring spaces, punctuation, and letter casing). The function should return True if the string is a palindrome, and False otherwise.
  
  def is_palindrome(s):
      s = s.lower()
      s = ''.join(filter(str.isalnum, s))
      return s == s[::-1]
  
  assert is_palindrome("racecar")==True
  assert is_palindrome("A man, a plan, a canal, Panama")==True
  assert is_palindrome("Hello, World!")==False
  assert is_palindrome("No lemon, no melon")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      s = s.lower()
      s = ''.join(filter(str.isalnum, s))
      return s == s[::-1]