input: Write a function that takes a string as input and determines whether 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).
  
  def is_palindrome(s):  
    s = s.lower()  
    s = ''.join(e for e in s if e.isalnum())  
    return s == s[::-1]
  
  assert is_palindrome("racecar")==True  
  assert is_palindrome("A man, a plan, a canal: Panama")==True  
  assert is_palindrome("hello")==False  
  assert is_palindrome("No 'x' in Nixon")==True  
  assert is_palindrome("")==True
output: def is_palindrome(s):  
    s = s.lower()  
    s = ''.join(e for e in s if e.isalnum())  
    return s == s[::-1]