input: 
  def is_palindrome(s):
      """ Check if the given string is a palindrome.
      A palindrome is a word, phrase, number, or other sequences of characters 
      which reads the same forward and backward (ignoring spaces, punctuation, 
      and letter casing).
      >>> is_palindrome("racecar")
      True
      >>> is_palindrome("hello")
      False
      """
      s = s.lower()
      left = 0
      right = len(s) - 1
      
      while left < right:
          if s[left] != s[right]:
              return False
          left += 1
          right -= 1
      return True
  
  print(is_palindrome("racecar"))
  
output:     s = ''.join(filter(str.isalnum, s))