"""
 Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

 Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome. 
"""

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        import re
        if not s:
            return True
        else:
            s = re.sub('[^0-9a-zA-Z]', '', s).lower()
            return s==s[::-1]

Create ValidPalindrome.py