input: Write a function to validate a password based on given rules.
  assert validate_password("Password123!") != None
  assert validate_password("password") == None
  assert validate_password("PASSWORD123") == None
  assert validate_password("Pass123!") == None
output: def validate_password(password):
      import re
      if (len(password) >= 8 and
          re.search(r'[A-Z]', password) and
          re.search(r'[a-z]', password) and
          re.search(r'[0-9]', password) and
          re.search(r'[\W_]', password)):
          return password
      return None