input: Write a function to remove non-alphanumeric characters using regex.
  assert remove_non_alphanum("Hello,@ $World.") == "Hello World."
  assert remove_non_alphanum("A*B&C^D") == "ABCD"
  assert remove_non_alphanum("123@456!789") == "123456789"
output: def remove_non_alphanum(s):
      import re
      return re.sub(r'[^a-zA-Z0-9\s]', '', s)