input: Write a python function to remove non-alphanumeric characters from a string using regular expressions.
  assert remove_non_alphanum("Hello, World!") == "HelloWorld"
  assert remove_non_alphanum("Python3.8") == "Python38"
  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)