input: Write a python function to tokenize a string using regular expressions.
  assert tokenize("This is a sentence.") == ['This', 'is', 'a', 'sentence', '.']
  assert tokenize("Hello, world!") == ['Hello', ',', 'world', '!']
  assert tokenize("Python 3.7") == ['Python', '3', '.', '7']
output: def tokenize(s):
      import re
      return re.findall(r"\w+|[^\w\s]", s)