input: Write a function to tokenize an English sentence.
  assert tokenize("This is a sentence.") == ['This', 'is', 'a', 'sentence', '.']
  assert tokenize("Hello, world!") == ['Hello', ',', 'world', '!']
  assert tokenize("How are you doing today?") == ['How', 'are', 'you', 'doing', 'today', '?']
output: def tokenize(sentence):
      import re
      return re.findall(r"\w+|[^\w\s]", sentence)