input: Write a python function to pre-process input for the classifier.
  assert pre_process('I like playing football') == ['like', 'playing', 'football']
  assert pre_process('Data science is an interesting field') == ['data', 'science', 'interesting', 'field']
  assert pre_process('The weather is nice today') == ['weather', 'nice', 'today']
output: def pre_process(text):
      import re
      # Convert to lowercase
      text = text.lower()
      # Remove punctuation using regex
      text = re.sub(r'[^\w\s]', '', text)
      # Split into words
      words = text.split()
      # Remove common stopwords
      stopwords = {'i', 'the', 'is', 'an'}
      words = [word for word in words if word not in stopwords]
      return words