input: Write a python function to generate a chatbot response based on user input after preprocessing it.
  assert chatbot_response('Hello, how are you?') in ['That is an interesting topic.', 'I see. Tell me more.', 'I am happy to help.']
  assert chatbot_response('Tell me a joke.') in ['That is an interesting topic.', 'I see. Tell me more.', 'I am happy to help.']
  assert chatbot_response('') == "Sorry, I didn't quite understand what you said."
output: def chatbot_response(user_input):
      import re
      user_input = user_input.strip().lower()
      user_input = re.sub(r'[^a-zA-Z\s]', '', user_input)
      
      if not user_input:
          return "Sorry, I didn't quite understand what you said."
      
      responses = ['That is an interesting topic.', 'I see. Tell me more.', 'I am happy to help.']
      return responses[hash(user_input) % len(responses)]