input: Write a python function to create a simple chatbot that responds to greetings using regular expressions and random responses.
  assert bot.process_message("hello bot") in ['Hello!', 'Hi there!', 'Greetings!', 'Nice to meet you!']
  assert bot.process_message("hey there") in ['Hello!', 'Hi there!', 'Greetings!', 'Nice to meet you!']
  assert bot.process_message("hi, how are you?") in ['Hello!', 'Hi there!', 'Greetings!', 'Nice to meet you!']
  assert bot.process_message("goodbye") is None
output: import re
  import random
  
  class SimpleChatbot:
      def __init__(self):
          self.greetings = ['hello', 'hi', 'hey', 'greetings']
          self.responses = ['Hello!', 'Hi there!', 'Greetings!', 'Nice to meet you!']
          
      def process_message(self, message):
          for greeting in self.greetings:
              if re.search(r'\b' + greeting + r'\b', message, re.IGNORECASE):
                  return random.choice(self.responses)
          return None
  
  bot = SimpleChatbot()