input: Write a python function to preprocess user input by removing special characters.
  assert remove_special_characters("Hello, World!") == "Hello World"
  assert remove_special_characters("Good morning! How are you?") == "Good morning How are you"
  assert remove_special_characters("This is an example: @test!") == "This is an example test"
output: def remove_special_characters(input_string):
      import re
      return re.sub(r'[^\w\s]', '', input_string)