code: def extract_expression(dirty_string, language):
      
      tokens = tokenize(dirty_string, language)
  
      start_index = 0
      end_index = len(tokens)
  
      for part in tokens:
          if is_symbol(part) or is_word(part, language):
              break
          else:
              start_index += 1
  
      for part in reversed(tokens):
          if is_symbol(part) or is_word(part, language):
              break
          else:
              end_index -= 1
  
      return ' '.join(tokens[start_index:end_index])
docs: Give a string such as: "What is 4 + 4?"
      Return the string "4 + 4"