input: Write a function named `search_string` that takes a list of strings and a user input string as parameters. The function should search the list to find if there are any matches of the input string within the list elements using regular expressions. If a match is found, the function should return "String matches"; otherwise, it should return "String doesn't match". If the input list is empty, the function should return "Empty list". Provide assert tests to verify your function.
  assert search_string(["First","Second","Third"], "Sec") == "String matches"
  assert search_string(["First","Second","Third"], "Fourth") == "String doesn't match"
  assert search_string([], "First") == "Empty list"
  assert search_string(["First","Second","Third"], "first") == "String doesn't match"
output: import re
  
  def search_string(lst, user_input):
      if not lst:
          return "Empty list"
      for item in lst:
          if re.search(user_input, item):
              return "String matches"
      return "String doesn't match"