input: Write a python function to extract a substring between two given substrings using regular expressions.
  assert extract_substring("This is an example string", "an", "string") == " example "
  assert extract_substring("Hello world, this is a test", "Hello", "test") == " world, this is a "
  assert extract_substring("Find me between the lines", "me", "the") == " between "
output: def extract_substring(s, start, end):
      import re
      pattern = f'{re.escape(start)}(.*?){re.escape(end)}'
      match = re.search(pattern, s)
      if match:
          return match.group(1)
      return ""