input: Write a python function to find a substring between two occurrences of a given string.
  assert get_substring("ab-Hello World-ab", "ab") == "Hello World"
  assert get_substring("xy-Hello-xy-World-xy", "xy") == "Hello"
  assert get_substring("test-123-test-456-test", "test") == "123"
  assert get_substring("no-match-here", "not") == ""
output: def get_substring(s, delimiter):
      start = s.find(delimiter)
      if start == -1:
          return ""
      start += len(delimiter)
      end = s.find(delimiter, start)
      if end == -1:
          return ""
      return s[start:end]