input: Write a function to convert a given HTML string to plain text without using any external libraries. If the given HTML string is empty then return an empty string.
  
  assert html_to_text("<a>Hello World</a>")=="Hello World"
  assert html_to_text("<p><strong>Note:</strong> This is an example.</p>")=="Note: This is an example."
  assert html_to_text("<ul><li>Item 1</li><li>Item 2</li></ul>")=="Item 1Item 2"
  assert html_to_text("<div>Content</div>")=="Content"
  assert html_to_text("")==""
output: def html_to_text(html):
      if not html:
          return ""
      import re
      text = re.sub(r'<[^>]+>', '', html)
      return text