input: Write a function to convert HTML to plain text without using any libraries. The function should strip all HTML tags and return the plain text content. 
  assert html_to_text("<h1>Hello World!</h1>")=="Hello World!"
  assert html_to_text("<p>This is a paragraph.</p>")=="This is a paragraph."
  assert html_to_text("<div><p>First paragraph.</p><p>Second paragraph.</p></div>")=="First paragraph.Second paragraph."
  assert html_to_text("<ul><li>Item 1</li><li>Item 2</li></ul>")=="Item 1Item 2"
  assert html_to_text("")==""
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)