input: Write a function to convert HTML to plain text without using any libraries. The function should strip out all HTML tags and leave only the plain text content.
  assert html_to_text("<h1>Hello, World!</h1>")=="Hello, World!"
  assert html_to_text("<p>This is a <strong>bold</strong> statement.</p>")=="This is a bold statement."
  assert html_to_text("<div><p>Some content</p></div>")=="Some content"
  assert html_to_text("")==""
  assert html_to_text("No tags here")=="No tags here"
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)