input: Write a function to convert an HTML string to plain text without using any libraries. If there are HTML tags in the input string, they should be removed while keeping the text content intact. If the input string is empty, the function should return an empty string.
  assert html_to_text("<p>Hello, <b>world</b>!</p>")=="Hello, world!"
  assert html_to_text("<div><h1>Title</h1><p>Paragraph</p></div>")=="TitleParagraph"
  assert html_to_text("<br>")==""
  assert html_to_text("No tags here")=="No tags here"
  assert html_to_text("")==""
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)