input: Write a function to convert HTML to plain text without using any libraries. The function should take in an HTML string and return a plain text string by removing all HTML tags.
  assert html_to_text("<p>Hello, <b>world</b>!</p>")=="Hello, world!"
  assert html_to_text("<h1>Title</h1><p>Paragraph</p>")=="TitleParagraph"
  assert html_to_text("<a href='https://example.com'>Link</a>")=="Link"
  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)