input: Write a function to convert HTML to plain text without using any libraries. The function should take an HTML string as input, strip all HTML tags, and return the plain text content.
  assert html_to_text("<p>Hello, World!</p>")=="Hello, World!"
  assert html_to_text("<div><h1>Title</h1><p>Paragraph</p></div>")=="Title Paragraph"
  assert html_to_text("<a href='https://example.com'>Link</a>")=="Link"
  assert html_to_text("<br>")==""
  assert html_to_text("No tags here")=="No tags here"
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)