input: Write a function to convert HTML to plain text without using any libraries. The function should remove 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("<a href='example.com'>Link</a>")=="Link"
  assert html_to_text("<div><p>Text</p><p>More text</p></div>")=="TextMore text"
  assert html_to_text("No tags here")=="No tags here"
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)