input: Write a python function to extract all email addresses from a given HTML string using regular expressions.
  assert extract_emails("<html><body><p>Email: contact@company.com</p><p>Email: support@company.com</p></body></html>") == ["contact@company.com", "support@company.com"]
  assert extract_emails("<html><body><p>Reach us at mail@domain.org or sales@domain.org</p></body></html>") == ["mail@domain.org", "sales@domain.org"]
  assert extract_emails("<html><body><p>No emails here!</p></body></html>") == []
output: def extract_emails(html_str):
      import re
      return re.findall(r'[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}', html_str)