input: Write a python function to extract the domain name from a URL using regular expressions.
  assert extract_domain_name("http://www.google.com") == "www.google.com"
  assert extract_domain_name("https://example.com/path/to/page") == "example.com"
  assert extract_domain_name("ftp://ftp.example.com/resource") == "ftp.example.com"
  assert extract_domain_name("http://subdomain.example.co.uk") == "subdomain.example.co.uk"
output: def extract_domain_name(url):
      import re
      pattern = r'^(?:http|https|ftp)://([^/]+)'
      match = re.search(pattern, url)
      return match.group(1) if match else None