input: Write a python function to check if an email address is valid using regex.
  assert is_valid_email("example@example.com")
  assert not is_valid_email("example.com")
  assert is_valid_email("user.name+tag+sorting@example.com")
  assert not is_valid_email("user.name@com")
  assert not is_valid_email("user.name@.com")
output: def is_valid_email(email):
      import re
      pattern = r'^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$'
      return re.match(pattern, email) is not None