import re


def resolve_token(host):
    """
    Resolve access token from host string
    Format: https://<token>@<url>
    :param host: host as a string
    :return: token or None
    """
    host_re = r"^https?://([a-zA-Z0-9\-_]+?\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+)\@.+$"
    match = re.match(host_re, host)
    return match.group(1) if match else None