import string


def transport_name(address):
    """
    If the address of the bridge starts with a valid c identifier then
    we consider it to be a bridge.
    Returns:
        The transport_name if it's a transport.
        None if it's not a obfsproxy bridge.
    """
    transport_name = address.split(' ')[0]
    transport_name_chars = string.ascii_letters + string.digits
    if all(c in transport_name_chars for c in transport_name):
        return transport_name
    else:
        return None