def extract_property_class(prop):
    """
    This function extracts the property class from the property string.
    Property strings have the format of -([<function>.]<property_class_id>.<counter>)
    """
    prop_class = prop["property"].rsplit(".", 3)
    # Do nothing if prop_class is diff than cbmc's convention
    class_id = prop_class[-2] if len(prop_class) > 1 else None
    return class_id