def replace_tags(string, from_tag="i", to_tag="italic"):
    """
    Replace tags such as <i> to <italic>
    <sup> and <sub> are allowed and do not need to be replaced
    This does not validate markup
    """
    string = string.replace("<" + from_tag + ">", "<" + to_tag + ">")
    string = string.replace("</" + from_tag + ">", "</" + to_tag + ">")
    return string