正则表达式实现

"/testing/parent/default.aspx"  --> "/testing/default.aspx"
"/testing/parent.aspx"          --> "/testing/default.aspx"
"/testing/default.aspx"         --> "/default.aspx"
"/default.aspx"                 --> null

...

private string GetParentUrl(string url)
{
    string parentUrl = url;

    if (parentUrl.EndsWith("Default.aspx", StringComparison.OrdinalIgnoreCase))
    {
        parentUrl = parentUrl.Substring(0, parentUrl.Length - 12);

        if (parentUrl.EndsWith("/"))
            parentUrl = parentUrl.Substring(0, parentUrl.Length - 1);
    }

    int i = parentUrl.LastIndexOf("/");

    if (i < 2) return null;

    parentUrl = parentUrl.Substring(0, i + 1);

    return string.Format(CultureInfo.InvariantCulture, "{0}Default.aspx", parentUrl);
}

这个代码有效,但我觉得很难闻。它不适用于具有查询字符串的URL。如何使用regex改进它?