How do I match a URL starting with a literal tilde character (~) in System.Web.Routing / ASP.NET MVC?

I'm converting some old-school code to ASP.NET MVC, and have hit a snag caused by our URL formats. We specify thumbnail width, height, etc. in a URL by prefixing the special URL path with a tilde, as in this example:

http://www.mysite.com/photo/~200x400/crop/some_photo.jpg

At the moment, this is resolved by a custom 404 handler in IIS, but now I want to replace /photo/ with an ASP.NET and use System.Web.Routing to extract the width, height, etc. from the incoming URL.

Problem is - I can't do this:

routes.MapRoute(
  "ThumbnailWithFullFilename",
  "~{width}x{height}/{fileNameWithoutExtension}.{extension}",
  new { controller = "Photo"开发者_如何学运维, action = "Thumbnail" }
);

because System.Web.Routing won't allow a route to start with a tilde (~) character.

Changing the URL format isn't an option... we've supported this URL format publicly since 2000 and the web is probably rife with references to it. Can I add some kind of constrained wildcard to the route?

You could write a custom crop route:

public class CropRoute : Route
{
    private static readonly string RoutePattern = "{size}/crop/{fileNameWithoutExtension}.{extension}";
    private static readonly string SizePattern = @"^\~(?<width>[0-9]+)x(?<height>[0-9]+)$";
    private static readonly Regex SizeRegex = new Regex(SizePattern, RegexOptions.Compiled);

    public CropRoute(RouteValueDictionary defaults)
        : base(
            RoutePattern,
            defaults,
            new RouteValueDictionary(new
            {
                size = SizePattern
            }),
            new MvcRouteHandler()
        )
    {
    }

    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        var rd = base.GetRouteData(httpContext);
        if (rd == null)
        {
            return null;
        }
        var size = rd.Values["size"] as string;
        if (size != null)
        {
            var match = SizeRegex.Match(size);
            rd.Values["width"] = match.Groups["width"].Value;
            rd.Values["height"] = match.Groups["height"].Value;
        }
        return rd;
    }
}

which you would register like this:

routes.Add(
    new CropRoute(
        new RouteValueDictionary(new
        {
            controller = "Photo",
            action = "Thumbnail"
        })
    )
);

and inside the Thumbnail action of Photo controller you should get all you need when you request /~200x400/crop/some_photo.jpg:

public ActionResult Thumbnail(
    string fileNameWithoutExtension, 
    string extension, 
    string width, 
    string height
)
{
    ...
}