Regex – Why is ASP.NET-MVC Routing’s UrlParameter.Optional ignored when using this Regex

This is a stripped down example of a problem I was having this morning with ASP.NET MVC's URL routing.

Fairly simple, I wanted a route's Action to be called, whether or not the parameter on the end was supplied.

This route works fine, matching both /apple/ and /apple/test/

routes.MapRoute(
    "Working Route",
    "apple/{parameter}",
    new { 
        controller = "Apple", 
        action = "Action", 
        parameter = UrlParameter.Optional
    },
    new { parameter = @"([a-z0-9\.-]+)" }
);

However, this second route will only match /banana/test/ and the like. When I try /banana/, the router just passes right over it and returns the catch-all 404 error.

routes.MapRoute(
    "Non-Working Route",
    "banana/{parameter}",
    new { 
        controller = "Banana", 
        action = "Action", 
        parameter = UrlParameter.Optional
    },
    new { parameter = @"([a-z0-9]+)" }
);

The only difference is the Regex validation of the parameter, but as it's quite a simple Regex match, they should both work perfectly fine for a URL like /banana/, yet the second route just fails to recognise it.

I side-stepped my problem by just changing the Regex on route #2 to match that on route #1, and accept the '.' and '-' characters, I just wondered if anyone knows why this seems to be happening.

EDIT:

Here are the Controllers and Actions that I'm using for my example. Nothing fancy here.

public class AppleController : Controller
{
    public ActionResult Action(string parameter)
    {
        if (parameter == null)
        {
            parameter = "No parameter specified.";
        }
        ViewData["parameter"] = parameter;
        return View();
    }
}

public class BananaController : Controller
{
    public ActionResult Action(string parameter)
    {
        if (parameter == null)
        {
            parameter = "No parameter specified.";
        }
        ViewData["parameter"] = parameter;
        return View();
    }
}

So my problem is that /apple/ would display "No parameter specified.", while /banana/ gives me an undesired 404 instead.

So far..

Using parameter = URLParameter.Optional in the Route declaration:
Route #1 works perfectly, Route #2 doesn't match without the parameter.

Using parameter = "" in the Route declaration:
Both Route #1 & Route #2 fail to match when the parameter is left off the URL.

Declaring parameter = "" in the Action method signature:
Not possible due to .NET version.

Removing all other routes has no effect.