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Seb Nilsson

Rewrite Old URLs with Regex into RouteValueDictionary


Have you ever needed to rewrite/redirect old URLs that have very similar pattern as your new, modern URLs, but want to do it with simple, maintainable code?

In the following URL you very specifically can see an ID-value and a potential Category-value.

So you probably want to use Regex and point out named groups in above URL.


Then you want to get those named values into a RouteValueDictionary, with above named Keys, like <id> and <category> with the Regex-matched Values in the URL. You can then, for example, send it to a UrlHelper. This is what the following method does:

public static RouteValueDictionary GetRegexRouteValues(string url, string urlPattern)
    if (url == null)
        throw new ArgumentNullException("url");
    if (urlPattern == null)
        throw new ArgumentNullException("urlPattern");

    var regex = new Regex(urlPattern);
    var match = regex.Match(url);
    if (!match.Success)
        return null;

    var namedGroupNames = regex.GetGroupNames().Where(x => x != null && !Regex.IsMatch(x, @"^[0-9]+$"));
    var groups = (from groupName in namedGroupNames
                  let groupItem = match.Groups[groupName]
                  where groupItem != null
                  select new KeyValuePair<string, string>(groupName, groupItem.Value)).ToList();

    var routeValues = new RouteValueDictionary();
    groups.ForEach(x => routeValues.Add(x.Key, x.Value));

    return routeValues;

Now you can write code that is more easy to read and maintain to redirect specific URLs:

private static void RedirectProductDetails(string requestUrl) {
    string urlRegex = @"^(?:.*)\/?products\/details\.aspx\?id=(?<id>[\d]*)(&category=(?<category>[^&]*))?";
    var routeValues = GetRegexRouteValues(requestUrl, urlRegex);

    routeValues["controller"] = "Products";
    routeValues["action"] = "Details";
    return this.Url.RouteUrl(routeValues);

IIS URL Rewrite-Rules Skipping Files-types

IIS Welcome

If you need to put in a rule for the IIS URL Rewrite Module, but need the rule to skip some file-endings and/or targets that are directories or actual files on disk, this is the post for you.

Following some SEO best-practices that tells us to use trailing slashes on our URLs I used the IIS Manager and added the IIS URL Rewrite-module's built-in rule "Append or remove the trailing slash symbol", which creates the following rule:

 <rule name="Add trailing slash" stopProcessing="true">
  <match url="(.*[^/])$" />
    <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
    <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
  <action type="Redirect" redirectType="Permanent" url="{R:1}/" />

This rule takes into account and doesn't apply the rule to files and directories that exists on disk. But there is a big problem with this generic rule.

If you are dynamically serving up files with extensions, then an URL like:

will become:

Adding conditions for specific file-endings

To solve this you can add conditions for certain file-endings, like .html and .aspx:

  <!-- ... -->
  <add input="{REQUEST_FILENAME}" pattern="(.*?)\.html$" negate="true" />
  <add input="{REQUEST_FILENAME}" pattern="(.*?)\.aspx$" negate="true" />

Since the rules above already don't apply for files physically on disk, you don't need to add file-endings like .css, .png or .js.

Update: Match ANY file-ending

If you want to match just any file-ending at all, you use the following pattern:

  <!-- ... -->
  <add input="{URL}" pattern=".*/[^.]*\.[\d\w]+$" negate="true" />