Passing a regex to match prefix for nested routers

Fixes #134

Advanced Usage.md

@@ -149,6 +149,7 @@ Pre-conditions are defined in the routes object, using the `wrap` method exporte
 
 The pre-condition functions receive a dictionary `detail` with the same structure as the `routeLoaded` event:
 
+- `detail.route`: the route that was matched, exactly as defined in the route definition object
 - `detail.component`: the Svelte component that is being evaluated (this is a JavaScript function)
 - `detail.name`: name of the Svelte component (a string)
 - `detail.location`: the current path (just like the `$location` readable store)
@@ -333,6 +334,8 @@ Both routes first load the `Hello` route, as they both match `/hello/*` in the o
 
 Features like highlighting active links will still work, regardless of where those links are placed in the page (in which component).
 
+Note that if your parent router uses a route that contains parameters, such as `/user/:id`, then you must define a regular expression for `prefix`. For example: `prefix={/^\/user\/[0-9]+/}`.
+
 ## Route groups
 
 You can get route groups by creating a Svelte component which nests the other components. For example:

Router.svelte

@@ -304,8 +304,16 @@ class RouteItem {
      */
     match(path) {
         // If there's a prefix, remove it before we run the matching
-        if (prefix && path.startsWith(prefix)) {
-            path = path.substr(prefix.length) || '/'
+        if (prefix) {
+            if (typeof prefix == 'string' && path.startsWith(prefix)) {
+                path = path.substr(prefix.length) || '/'
+            }
+            else if (prefix instanceof RegExp) {
+                const match = path.match(prefix)
+                if (match && match[0]) {
+                    path = path.substr(match[0].length) || '/'
+                }
+            }
         }
 
         // Check if the pattern matches