magic router :D
If no route matched the request, then check if action=404 magic route exists.
If it does exist, return that one; else return null (which will fallback to the framework's 404 page).
If more than one route match the request, the more specific route wins. Note that specificity here only depends on the number of matching arguments (ie., foo=bar&qux=xyzzy is more specific than foo=bar).
Technically, there is no difference between explicit regex routes!