The manual section for Zend_Application_Resource_Router shows an example of how to create a route with parameter defaults, but it does not show how to add parameter requirements. This is important because while the key "defaults" is spelled out, for some reason the key "requirements" has been abbreviated in the code as "reqs". As a result, the correct syntax should be:

resources.router.routes.route_id.route = "/product/:product_id"
resources.router.routes.route_id.defaults.module = "default"
resources.router.routes.route_id.defaults.controller = "product"
resources.router.routes.route_id.defaults.action = "info"
resources.router.routes.route_id.reqs.product_id = "^\d+$"