diff --git a/lib/cartodb/controllers/layergroup/tile.js b/lib/cartodb/controllers/layergroup/tile.js
index 4008afa4..7837d116 100644
--- a/lib/cartodb/controllers/layergroup/tile.js
+++ b/lib/cartodb/controllers/layergroup/tile.js
@@ -41,7 +41,9 @@ module.exports = class TileLayergroupController {
     }
 
     register (mapRouter) {
+        // REGEXP matches with positive and negative numnbers
         const number = () => `(-?\\d+)`;
+        // REGEXP doesn't match with `val`
         const not = (val) => `(?!${val})([^\/]+?)`;
 
         mapRouter.get(