Fix interception routes with trailing slash configuration

packages/next/src/lib/build-custom-route.ts

@@ -37,14 +37,6 @@ export function buildCustomRoute(
     delimiter: '/', // default is `/#?`, but Next does not pass query info
   })
 
-  let source = compiled.source
-  if (!route.internal) {
-    source = modifyRouteRegex(
-      source,
-      type === 'redirect' ? restrictedRedirectPaths : undefined
-    )
-  }
-
   // If this is an internal rewrite and it already provides a regex, use it
   // otherwise, normalize the source to a regex.
   let regex: string
@@ -54,6 +46,14 @@ export function buildCustomRoute(
     !('regex' in route) ||
     typeof route.regex !== 'string'
   ) {
+    let source = compiled.source
+    if (!route.internal) {
+      source = modifyRouteRegex(
+        source,
+        type === 'redirect' ? restrictedRedirectPaths : undefined
+      )
+    }
+
     regex = normalizeRouteRegex(source)
   } else {
     regex = route.regex

packages/next/src/lib/generate-interception-routes-rewrites.ts

@@ -52,7 +52,6 @@ export function generateInterceptionRoutesRewrites(
             value: headerRegex,
           },
         ],
-        internal: true,
         regex: source.namedRegex,
       })
     }