diff --git a/docs/topics/2.4-accouncement.md b/docs/topics/2.4-accouncement.md
index 0cf50ce90..91472b9c4 100644
--- a/docs/topics/2.4-accouncement.md
+++ b/docs/topics/2.4-accouncement.md
@@ -2,3 +2,4 @@
 * List/detail routes.
 * 1.3 Support dropped, install six for <=1.4.?.
 * `allow_none` for char fields
+* `trailing_slash = True` --> `[^/]`, `trailing_slash = False` --> `[^/.]`, becomes simply `[^/]` and `lookup_value_regex` is added.
diff --git a/rest_framework/routers.py b/rest_framework/routers.py
index df1233fdd..406ebcf77 100644
--- a/rest_framework/routers.py
+++ b/rest_framework/routers.py
@@ -220,12 +220,10 @@ class SimpleRouter(BaseRouter):
         https://github.com/alanjds/drf-nested-routers
         """
         base_regex = '(?P<{lookup_prefix}{lookup_field}>{lookup_value})'
+        # Use `pk` as default field, unset set.  Default regex should not
+        # consume `.json` style suffixes and should break at '/' boundaries.
         lookup_field = getattr(viewset, 'lookup_field', 'pk')
-        try:
-            lookup_value = viewset.lookup_value_regex
-        except AttributeError:
-            # Don't consume `.json` style suffixes
-            lookup_value = '[^/.]+'
+        lookup_value = getattr(viewset, 'lookup_value_regex', '[^/.]+')
         return base_regex.format(
             lookup_prefix=lookup_prefix,
             lookup_field=lookup_field,