Ensure that malicious attempts to use the branch name mechanism don't get anywhere

Author: robredpath

prefix_finder/frontend/urls.py

@@ -6,7 +6,7 @@
 urlpatterns = [
     url(r'^$', views.home, name='home'),
     url(r'^_update_lists$', views.update_lists, name='update_lists'),
-    url(r'^_preview_branch/(.+)$', views.preview_branch, name='preview_branch'),
+    url(r'^_preview_branch/([A-Za-z0-9-]+)$', views.preview_branch, name='preview_branch'),
     url(r'^terms', TemplateView.as_view(template_name='terms.html'), name='terms'),
     url(r'^about', TemplateView.as_view(template_name='about.html'), name='about'),
     url(r'^list/(.+)$', views.list_details, name='list'),