diff --git a/app/views/devise/shared/_signup_box.html.haml b/app/views/devise/shared/_signup_box.html.haml
index 7c68e3266e5177ffecbdf37e516d4e05af76fefb..3133f6de2e844d208d24b665504f87bfd575004c 100644
--- a/app/views/devise/shared/_signup_box.html.haml
+++ b/app/views/devise/shared/_signup_box.html.haml
@@ -8,7 +8,7 @@
         = f.text_field :name, class: "form-control top", required: true, title: "This field is required."
       %div.username.form-group
         = f.label :username
-        = f.text_field :username, class: "form-control middle", pattern: "[a-zA-Z0-9]+", required: true, title: 'Please create a username with only alphanumeric characters.'
+        = f.text_field :username, class: "form-control middle", pattern: Gitlab::Regex::NAMESPACE_REGEX_STR_SIMPLE, required: true, title: 'Please create a username with only alphanumeric characters.'
         %p.validation-error.hide Username is already taken.
         %p.validation-success.hide Username is available.
         %p.validation-pending.hide Checking username availability...
diff --git a/lib/gitlab/regex.rb b/lib/gitlab/regex.rb
index 155ca47e04c819c8aa6537fc861dc7419adc652c..47ea8b7e82e6d104d9ed9d08c55c344551412abf 100644
--- a/lib/gitlab/regex.rb
+++ b/lib/gitlab/regex.rb
@@ -2,7 +2,14 @@ module Gitlab
   module Regex
     extend self
 
-    NAMESPACE_REGEX_STR = '(?:[a-zA-Z0-9_\.][a-zA-Z0-9_\-\.]*[a-zA-Z0-9_\-]|[a-zA-Z0-9_])(?<!\.git|\.atom)'.freeze
+    # The namespace regex is used in Javascript to validate usernames in the "Register" form. However, Javascript
+    # does not support the negative lookbehind assertion (?<!) that disallows usernames ending in `.git` and `.atom`.
+    # Since this is a non-trivial problem to solve in Javascript (heavily complicate the regex, modify view code to
+    # allow non-regex validatiions, etc), `NAMESPACE_REGEX_STR_SIMPLE` serves as a Javascript-compatible version of
+    # `NAMESPACE_REGEX_STR`, with the negative lookbehind assertion removed. This means that the client-side validation
+    # will pass for usernames ending in `.atom` and `.git`, but will be caught by the server-side validation.
+    NAMESPACE_REGEX_STR_SIMPLE = '[a-zA-Z0-9_\.][a-zA-Z0-9_\-\.]*[a-zA-Z0-9_\-]|[a-zA-Z0-9_]'.freeze
+    NAMESPACE_REGEX_STR = "(?:#{NAMESPACE_REGEX_STR_SIMPLE})(?<!\.git|\.atom)".freeze
 
     def namespace_regex
       @namespace_regex ||= /\A#{NAMESPACE_REGEX_STR}\z/.freeze
diff --git a/spec/features/users_spec.rb b/spec/features/users_spec.rb
index 111ca7f7a703a05963cb8f8e26e1aba47afab244..afa98f3f715c8c4612b47cc462150ba865058d8e 100644
--- a/spec/features/users_spec.rb
+++ b/spec/features/users_spec.rb
@@ -74,16 +74,29 @@ feature 'Users', feature: true, js: true do
       visit new_user_session_path
       click_link 'Register'
     end
+
+    scenario 'doesn\'t show an error border if the username is available' do
+      fill_in username_input, with: 'new-user'
+      wait_for_ajax
+      expect(find('.username')).not_to have_css '.gl-field-error-outline'
+    end
+
+    scenario 'does not show an error border if the username contains dots (.)' do
+      fill_in username_input, with: 'new.user.username'
+      wait_for_ajax
+      expect(find('.username')).not_to have_css '.gl-field-error-outline'
+    end
+
     scenario 'shows an error border if the username already exists' do
       fill_in username_input, with: user.username
       wait_for_ajax
       expect(find('.username')).to have_css '.gl-field-error-outline'
     end
 
-    scenario 'doesn\'t show an error border if the username is available' do
-      fill_in username_input, with: 'new-user'
+    scenario 'shows an  error border if the username contains special characters' do
+      fill_in username_input, with: 'new$user!username'
       wait_for_ajax
-      expect(find('#new_user_username')).not_to have_css '.gl-field-error-outline'
+      expect(find('.username')).to have_css '.gl-field-error-outline'
     end
   end