diff --git a/app/views/invitations/edit.html.haml b/app/views/invitations/edit.html.haml
index 1eac967f6..7b315050b 100644
--- a/app/views/invitations/edit.html.haml
+++ b/app/views/invitations/edit.html.haml
@@ -1,3 +1,5 @@
+= javascript_include_tag "validation"
+
 = form_for(resource, :as => resource_name, :url => invitation_path(resource_name), :html => {:method => :put }) do |f|
   %p
     = f.label :username
diff --git a/app/views/registrations/new.html.haml b/app/views/registrations/new.html.haml
index a8f54e153..0cf3fc759 100644
--- a/app/views/registrations/new.html.haml
+++ b/app/views/registrations/new.html.haml
@@ -1,3 +1,5 @@
+= javascript_include_tag "validation"
+
 .span-12.prepend-6.last
   .floating
     %h3
diff --git a/public/javascripts/validation.js b/public/javascripts/validation.js
new file mode 100644
index 000000000..0a7279444
--- /dev/null
+++ b/public/javascripts/validation.js
@@ -0,0 +1,23 @@
+/*   Copyright (c) 2010, Diaspora Inc.  This file is
+ *   licensed under the Affero General Public License version 3 or later.  See
+ *   the COPYRIGHT file.
+ */
+var Validation = {
+  rules: { 
+    username: {
+      characters: /^(|[A-Za-z0-9_]{0,32})$/,
+      length: [6, 32]
+    }
+  },
+  events: { 
+    usernameKeypress: function(evt) {
+      if(!Validation.rules.username.characters.test(this.value + String.fromCharCode(evt.charCode))) {
+        evt.preventDefault();
+      }
+    }
+  }
+};
+
+$(function() { 
+  $("#user_username").keypress(Validation.events.usernameKeypress);
+});
diff --git a/spec/javascripts/support/jasmine.yml b/spec/javascripts/support/jasmine.yml
index 61c9341fe..1b5cb5a34 100644
--- a/spec/javascripts/support/jasmine.yml
+++ b/spec/javascripts/support/jasmine.yml
@@ -14,6 +14,7 @@ src_files:
   - public/javascripts/vendor/jquery144.js
   - public/javascripts/vendor/jquery-ui-1.8.6.custom.min.js
   - public/javascripts/vendor/jquery.tipsy.js
+  - public/javascripts/validation.js
   - public/javascripts/diaspora.js
   - public/javascripts/mobile.js
   - public/javascripts/aspect-edit.js
diff --git a/spec/javascripts/validation-spec.js b/spec/javascripts/validation-spec.js
new file mode 100644
index 000000000..e896d0db5
--- /dev/null
+++ b/spec/javascripts/validation-spec.js
@@ -0,0 +1,20 @@
+describe("Validation", function() { 
+  describe("rules", function() {
+    describe("username", function() {
+      describe("characters", function() {
+        it("is the regex for checking if we allow what the user typed", function() { 
+          expect(typeof Validation.rules.username.characters).toEqual("function");
+        });
+      });
+    });
+  });
+  describe("events", function() { 
+    describe("usernameKeypress", function() { 
+      it("doesn't allow the user to type anything but letters, numbers and underscores", function() { 
+        expect(Validation.rules.username.characters.test("*")).toBeFalsy();
+        expect(Validation.rules.username.characters.test("Aa_")).toBeTruthy();
+        expect(Validation.rules.username.characters.test("ffffffffffffffffffffffffffffffffff")).toBeFalsy();
+      }); 
+    });
+  });
+});