diff --git a/app/models/user.rb b/app/models/user.rb
index 40d5f20..4f49b81 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -12,6 +12,13 @@ class User < ApplicationRecord
 
   validates_uniqueness_of :cn
   validates_length_of :cn, :minimum => 3
+  validates_format_of :cn, with: /\A([a-z0-9\-])*\z/,
+                           if: Proc.new{ |u| u.cn.present? },
+                           message: "is invalid. Please use only letters, numbers and -"
+  validates_format_of :cn, without: /\A-/,
+                           if: Proc.new{ |u| u.cn.present? },
+                           message: "is invalid. Usernames need to start with a letter."
+
   validates_uniqueness_of :email
   validates :email, email: true
 
diff --git a/spec/features/signup_spec.rb b/spec/features/signup_spec.rb
index 8b730c2..9dff0df 100644
--- a/spec/features/signup_spec.rb
+++ b/spec/features/signup_spec.rb
@@ -71,6 +71,12 @@ RSpec.describe "Signup", type: :feature do
       fill_in "user_cn", with: "t"
       click_button "Continue"
       expect(page).to have_content("Username is too short")
+      fill_in "user_cn", with: "-tony"
+      click_button "Continue"
+      expect(page).to have_content("Username is invalid")
+      fill_in "user_cn", with: "$atoshi"
+      click_button "Continue"
+      expect(page).to have_content("Username is invalid")
       fill_in "user_cn", with: "jimmy"
       click_button "Continue"
       expect(page).to have_content("Username has already been taken")