Add username format restrictions

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