Well, 2 letter usernames are fine too

app/models/user.rb

@@ -1,6 +1,6 @@
 class User < ActiveRecord::Base
 
-  USERNAME_FORMAT = /\A[a-zA-Z0-9][a-zA-Z0-9-]+[a-zA-Z0-9]\z/
+  USERNAME_FORMAT = /\A[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9]\z/
 
   has_many :api_tokens, :dependent => :destroy
   has_many :asciicasts, :dependent => :destroy
@@ -13,7 +13,7 @@ class User < ActiveRecord::Base
   validates :username, presence: true
   validates :username, uniqueness: { scope: :dummy, case_sensitive: false },
                        format: { with: USERNAME_FORMAT },
-                       length: { minimum: 3, maximum: 16 },
+                       length: { minimum: 2, maximum: 16 },
                        unless: :dummy
   validates :email, presence: true, uniqueness: true, unless: :dummy
 

spec/models/user_spec.rb

@@ -51,13 +51,13 @@ describe User do
 
       it { should allow_value('sickill').for(:username) }
       it { should allow_value('sick-ill').for(:username) }
-      it { should allow_value('abc').for(:username) }
+      it { should allow_value('ab').for(:username) }
       it { should allow_value('s' * 16).for(:username) }
       it { should allow_value('Sickill').for(:username) }
       it { should_not allow_value('sick.ill').for(:username) }
       it { should_not allow_value('-sickill').for(:username) }
       it { should_not allow_value('sickill-').for(:username) }
-      it { should_not allow_value('ab').for(:username) }
+      it { should_not allow_value('a').for(:username) }
       it { should_not allow_value('s' * 17).for(:username) }
 
       it "checks username uniqueness" do