diff --git a/app/models/user.rb b/app/models/user.rb
index a5b4638..cf12ce0 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -55,8 +55,9 @@ class User < ActiveRecord::Base
 
   validates :password, :presence => true, :on => :create
 
+  VALID_USERNAME = /[A-Za-z0-9][A-Za-z0-9_-]{0,24}/
   validates :username,
-    :format => { :with => /\A[A-Za-z0-9][A-Za-z0-9_-]{0,24}\Z/ },
+    :format => { :with => /\A#{VALID_USERNAME}\Z/ },
     :uniqueness => { :case_sensitive => false }
 
   validates_each :username do |record,attr,value|
diff --git a/extras/markdowner.rb b/extras/markdowner.rb
index 2194dfb..7af5fe3 100644
--- a/extras/markdowner.rb
+++ b/extras/markdowner.rb
@@ -44,7 +44,7 @@ class Markdowner
 
   def self.postprocess_text_node(node)
     while node
-      return unless node.string_content =~ /\B(@[\w\-]+)/
+      return unless node.string_content =~ /\B(@#{User::VALID_USERNAME})/
       before, user, after = $`, $1, $'
 
       node.string_content = before