Username Validation In Yii Model

From my sourcecode, I gave the information for username validation in yii framework.

<?php
class User extends CActiveRecord
{
  public static function model($className=__CLASS__)
  {
      return parent::model($className);
  }
  public function tableName()
  {
      return 'tbl_user';
  }
  public function rules()
  {
          array('username', 'match', 'pattern' => '/^[A-Za-z0-9_]+$/u',
                 'message'=>'Username can contain only alphanumeric 
                             characters and hyphens(-).'),
          array('username','unique'),
  }
}
?>