silverlight 文本框只能输入汉字

 private void txtName_KeyDown(object sender, KeyEventArgs e)
        {
            Regex rg = new Regex("^[\u4e00-\u9fa5\b]$"); //\b是退格键
            if (!rg.IsMatch(e.Key.ToString()))
            {
                e.Handled = true;

            }
            if (txtName.Text.Length >= )
            {
                txtName.Text = txtName.Text.Substring(, );
                EasySL.Controls.Window.Alert("请输入20内的名称", this.floatePanel);
            }

        }