C#如何控制文本框输入大写字符？

 //控制密码的大小写：不允许使用大写字母  

    string pattern = @"^[^A-Z]*$";
   Match m = Regex.Match(this.textBoxNew.Text.Trim(), pattern);   // 匹配正则表达式
                        if (!m.Success) 
                        {
                            MessageBox.Show("您的密码输入了大写字母或开启了Caps Lock键，请重新输入！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                            this.textBoxNew.Focus();
                            return;
                        }