如何在WPF中使用textbox的掩码/验证？

<Window x:Class="WpfApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="WPF TextBox Mask/Validation" Height="250" Width="400">
    <Grid>
        <TextBox x:Name="txtInput" Width="200" Height="30" Margin="10"
                 Text="{Binding Path=InputText, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
    </Grid>
</Window>

using System.ComponentModel;
using System.Text.RegularExpressions;
using System.Windows;

namespace WpfApp
{
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private string inputText;

        public string InputText
        {
            get { return inputText; }
            set
            {
                inputText = value;
                OnPropertyChanged("InputText");
            }
        }

        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public class CustomValidationRule : ValidationRule
        {
            public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
            {
                string input = value as string;

                // 检查输入是否为空
                if (string.IsNullOrEmpty(input))
                    return new ValidationResult(false, "输入不能为空");

                // 检查输入是否符合指定的正则表达式
                Regex regex = new Regex(@"^\d{4}$");
                if (!regex.IsMatch(input))
                    return new ValidationResult(false, "输入必须为4位数字");

                return ValidationResult.ValidResult;
            }
        }
    }
}