public partial class MainWindow : ReactiveWindow<MainWindowViewModel>
{
	public MainWindow()
	{
	    InitializeComponent();
	    ViewModel = new MainWindowViewModel();
	    this.WhenActivated(cleanup =>
	    {
				this.BindWithValidation(
	            ViewModel,
	            vm => vm.NoSymbolsTextProperty.Value,
	            view => view.NoSymbolsEntry.Text,
	            .DisposeWith(cleanup);
	    });
	}
}

using System.ComponentModel.DataAnnotations;
using ReactiveUI;

public class MainWindowViewModel : ReactiveObject
{
    [RegularExpression(@"^[^!@#$%^&*()]*$", ErrorMessage = "Symbols not allowed!")]
    public ReactiveProperty<string> NoSymbolsTextProperty { get; }

    public MainWindowViewModel()
    {
        NoSymbolsTextProperty = new ReactiveProperty<string>().AddValidation(() => NoSymbolsTextProperty);
    }
}