public class UnNumberBehavior : Behavior<TextBox>
    {
        protected override void OnAttached()
        {
            base.OnAttached();
            this.AssociatedObject.PreviewTextInput += OnTextChanched;
        }

        private void OnTextChanched(object sender, TextCompositionEventArgs e)
        {
            if (sender is TextBox)
            {
                Regex regex = new Regex("[А-Яа-я]");
                e.Handled = regex.IsMatch(e.Text);
            }
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();
            this.AssociatedObject.TextInput -= OnTextChanched;
        }

    }