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

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

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

    }