Filtered WPF textbox

I am trying to make a textbox which filters user input to match specified type, so I can discard some of my validation logic.

For example, if I specify ushort I want my textbox to only accept text changes which result in a valid ushort value and nothing else.

This is what I've ended up with:

public abstract class CustomTextBox<T> : TextBox
{
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(T), typeof(CustomTextBox<T>), new FrameworkPropertyMetadata(default(T), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Inherits, OnValueChanged));
    public T Value
    {
        get { return (T)GetValue(ValueProperty); }
        set { SetValue(ValueProperty, value); }
    }

    protected override void OnPreviewTextInput(System.Windows.Input.TextCompositionEventArgs e)
    {
        if (_regex != null && !_regex.IsMatch(e.Text))
        {
            e.Handled = true;
        }
        else
        {
            base.OnPreviewTextInput(e);
        }
    }

    protected override void OnTextChanged(TextChangedEventArgs e)
    {
        if (String.IsNullOrEmpty(Text))
        {
            return;
        }

        T val;
        if (!TryParse(out val))
        {
            var index = CaretIndex;
            Text = _validText;
            CaretIndex = index > 0 ? index - 1 : 0;
            e.Handled = true;
        }
        else
        {
            Value = val;
            _validText = Text;

        }
    }

    protected CustomTextBox(string regexPattern = null)
    {
        if (!String.IsNullOrEmpty(regexPattern))
        {
            _regex = new Regex(regexPattern);
        }
        _validText = ToString(Value);

        Loaded += OnTextboxLoaded;
    }

    protected override void OnLostFocus(RoutedEventArgs e)
    {
        ValidateText();

        base.OnLostFocus(e);
    }

    protected virtual string ToString(T value)
    {
        return value.ToString();
    }

    protected abstract bool TryParse(out T val);

    private readonly Regex _regex;
    private string _validText;

    private void ValidateText()
    {
        T val;
        if (!TryParse(out val))
        {
            Text = ToString(Value);
        }
    }

    private void OnTextboxLoaded(object sender, RoutedEventArgs e)
    {
        ValidateText();
    }

    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var tb = (CustomTextBox<T>)d;
        if (!e.OldValue.Equals(e.NewValue))
        {
            var str = tb.ToString((T)e.NewValue);
            if (!str.Equals(tb.Text))
            {
                tb.Text = str;
            }              
        }
    }
}

ushort implementation:

public sealed class UshortTextBox : CustomTextBox<ushort>
{
    public UshortTextBox()
        : base(@"^[0-9]$")
    {
    }

    protected override bool TryParse(out ushort val)
    {
        return UInt16.TryParse(Text, out val);
    }
}

I'm new to this whole custom-control-making, so if you see some rookie mistakes or ways to improve my code, please let me know.