WPF TextBox to enter decimal values

I currently use this behavior for digital and decimal input:

public class TextBoxInputBehavior : Behavior<TextBox>
{
    const NumberStyles validNumberStyles = NumberStyles.AllowDecimalPoint |
                                               NumberStyles.AllowThousands |
                                               NumberStyles.AllowLeadingSign;
    public TextBoxInputBehavior()
    {
        this.InputMode = TextBoxInputMode.None;
        this.JustPositivDecimalInput = false;
    }

    public TextBoxInputMode InputMode { get; set; }


    public static readonly DependencyProperty JustPositivDecimalInputProperty =
     DependencyProperty.Register("JustPositivDecimalInput", typeof(bool),
     typeof(TextBoxInputBehavior), new FrameworkPropertyMetadata(false));

    public bool JustPositivDecimalInput
    {
        get { return (bool)GetValue(JustPositivDecimalInputProperty); }
        set { SetValue(JustPositivDecimalInputProperty, value); }
    }

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.PreviewTextInput += AssociatedObjectPreviewTextInput;
        AssociatedObject.PreviewKeyDown += AssociatedObjectPreviewKeyDown;

        DataObject.AddPastingHandler(AssociatedObject, Pasting);

    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.PreviewTextInput -= AssociatedObjectPreviewTextInput;
        AssociatedObject.PreviewKeyDown -= AssociatedObjectPreviewKeyDown;

        DataObject.RemovePastingHandler(AssociatedObject, Pasting);
    }

    private void Pasting(object sender, DataObjectPastingEventArgs e)
    {
        if (e.DataObject.GetDataPresent(typeof(string)))
        {
            var pastedText = (string)e.DataObject.GetData(typeof(string));

            if (!this.IsValidInput(this.GetText(pastedText)))
            {
                System.Media.SystemSounds.Beep.Play();
                e.CancelCommand();
            }
        }
        else
        {
            System.Media.SystemSounds.Beep.Play();
            e.CancelCommand();
        }
     }

     private void AssociatedObjectPreviewKeyDown(object sender, KeyEventArgs e)
     {
        if (e.Key == Key.Space)
        {
            if (!this.IsValidInput(this.GetText(" ")))
            {
                System.Media.SystemSounds.Beep.Play();
                e.Handled = true;
            }
        }
     }

     private void AssociatedObjectPreviewTextInput(object sender, TextCompositionEventArgs e)
     {
        if (!this.IsValidInput(this.GetText(e.Text)))
        {
            System.Media.SystemSounds.Beep.Play();
            e.Handled = true;
        }
     }

     private string GetText(string input)
     {
        var txt = this.AssociatedObject;

        int selectionStart = txt.SelectionStart;
        if (txt.Text.Length < selectionStart) 
            selectionStart = txt.Text.Length;

        int selectionLength = txt.SelectionLength;
        if (txt.Text.Length < selectionStart + selectionLength) 
            selectionLength = txt.Text.Length - selectionStart;

        var realtext = txt.Text.Remove(selectionStart, selectionLength);

        int caretIndex = txt.CaretIndex;
        if (realtext.Length < caretIndex) 
            caretIndex = realtext.Length;

        var newtext = realtext.Insert(caretIndex, input);

        return newtext;
     }

     private bool IsValidInput(string input)
     {
        switch (InputMode)
        {
            case TextBoxInputMode.None:
                return true;
            case TextBoxInputMode.DigitInput:
                return CheckIsDigit(input);

            case TextBoxInputMode.DecimalInput:
                decimal d;
                //wen mehr als ein Komma
                if (input.ToCharArray().Where(x => x == ',').Count() > 1)
                    return false;


                if (input.Contains("-"))
                {
                     if (this.JustPositivDecimalInput) 
                        return false;


                     if (input.IndexOf("-",StringComparison.Ordinal) > 0) 
                          return false;

                      if(input.ToCharArray().Count(x=>x=='-') > 1)
                          return false;

                        //minus einmal am anfang zulässig
                       if (input.Length == 1) 
                           return true;
                    }

                    var result = decimal.TryParse(input, validNumberStyles, CultureInfo.CurrentCulture, out d);
                    return result;



            default: throw new ArgumentException("Unknown TextBoxInputMode");

        }
        return true;
     }

     private bool CheckIsDigit(string wert)
     {
        return wert.ToCharArray().All(Char.IsDigit);
     }
}

 public enum TextBoxInputMode
 {
  None,
  DecimalInput,
  DigitInput
  }

The XAML usage looks like this:

<TextBox Text="{Binding Sum}">
    <i:Interaction.Behaviors>
        <Behaviors:TextBoxInputBehavior InputMode="DecimalInput"/>
    </i:Interaction.Behaviors>
</TextBox>