WPF

namespace Goldlight.Behaviors
{
  using System.Windows;
  using System.Windows.Interactivity;
  using System.Windows.Input;
  using System.Windows.Controls;
  using System.Text.RegularExpressions;

  public class NumericTextBoxBehavior : Behavior<TextBox>
  {
    #region Overrides
    protected override void OnAttached()
    {
      base.OnAttached();
      AssociatedObject.PreviewTextInput += AssociatedObject_PreviewTextInput;
      DataObject.AddPastingHandler(AssociatedObject, OnClipboardPaste);
    }

    protected override void OnDetaching()
    {
      AssociatedObject.PreviewTextInput -= AssociatedObject_PreviewTextInput;
      DataObject.RemovePastingHandler(AssociatedObject, OnClipboardPaste);
    }
    #endregion

    #region Private Methods
    private void AssociatedObject_PreviewTextInput(object sender, TextCompositionEventArgs e)
    {
      var tb = AssociatedObject;

      e.Handled = !Validate(e.Text);
    }


    private void OnClipboardPaste(object sender, DataObjectPastingEventArgs e)
    {
      var tb = sender as TextBox;
      string text = e.SourceDataObject.GetData(e.FormatToApply) as string;

      if (!string.IsNullOrEmpty(text) && !Validate(text))
      {
        e.CancelCommand();
      }
    }

    private bool Validate(string newContent)
    {
      TextBox tb = AssociatedObject;
      string testString = string.Empty;
      string pre = string.Empty;
      string post = string.Empty;
      // replace selection with new text.
      if (!string.IsNullOrEmpty(tb.SelectedText))
      {
        pre = tb.Text.Substring(0, tb.SelectionStart);
        post = tb.Text.Substring(tb.SelectionStart + tb.SelectionLength,
          tb.Text.Length - (tb.SelectionStart + tb.SelectionLength));
      }
      else
      {
        pre = tb.Text.Substring(0, tb.CaretIndex);
        post = tb.Text.Substring(tb.CaretIndex, tb.Text.Length - tb.CaretIndex);
      }
      testString = pre + newContent + post;

      Regex regExpr = new Regex(@"^([-+]?)(\d*)([,.]?)(\d*)$");
      return (regExpr.IsMatch(testString));
    }
    #endregion
  }
}