private void macEntered(object sender, KeyEventArgs e) { TextBox box = sender as TextBox; Regex rxMacAddress = new Regex(@"^[0-9a-fA-F]{2}(((:[0-9a-fA-F]{2}){5})|((:[0-9a-fA-F]{2}){5}))$"); SetupModel sm = (SetupModel)(this.DataContext); if (e.Key == Key.Return) { if ((!rxMacAddress.IsMatch(box.Text) || sm.macRepeated(box.Text.ToString())) && box.Text.Length != 0) { Storyboard redFlash = box.TryFindResource("redFlash") as Storyboard; box.Text = ""; redFlash.Begin(box); } else { BindingExpression binding = box.GetBindingExpression(TextBox.TextProperty); Keyboard.ClearFocus(); binding.UpdateSource(); sm.checkMacs(); } } }