Attached Property based TextBox RegEx mask

                {
                    var localTxt = lastValidText;
                    var proposedText = GetProposedText(tbInstance, e1.Text);
                    if (!maskRegEx.IsMatch(proposedText))
                    {
                        e1.Handled = true;
                    }
                };
                
                tbInstance.Loaded += (s, e1) =>
                {
                    if (maskRegEx.IsMatch(tbInstance.Text))
                    {
                        lastValidText = tbInstance.Text;
                        SetHasRegExMisMatch(tbInstance,false);
                    }
                    else
                    {
                        SetHasRegExMisMatch(tbInstance, true);
                    }
                };

                tbInstance.TextChanged += (s, e1) =>
                {
                    if (maskRegEx.IsMatch(tbInstance.Text))
                    {
                        lastValidText = tbInstance.Text;
                        SetHasRegExMisMatch(tbInstance, false);
                    }
                    else
                    {
                        SetHasRegExMisMatch(tbInstance, true);
                    }
                };

                tbInstance.PreviewLostKeyboardFocus += (s, e1) =>
                {
                    var localTxt = lastValidText;
                    if (!maskRegEx.IsMatch(tbInstance.Text))
                    {
                        SetHasRegExMisMatch(tbInstance, true);
                        e1.Handled = true;
                        tbInstance.Text = localTxt;
                        tbInstance.CaretIndex = 99;
                    }
                    else
                    {
                        SetHasRegExMisMatch(tbInstance, false);
                    }
                };            
        }

        private static string GetProposedText(TextBox textBox, string newText)
        {
            var text = textBox.Text;

            if (textBox.SelectionStart != -1)
            {
                text = text.Remove(textBox.SelectionStart, textBox.SelectionLength);
            }

            text = text.Insert(textBox.CaretIndex, newText);

            return text;
        }
    }