Behavior of TextBox elements

When working with input data (TextBox) you will usually have a certain data type that you expect from the end user. This may be an integer or string type. Even if it’s a string then you may want to check if it matches a certain pattern.
You can of course use a validation and that will simply color in red the faulty lines but that was not what I was looking for, I needed something that will not allow the user to insert a string in a number field.

There is a nice library (Interactivity) where you can define behaviours, but I was creating an add-in and this proved to be a problem, so I decided to make something simple but pretty powerfull. Of course you may extend it as you like:

public class TextboxStrict
    {
        private Regex validate;
        private string lastKnowText = "";
        private string defaultText = "";
        private int defaultCaretIndex = 0;
        private int lastKnownCaretIndex = 0;

        /// inputChange - marks that the change was made by this class so that 
        /// when text changed is fired again - caret index will not move it's position
        private bool inputChange = false;

        public TextboxStrict(string regexPattern, string startWith = "")
        {
            validate = new Regex(regexPattern);
            ///lastKnownText will be the placeholder of any good changes in text status
            this.lastKnowText = startWith;
            ///default text will be the holder of the default text on initialize and on text == ""
            this.defaultText = startWith;

            this.lastKnownCaretIndex = startWith.Length;
            this.defaultCaretIndex = startWith.Length;
        }

        public void AttachTo(TextBox textBox)
        {
            ///Initializing text box data with default entries
            textBox.Text = this.defaultText;
            textBox.CaretIndex = this.defaultCaretIndex;

            textBox.TextChanged += EvtTextChanged;
            textBox.SelectionChanged += EvtSelectionChanged;
        }

        /// <summary>
        /// Validate function
        /// </summary>
        private bool Validate(string text)
        {
            return validate.IsMatch(text);
        }

        /// <summary>
        /// Making sure last known caret index keeps track
        /// </summary>
        private void EvtSelectionChanged(object sender, RoutedEventArgs e)
        {
            TextBox textBox = (TextBox)sender;
            lastKnownCaretIndex = textBox.CaretIndex;
        }
        /// <summary>
        /// Validates text and keeps last known good one. Keeps track of caret index also
        /// </summary>
        private void EvtTextChanged(object sender, TextChangedEventArgs e)
        {
            TextBox textbox = (TextBox)sender;

            if (textbox.Text == "")
            {
                inputChange = true;
                textbox.Text = this.defaultText;
                textbox.CaretIndex = this.defaultCaretIndex;
            }
            ///If it doesn't validate - change back. Mark as internal change to not override the last index
            else if (!this.Validate(textbox.Text))
            {
                inputChange = true;
                textbox.Text = lastKnowText;
                textbox.CaretIndex = lastKnownCaretIndex;
            }
            else
            {
                if (!inputChange)
                {
                    lastKnowText = textbox.Text;
                    lastKnownCaretIndex = textbox.CaretIndex;
                }
                inputChange = false;
            }
        }
    }

public partial class MainWindow : Window
    {
        TextboxStrict inputLimit = new TextboxStrict("^[0-9]{1,10}$", "0");

        public MainWindow()
        {
            InitializeComponent();
            inputLimit.AttachTo(TestTextBox);
        }
    }