How can I do it ? Get the word under the cursor in Window

private void txtHoverWord_MouseMove(object sender, MouseEventArgs e)
{
    if (!(sender is TextBox)) return;
    var targetTextBox = sender as TextBox;
    if (targetTextBox.TextLength < 1) return;

    var currentTextIndex = targetTextBox.GetCharIndexFromPosition(e.Location);
    var wordRegex = new Regex(@"(\w+)");
    var words = wordRegex.Matches(targetTextBox.Text);
    if (words.Count < 1) return;

    var currentWord = string.Empty;
    for (var i = words.Count - 1; i >= 0; i--)
    {
        if (words[i].Index <= currentTextIndex)
        {
            currentWord = words[i].Value;
            break;
        }
    }

    if (currentWord == string.Empty) return;
    tooltip1.SetToolTip(targetTextBox, currentWord);
}