HighlightTextBox.cs

private void Update()
{
    if (string.IsNullOrEmpty(Text) || string.IsNullOrEmpty(HighlightedText))
    {
        TB.Inlines.Clear();
        return;
    }

    TB.Inlines.Clear();
    var parts = Regex.Split(Text, HighlightedText, RegexOptions.IgnoreCase);
    var len = 0;
    foreach (var part in parts)
    {
        len = len + part.Length + 1;

        TB.Inlines.Add(new Run
        {
            Text = part
        });

        if (Text.Length >= len)
        {
            var highlight = Text.Substring(len - 1, HighlightedText.Length); //to match the case

            TB.Inlines.Add(new Run
            {
                Text = highlight,
                Foreground = HighlightBrush
            });
        }
    }
}