WPF

void HighlightRegexMatches()
{
    if (!pHighlighting)
    {
        pHighlighting = true;

        Regex r = null;

        string p = new TextRange(
            rtbPattern.Document.ContentStart,
            rtbPattern.Document.ContentEnd
            ).Text.Replace("\r\n", "");

        try
        {
            r = new Regex(p);
        }
        catch (Exception ex)
        {
            // User typed an invalid regex, so cancel.

            pHighlighting = false;
        }

        if (r == null) return;

        try
        {
            string s = new TextRange(
                rtbTestData.Document.ContentStart,
                rtbTestData.Document.ContentEnd
                ).Text;

            MatchCollection c = r.Matches(s);

            FlowDocument d = rtbTestData.Document;

            TextRange allText = new TextRange(d.ContentStart, d.ContentEnd);

            allText.ApplyPropertyValue(TextElement.BackgroundProperty, Brushes.Transparent);

            int count = 0;

            foreach (Match m in c)
            {
                TextPointer start = d.ContentStart.GetPositionAtOffset(m.Index);

                TextPointer end = d.ContentStart.GetPositionAtOffset(m.Index + m.Length);

                TextRange t = new TextRange(start, end);

                t.ApplyPropertyValue(TextElement.BackgroundProperty, Brushes.HotPink);

                count++;
            }
        }
        catch (Exception ex)
        {
        }
        finally
        {
            pHighlighting = false;
        }
    }
}