Lucene.Net Search result to highlight search keywords

public string GeneratePreviewText(Query q, string text)
{
    QueryScorer scorer = new QueryScorer(q);
    Formatter formatter = new SimpleHTMLFormatter(highlightStartTag, highlightEndTag);
    Highlighter highlighter = new Highlighter(formatter, scorer);
    highlighter.SetTextFragmenter(new SimpleFragmenter(fragmentLength));
    TokenStream stream = new StandardAnalyzer().TokenStream(new StringReader(text));
    return highlighter.GetBestFragments(stream, text, fragmentCount, fragmentSeparator);
}