Markdown in .NET

pattern = string.Format(@"
            (?:
                (?<=nn)           # Starting after a blank line
                |                   # or
                An?               # the beginning of the doc
            )
            (                       # save in $1
                [ ]{{0, {0}}}
                <(hr)               # start tag = $2
                b                  # word break
                ([^<>])*?           #
                /?>                 # the matching end tag
                [ t]*
                (?=n{{2,}}|Z)     # followed by a blank line or end of document
            )", tabWidth - 1);
        text = Regex.Replace(text, pattern, new MatchEvaluator(HtmlEvaluator), RegexOptions.IgnorePatternWhitespace);