<li data-bm="6" class="b_algo"><h2><a href="http://php.net/manual/en/index.php" h="ID=SERP,5100.1"><strong>PHP</strong>: <strong>PHP Manual</strong> - Manual</a></h2></li>

var result = Regex.Match(input,
                         @"<a [^>]+? href s*=s* ([""']) (?<url>[^1]*?) (1)",
                         RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);
if (result.Success)
{
    return result.Groups["url"].Value;
}