void Main()
{
    string yourhtml=@"<div>
<img src=""bbs.jpg"" alt=""gbbs"" /> vbbbs mobildbbs
</div>";
 Regex reg=new Regex("(?i)(?<!<[^>]*?)bbs");
 yourhtml=reg.Replace(yourhtml,"<a href=\"http://$0.com\">$0</a>",1);
 Console.WriteLine(yourhtml);
}

/*
<div>
<img src="bbs.jpg" alt="gbbs" /> vb<a href="http://bbs.com">bbs</a> mobildbbs
</div>
 <li class="bbs">
*/