Using a regular expression to parse HTML is fraught with pitfalls. HTML is not a regular language and hence can’t be 100% correctly parsed with a regex. This is just one of many problems you will run into. The best approach is to use an HTML / XML parser to do this for you.
Here is a link to a blog post I wrote awhile back which goes into more details about this problem.
That being said, here’s a solution that should fix this particular problem. It in no way is a perfect solution though.
var pattern = @"<(img|a)[^>]*>(?<content>[^<]*)<";
var regex = new Regex(pattern);
var m = regex.Match(sSummary);
if ( m.Success ) {
sResult = m.Groups["content"].Value;