How to get span tag elements with regex expression matching..

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C#5.0

I have an html o/p..now i need to find a span tag and ..i need to get child elements in span tag...
here is my code

   List<string> pages = new List<string>();
            String regex = "<span class=\"prime-message\"> $1 </span>";
            MatchCollection th = Regex.Matches(data, regex, RegexOptions.Singleline);
            foreach (Match i in th)
	        {
		        string target=i.Groups[0].Value;
                target=target.Replace("<span>",string.Empty);
                target=target.Replace("</span>",string.Empty);
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here i could not getting span elements tags in side of "i" in foreach loop...and also i was not getting any elements in "th"...how coud i achieve this...i just need to get spam tag in side of "i" in foreach loop....i wasso thankfull for any sugestions and advices...