Wanted the space between the span tags to be replaced by  
<span style="font-size:11;color:black"> </span>
For that i wrote teh following regex but it give me the out put as below :
regex :
Regex.Replace(xml, @"<span[^>]*?>\s*</span>", (match) => match.Value.Replace(" ", "&nbsp;"), RegexOptions.IgnoreCase);
output :
<span&nbsp;style="font-size:11;color:black">&nbsp;</span>