One last thought. Try this:
boost::regex re ("<div\\sclass=\"Summary\">(.*?)</div>", boost::regex::mod_s);
If that doesn't work, try double-backslashes before the forward slash.
The mod_s switch ensures that the period can match newlines.
You've spurred me into installing boost (for the regexes if nothing else),
but I haven't done it yet, so I can't test it myself!