boost::regex вытянуть нужные данные....

#include <iostream>
#include <boost\regex.hpp>
 
 
int main()
{
    std::string str("widget_vars.courses_cash[\"USD\"] = {\"buy\":\"8.010\", \"sell\":\"8.045\"};");
    boost::regex wablon("/^\"buy\":\"([\\d].[\\d]+)\"$/");
    boost::smatch what;
    boost::regex_match(str,what,wablon);
    for(int i=0;i<(int)what.size();i++)
        std::cout<<std::string(what[i].first,what[i].second)<<std::endl;    
    return 0;
}

#include <iostream>
#include <string>
#include <boost/regex.hpp>
 
int main()
{
   const std::string str = "widget_vars.courses_cash[\"USD\"] = {\"buy\":\"8.010\", \"sell\":\"8.045\"};";
   const std::string pattern = "\\{\"buy\":\"(?<buy_amount>(\\d|\\.)+)\"";
   boost::smatch what; 
   boost::regex_search(str, what, boost::regex(pattern));
   if (!what.empty())
   {
      std::cout << what["buy_amount"] << std::endl;
   }
}

?<buy_amount>

boost::match_results<std::string::const_iterator> matches;

 #define BOOST_REGEX_MATCH_EXTRA

 (?<buy_amount>

(?<buy_amount>(\\d|\\.)+)

what["buy_amount"]

#include <boost/regex.hpp>
#include <iostream>
 
 
void print_captures(const std::string& regx, const std::string& text)
{
   boost::regex e(regx);
   boost::match what;
   std::cout << "Expression:  \"" << regx << "\"\n";
   std::cout << "Text:        \"" << text << "\"\n";
   if(boost::regex_match(text, what, e, boost::match_extra))
   {
      unsigned i, j;
      std::cout << "** Match found **\n   Sub-Expressions:\n";
      for(i = 0; i < what.size(); ++i)
         std::cout << "      $" << i << " = \"" << what[i] << "\"\n";
      std::cout << "   Captures:\n";
      for(i = 0; i < what.size(); ++i)
      {
         std::cout << "      $" << i << " = {";
         for(j = 0; j < what.captures(i).size(); ++j)
         {
            if(j)
               std::cout << ", ";
            else
               std::cout << " ";
            std::cout << "\"" << what.captures(i)[j] << "\"";
         }
         std::cout << " }\n";
      }
   }
   else
   {
      std::cout << "** No Match found **\n";
   }
}
 
int main(int , char* [])
{
   print_captures("(([[:lower:]]+)|([[:upper:]]+))+", "aBBcccDDDDDeeeeeeee");
   print_captures("a(b+|((c)*))+d", "abd");
   print_captures("(.*)bar|(.*)bah", "abcbar");
   print_captures("(.*)bar|(.*)bah", "abcbah");
   print_captures("^(?:(\\w+)|(?>\\W+))*$", "now is the time for all good men to come to the aid of the party");
   print_captures("^(?>(\\w+)\\W*)*$", "now is the time for all good men to come to the aid of the party");
   print_captures("^(\\w+)\\W+(?>(\\w+)\\W+)*(\\w+)$", "now is the time for all good men to come to the aid of the party");
   print_captures("^(\\w+)\\W+(?>(\\w+)\\W+(?:(\\w+)\\W+){0,2})*(\\w+)$", "now is the time for all good men to come to the aid of the party");
   return 0;
}

#define BOOST_REGEX_MATCH_EXTRA