#include <iostream>
#include <boost/regex.hpp>

int main()
{
    using std::cerr;
    using boost::regex;
    using boost::regex_error;
    
    try 
    {
	regex m1("(?xism:\\d)", regex::perl); // ok
	regex m2("(?-x:\\d)", regex::perl);   // ok
	regex m3("(?-xism\\d)", regex::perl); // bad regexp
    }
    catch (boost::regex_error err)
    {
	cerr << "Bad regular expression: " << err.what() << "\n";
    }
}