#include <iostream>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;
using qi::omit;
using qi::repeat;
using std::cout;
using std::endl;
using qi::string;
using qi::char_;

typedef qi::rule<std::string::const_iterator, std::string()> strrule_t;

void test(const std::string input, strrule_t rule) 
{
    std::string target;
    std::string::const_iterator i = input.begin(), ie = input.end();

    if (qi::parse(i, ie, rule, target)) 
    {
        cout << "Success: '" << target << "'" << endl;
    }
    else 
    {
        cout << "Failed to match." << endl;
    }
}

int main() 
{
    strrule_t shortHexValue = repeat(4)[char_("a-fA-F0-9")];
    strrule_t longHexValue = repeat(8)[char_("a-fA-F0-9")];
    strrule_t hexValue1 = omit[string("0x")] >> ( longHexValue | shortHexValue );
    strrule_t hexValue2 = omit[string("0x")] >> ( shortHexValue | longHexValue );

    test( "0x123FAA22", hexValue1 );
    test( "0x12AA", hexValue1 );
    test( "0x123FAA22", hexValue2 );
    test( "0x1A22", hexValue2 );
    
    return 0;
}