boost::xpressive's bug?

gistfile1.txt

#include"boost/xpresaive/xpresaive.hpp"
namespace bx = boost::xpressive ;
auto main ( ) -> int
{
    bx::sregex regex1 = bx::bos >> ( "instance" | + ( bx::range ( 'a' , 'z' ) | bx::range ( 'A' , 'Z' ) ) ) ;
    bx::smatch result ;
    if ( bx::regex_search ( std::string { "instanceof" } , result , regex1 ) )
    {
        std::cout << result.str ( ) << std::endl ; // output : "instance"
    }
    bx::sregex regex2 = bx::bos >> ( + ( bx::range ( 'a' , 'z' ) | bx::range ( 'A' , 'Z' ) ) | "instance" ) ;
    if ( bx::regex_search ( std::string { "instanceof" } , result , regex2 ) )
    {
        std::cout << result.str ( ) << std::endl ; // output : "instanceof"
    }
}