I am trying to parse the lines :: 10.1.1.1/32 10.10.1.1 29/nolabel 10.7.7.7/32 150.2.2.2 nolabel/30 Which are space separated and putting them in a vector of strings :: sp %= lit(' '); ip_address %= +char_("0-9") >> char_('.') >> +char_("0-9") >> char_('.') >> +char_("0-9") >> char_('.') >> +char_("0-9") >> *char_("/") >> *char_("0-9"); data %= *sp >> ( xr_special | ip_address | +lit("/") | +char_("0-9A-Za-z()")); start %= +(data ) >> *sp >> eoi; qi::rule<Iterator, void()> sp; qi::rule<Iterator, std::string()> ip_address; qi::rule<Iterator, std::string()> data; qi::rule<Iterator, test::Parsed_Rec()> start; (Parsed Rec carries a vector of strings ) Grammar is able to parse the lines but while parsing 29 in first line & 30 in last line, the attributes are incorrectly filled. The attributes are filled twice ( Debug File for this is debug_file_incorrect). Snippet :: <data> <try> 29/nolabel</try> <xr_special> <try>29/nolabel</try> <fail/> </xr_special> <ip_address> <try>29/nolabel</try> <fail/> </ip_address> <success>/nolabel</success> <attributes>[[2, 9, 2, 9]]</attributes> </data> While if I change my grammar to explicitly fill the attributes, they are filled correctly. File attached : debug_file_correct data = *sp >> ( xr_special | ip_address[_val= _1] | +lit("/") | +char_("0-9A-Za-z()")[_val+= _1] ); <data> <try> 29/nolabel</try> <xr_special> <try>29/nolabel</try> <fail/> </xr_special> <ip_address> <try>29/nolabel</try> <fail/> </ip_address> <success>/nolabel</success> <attributes>[[2, 9]]</attributes> </data> Thanks Preeti