the logical not operator not working in boost::spirit::qi

Question

if in a qi::grammar I use this base rule

expression = (boost::spirit::ascii::string("aaa"));

it will parse "aaa" and nothing else

when I use this one ( notice the ! ) it parses nothing at all while I expect it to be successful on everything but "aaa"

expression = !(boost::spirit::ascii::string("aaa"));

Could I be missing some include? I'm using boost 1.54.0.

EDIT:

Sorry this is a bit drafty I modified the calculator example for my first trials...

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>

#include <iostream>
#include <string>
#include <boost/spirit/include/qi_lit.hpp>
#include <boost/spirit/include/qi_not_predicate.hpp>
/*
 * \
    __grammar_calculator.cpp

HEADERS += \
    __grammar_calculator.h
 */
namespace client
{
    namespace qi = boost::spirit::qi;
    namespace ascii = boost::spirit::ascii;

    ///////////////////////////////////////////////////////////////////////////
    //  Our calculator grammar
    ///////////////////////////////////////////////////////////////////////////
    template <typename Iterator>
    struct calculator : qi::grammar<Iterator, int(), ascii::space_type>
    {
        calculator() : calculator::base_type(expression)
        {
            using qi::_val;
            using qi::_1;
            using qi::uint_;
            using boost::spirit::qi::lit;
            using boost::spirit::ascii::string;

            expression = !(boost::spirit::ascii::string("aaa"));
        }

        qi::rule<Iterator, int(), ascii::space_type> expression, term, factor;
    };
}

///////////////////////////////////////////////////////////////////////////////
//  Main program
///////////////////////////////////////////////////////////////////////////////
int
main()
{
    std::cout << "/////////////////////////////////////////////////////////\n\n";
    std::cout << "Expression parser...\n\n";
    std::cout << "/////////////////////////////////////////////////////////\n\n";
    std::cout << "Type an expression...or [q or Q] to quit\n\n";

    using boost::spirit::ascii::space;
    typedef std::string::const_iterator iterator_type;
    typedef client::calculator<iterator_type> calculator;

    calculator calc; // Our grammar

    std::string str;
    int result;
    while (std::getline(std::cin, str))
    {
        if (str.empty() || str[0] == 'q' || str[0] == 'Q')
            break;

        std::string::const_iterator iter = str.begin();
        std::string::const_iterator end = str.end();
        bool r = phrase_parse(iter, end, calc, space, result);

        if (r && iter == end)
        {
            std::cout << "-------------------------\n";
            std::cout << "Parsing succeeded\n";
            //std::cout << "result = " << result << std::endl;
            std::cout << "-------------------------\n";
        }
        else
        {
            std::string rest(iter, end);
            std::cout << "-------------------------\n";
            std::cout << "Parsing failed\n";
            //std::cout << "stopped at: \": " << rest << "\"\n";
            std::cout << "-------------------------\n";
        }
    }

    std::cout << "Bye... :-) \n\n";
    return 0;
}

EDIT 2:

Same one a bit cleaner:

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>

#include <iostream>
#include <string>
#include <boost/spirit/include/qi_not_predicate.hpp>

namespace client
{
    namespace qi = boost::spirit::qi;
    namespace ascii = boost::spirit::ascii;

    template <typename Iterator>
    struct test : qi::grammar<Iterator>
    {
        test() : test::base_type(expression)
        {
            using boost::spirit::ascii::string;

            expression = (boost::spirit::ascii::string("aaa"));
        }

        qi::rule<Iterator> expression;
    };
}

int main()
{
    std::cout << "/////////////////////////////////////////////////////////\n\n";
    std::cout << "Expression parser...\n\n";
    std::cout << "/////////////////////////////////////////////////////////\n\n";
    std::cout << "Type an expression...or [q or Q] to quit\n\n";

    using boost::spirit::ascii::space;
    typedef std::string::const_iterator iterator_type;
    typedef client::test<iterator_type> test;

    test tester; // Our grammar

    std::string str;
    while (std::getline(std::cin, str))
    {
        if (str.empty() || str[0] == 'q' || str[0] == 'Q')
            break;

        std::string::const_iterator iter = str.begin();
        std::string::const_iterator end = str.end();
        bool r = phrase_parse(iter, end, tester, space);

        if (r && iter == end)
        {
            std::cout << "-------------------------\n";
            std::cout << "Parsing succeeded\n";
            std::cout << "-------------------------\n";
        }
        else
        {
            std::string rest(iter, end);
            std::cout << "-------------------------\n";
            std::cout << "Parsing failed\n";
            std::cout << "-------------------------\n";
        }
    }

    std::cout << "Bye... :-) \n\n";
    return 0;
}

ANSWER:

the problem came from the test:

if (r && iter == end)

as pointed out operator consumed nothing so iter!=end

in the below, sehe provided some alternatives.

Answer 1

For what it's worth:

• As others have pointed out operator& and operator! are zero-width lookahead assertions (they don't match but 'peek' and either succeed or fail on match).

You'll want to know about

• negating character-sets:

  qi::char_("a-z")    // matches lowercase letters
  

  ~qi::char_("a-z") // matches anything but lowercase letters

• 'parser subtraction' - think of exceptions:

  qi::char_ - qi::char_("a-z")  // equivalent to ~qi::char_("a-z") 
  qi::char_("a-z") - "keyword"  // any lowercase letters, but not if it spells "keyword" 
  

---

Edit so to scan forward to the next "%{" you'd do something like

qi::omit [ qi::char_ - "{%" ] >> "{%"

Note that you don't always need to 'wrap' literals in qi::lit unless the expression doesn't already involve proto-expressions in the Qi domain.

---

Edit 2 An example of how to use ! and &:

&qi::int_ >> +qi::char_   // parse a string, **iff** it starts with an integer

or

!keyword_list >> identifier // parse any identifier that's not a known keyword