Boost Spirit failing on empty string input - c++11

Question

I am trying to parse the following string and extract the parts inside the parenthesis.
This string fails:
_FIND('Something', '')_
Should return
part1 = 'Something'
part2 = ''
This string passes:
_FIND('Something', '*')_
Returns
part1 = 'Something'
part2 = '*'
I assume the problem lies with the "quoted_string"
find_parser() : find_parser::base_type(start)
{
using qi::lit;
using qi::lexeme;
using standard_wide::char_;
/// simple quoted string.
quoted_string %= lexeme['\'' >> +(char_ - '\'') >> '\''];
start %=
-(lit("$(")) // optional
>> lit("_FIND")
>> '('
>> quoted_string
>> -(',' >> quoted_string) // 2nd parameter optional
>> ")_"
>> -(lit(")")) // optional
;
}
I tried added an "empty" string lexeme like this, but it does not work.
quoted_string %= lexeme['\'' >> +(char_ - '\'') >> '\''];
empty_quoted_string %= lexeme['\'' >> +(qi::space - '\'') >> '\''];
start %=
lit("_FIND")
>> '('
>> (quoted_string|empty_quoted_string)
>> -(',' >> (quoted_string|empty_quoted_string)) // 2nd parameter optional
>> ")_"
;
I know it must be a simple thing, but I cannot put my finger on it.
Thanks for any inputs, hints or tips.

Answer 1

lexeme['\'' >> +(char_ - '\'') >> '\''];
+p means that p must match one-or-more times. If an empty string must be accepted, use the Kleene-star operator, which allows zero-or-more matches.
lexeme['\'' >> *(char_ - '\'') >> '\''];
Live Demo
Some inefficiencies/style issues resolves
Also, an incorrectness, where "$(_FIND('')" or "_FIND('')" would parse as "correct"
Live On Coliru
#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/adapted/std_pair.hpp>
using Params = std::pair<std::string, std::string>;
namespace qi = boost::spirit::qi;
template <typename It>
struct find_parser : qi::grammar<It, Params()> {
find_parser() : find_parser::base_type(start)
{
using namespace qi;
start = skip(space) [ "$(" >> find >> ")" | find ];
find
= '_' >> lit("FIND") >> lit('(')
>> quoted_string >> -(',' >> quoted_string) // 2nd parameter optional
>> ')' >> '_'
;
quoted_string = "'" >> *~char_("'") >> "'";
BOOST_SPIRIT_DEBUG_NODES((start)(find)(quoted_string))
}
private:
qi::rule<It, Params()> start;
// rules with skipper
qi::rule<It, Params(), qi::space_type> find;
// implicit lexemes
qi::rule<It, std::string()> quoted_string;
};
int main() {
using It = std::string::const_iterator;
find_parser<It> const p;
for (std::string const input : {
"_FIND('Something', 'Something else')_",
"_ FIND('Something', 'Something else') _",
"$(_FIND('Something', 'Something else')_)",
"$( _FIND( 'Something', 'Something else' )_ )",
// second arg empty
"_FIND('Something', '')_",
"_ FIND('Something', '') _",
"$(_FIND('Something', '')_)",
"$( _FIND( 'Something', '' )_ )",
// optional args omitted
"_FIND('Something')_",
"_ FIND('Something') _",
"$(_FIND('Something')_)",
"$( _FIND( 'Something' )_ )",
})
{
std::cout << "-------- " << input << " ------------\n";
It f = input.begin(), l = input.end();
Params parsed;
if (parse(f, l, p, parsed))
std::cout << "Parsed: '" << parsed.first << "', '" << parsed.second << "'\n";
else
std::cout << "Parsing failed\n";
if (f!=l)
std::cout << " -- Remaining unparsed: '" << std::string(f,l) << "'\n";
}
}