Boost.Spirit bug when mixing "alternates" with "optionals"?
boost, boost-spirit, boost-spirit-qi, c++
Solution
That is because you hit a bug in Spirit's directive `repeat[]`. Thanks for the report, I fixed this problem in SVN (rev. [66167]) and it will be available in Boost V1.45. At the same time I would like to add your small test as a regression test to Spirit's test suite. I hope you don't mind me doing so.
Problem
I've only been working with Boost.Spirit (from Boost 1.44) for three days, trying to parse raw e-mail messages by way of the exact grammar in RFC2822. I thought I was starting to understand it and get somewhere, but then I ran into a problem: ``` #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; 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 obsolete_year = omit[-qi::char_(" \t")] >> repeat(2)[qi::digit] >> omit[-qi::char_(" \t")]; strrule_t correct_year = repeat(4)[qi::digit]; test("1776", correct_year | repeat(2)[qi::digit]); // 1: Works, reports 1776. test("76", obsolete_year); // 2: Works, reports 76. test("76", obsolete_year | correct_year); // 3: Works, reports 76. test(" 76", correct_year | obsolete_year); // 4: Works, reports 76. test("76", correct_year | obsolete_year); // 5: Fails. test("76", correct_year | repeat(2)[qi::digit]); // 6: Also fails. } ``` If test #3 works, then why does test #5 -- the exact same test with the two alternatives reversed -- fail? By the same token, if you'll pardon the expression: if test #4 works, and the space at the beginning is marked as optional, then why does test #5 (the exact same test with the exact same input, save that there's no leading space in the input) fail? And finally, if this is a bug in Boost.Spirit (as I suspect it must be), how can I work around it?