c++ - 如何避开贪婪的rd？

我想解析一个可以包含“-”的字符串，但不能以它开头或结尾。

我希望这个解析器能够工作:

auto const parser = alnum >> -(*(alnum | char_('-')) >> alnum);

但是在我的测试输入中，“something”仅解析“so”，不占用其余部分。

问题在于中间的*(alnum | char_('-'))一直吃到最后(包括最后一个字符，因此整个可选括号都失败了)。

here和here的解释方式和原因

我想知道的是，我如何解决它并使它成为解析器？

现场观看:http://coliru.stacked-crooked.com/a/833cc2aac7ba5e27

最佳答案

我个人会“积极地”写它:

auto const rule = raw [ lexeme [
    alnum >> *('-' >> alnum | alnum) >> !(alnum|'-')
] ];

这使用

lexeme处理空格的重要性

raw避免必须主动匹配每个字符作为输出的一部分(您只想要所有字符)。

'-' >> alnum积极要求任何破折号后面应加一个数字。请注意，这也禁止输入中的"--"。请参阅

下的 VARIANT

Live On Coliru

#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <string>
#include <algorithm>

namespace x3 = boost::spirit::x3;

namespace parser {
    using namespace boost::spirit::x3;

    auto const rule = raw [ lexeme [
        alnum >> *('-' >> alnum | alnum) >> !(alnum|'-')
    ] ];
}

int main() {
    struct test { std::string input; bool expected; };

    for (auto const t : {
            test { "some-where", true },
            test { " some-where", true },
            test { "some-where ", true },
            test { "s", true },
            test { " s", true },
            test { "s ", true },
            test { "-", false },
            test { " -", false },
            test { "- ", false },

            test { "some-", false },
            test { " some-", false },
            test { "some- ", false },

            test { "some--where", false },
            test { " some--where", false },
            test { "some--where ", false },
        })
    {
        std::string output;
        bool ok = x3::phrase_parse(t.input.begin(), t.input.end(), parser::rule, x3::space, output);
        if (ok != t.expected)
            std::cout << "FAILURE: '" << t.input << "'\t" << std::boolalpha << ok << "\t'" << output << "'\n";
    }
}

变体

为了也允许some--thing和类似的输入，我将'-'更改为+lit('-'):

alnum >> *(+lit('-') >> alnum | alnum) >> !(alnum|'-')

Live On Coliru

#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <string>
#include <algorithm>

namespace x3 = boost::spirit::x3;

namespace parser {
    using namespace boost::spirit::x3;

    auto const rule = raw [ lexeme [
        alnum >> *(+lit('-') >> alnum | alnum) >> !(alnum|'-')
    ] ];
}

int main() {
    struct test { std::string input; bool expected; };

    for (auto const t : {
            test { "some-where", true },
            test { " some-where", true },
            test { "some-where ", true },
            test { "s", true },
            test { " s", true },
            test { "s ", true },
            test { "-", false },
            test { " -", false },
            test { "- ", false },

            test { "some-", false },
            test { " some-", false },
            test { "some- ", false },

            test { "some--where", true },
            test { " some--where", true },
            test { "some--where ", true },
        })
    {
        std::string output;
        bool ok = x3::phrase_parse(t.input.begin(), t.input.end(), parser::rule, x3::space, output);
        if (ok != t.expected)
            std::cout << "FAILURE: '" << t.input << "'\t" << std::boolalpha << ok << "\t'" << output << "'\n";
    }
}