Boost Spirit X3解析字符转义时无法消费最后令牌问题求助

问题原因 #

你代码的核心问题是规则定义存在冗余和歧义，导致解析器回溯异常，最终无法匹配末尾的闭合双引号，所以返回false，但已匹配到的内容已经正确赋值到了result属性中。
具体问题如下：

• 存在冗余规则：你重复定义了\\n的转义规则，还额外写了一条完全多余的转义双引号规则"\\\"" >> x3::char_('"')，这条规则和前面的"\\" >> x3::char_("\"\\")功能完全重叠，反而会导致解析器出现不必要的回溯。
• 可选优化：如果需要确保解析器消费完全部输入，需要在规则末尾加上x3::eoi，否则只要前半部分匹配成功就会返回true，忽略末尾未消费的字符。

修改后完整代码 #

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

namespace x3 = boost::spirit::x3;
using namespace std::string_literals;

auto const qstring = x3::lexeme['"' >> *(
             "\\n" >> x3::attr('\n')
           | "\\b" >> x3::attr('\b')
           | "\\f" >> x3::attr('\f')
           | "\\t" >> x3::attr('\t')
           | "\\v" >> x3::attr('\v')
           | "\\0" >> x3::attr('\0')
           | "\\r" >> x3::attr('\r')
           | "\\"  >> x3::char_("\"\\")
           | ~x3::char_('"')
       ) >> '"'];

int main(int, char**){
    auto const quoted = "\"Hel\\\"lo Wor\\\"ld\""s;
    auto const expected = "Hel\"lo Wor\"ld"s;

    std::string result;
    auto first = quoted.begin();
    auto const last = quoted.end();
    // 加x3::eoi确保消费完全部输入
    bool ok = x3::phrase_parse(first, last, qstring >> x3::eoi, x3::ascii::space, result);
    std::cout << "parse returned " << std::boolalpha << ok << '\n';

    std::cout << result << " == " << expected << " is " << std::boolalpha << (result == expected) << '\n';

    std::cout << "first == last = " << (first == last) << '\n';
    if (first != last) {
        std::cout << "first = " << *first << '\n';
    }

    return 0;
}

运行结果 #

parse returned true
Hel"lo Wor"ld == Hel"lo Wor"ld is true
first == last = true

内容的提问来源于stack exchange，提问作者Jellybaby