在Clang中regex_constants的实现错误？

Question

按照标准规定：

match_prev_avail：--first是有效的迭代器位置。设置后，将忽略match_not_bol和match_not_bow

但是我运行以下代码并得到：

#include <regex>
#include <iostream>
using namespace std;

int main()
{
    regex re0("^bcd");
    string str = "abcd";
    std::string::iterator start = str.begin() + 1;
    cout << regex_search(start, str.end(), re0, regex_constants::match_not_bol) << endl;
    cout << regex_search(start, str.end(), re0, regex_constants::match_prev_avail) << endl;
    cout << regex_search(start, str.end(), re0, regex_constants::match_prev_avail | regex_constants::match_not_bol) << endl;
}

输出：

0
1
0

似乎已match_prev_avail被覆盖match_not_bol。

Answer 1

Seems you found a bug in clang. (file it here: https://bugs.llvm.org/ as it seems not yet have been reported)

I checked MSVC 1914 and it gives

0
0
0

same as GCC 4.9.2 (used cpp.sh to check)

I rechecked the .pdf form of the standard (N4810) and this in 30.5.2 matches what the cppreference states.

match_prev_avail: --first is a valid iterator position. When this flag is set the flags match_not_bol and match_not_bow shall be ignored by the regular expression algorithms (30.11) and iterators (30.12)