C++ 正则表达式在循环中

Question

The following attempts to get the "a" from "A:a" and "b" from "B:b" and "c" from "C:c" by performing a regex search in a loop. In the first iteration of the outer loop, "a" appears in the second group of m. However for the second iteration of the outer loop, "b" appears in the third group of m. And for the third iteration of the outer loop, "c" appears in the fourth group of m. Why is that? Also, the first iteration of the outer loop has 4 groups in m. Why is that? Shouldn't there be two groups?

std::string s = "abc\n"
    "A:a\n"
    "def\n"
    "B:b\n"
    "ghi\n"
    "C:c\n\n"
    "jkl\n";
std::regex p("\nA:\\s*([^\n]+)|\nB:\\s*([^\n]+)|\nC:\\s*([^\n]+)");
std::smatch m;

while (std::regex_search(s, m, p))
{
    int count = 0;
    for (auto x:m)
    {
        std::cout << count << "->" << x << ", ";
        count++;
    }
    s = m.suffix().str();
}

The output is

0->
A:a, 1->a, 2->, 3->, 0->
B:b, 1->, 2->b, 3->, 0->
C:c, 1->, 2->, 3->c, 

Expected:

0->
A:a, 1->a, 0->
B:b, 1->b, 0->
C:c, 1->c, 

Answer 1

Group numbers don't reset in each alternative.

Since all your alternatives are the same except for the letter at the beginning, you can simply combine them into a single alternative and match the letter with [ABC] .

std::regex p("\n[ABC]:\\s*([^\n]+)");