Parse numbers by dot with std::regex

Question

I am trying to parse numbers separated with dots and get each number as sepate string. That is my sample program:

#include <iostream>
#include <string>
#include <regex>

int main(int argc, char* argv[])
{
    try
    {
        std::string data("123.321.456.654");
        std::regex reg(R"(([0-9]+)\.(\1)\.(\1)\.(\1))");
        std::smatch m;
        if (std::regex_match(data, m, reg))
        {
            std::cout << m.size() << std::endl;
            //std::cout << m[0].str() << std::endl;
            std::cout << m[1].str() << std::endl;
            std::cout << m[2].str() << std::endl;
            std::cout << m[3].str() << std::endl;
            std::cout << m[4].str() << std::endl;
        }
    }
    catch (const std::exception& e)
    {
        std::cerr << "Error occurred: " << e.what() << std::endl;
    }

    return EXIT_SUCCESS;
}

In this sample I am using three regex:

works:

std::regex reg(R"((\d+)\.(\d+)\.(\d+)\.(\d+))");

works:

std::regex reg(R"(([0-9]+)\.([0-9]+)\.([0-9]+)\.([0-9]+))");

do not work at all:

std::regex reg(R"(([0-9]+)\.(\1)\.(\1)\.(\1))");

My question is why last regex is not working and where is error in syntax? Thanks.

Because `\1` is a backreference, it is not a subroutine call that recurses the *pattern*. It matches the value captured with the first capturing group. — Wiktor Stribiżew, May 08 '17 at 10:36
This is not a C++ question but a regex question; please get into the habit of breaking down your problems into constituent parts. — Lightness Races in Orbit, May 08 '17 at 11:14
@WiktorStribiżew Yes, so first group is `([0-9]+)` and this should be correct. — Space Rabbit, May 08 '17 at 11:14
@SpaceRabbit: No, the first backreference is "123". Not `[0-9]+`. See my answer and the resource I linked you to. — Lightness Races in Orbit, May 08 '17 at 11:15
@WiktorStribiżew: Then don't answer it; but, if you're going to (and you did), don't do it in the comments section! Furthermore, if you think that this is a duplicate, use your rep to cast a duplicate vote. — Lightness Races in Orbit, May 08 '17 at 11:24

score 1 · Accepted Answer · answered May 08 '17 at 11:13

1

The error is not in your syntax; it is in your logic.

Backreferences do not repeat a pattern; they match previously-matched text.

So, your pattern would match 123.123.123.123.

answered May 08 '17 at 11:13

Lightness Races in Orbit

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Parse numbers by dot with std::regex

1 Answers1