Regex_search c++

Question

#include <iostream>
#include <regex>

int main() {

    std::string s = "{\"|1|\":\"A\",\"|2|\":\"B\",\"|37|\":\"4234235\",\"|4|\":\"C\"}";

    std::regex regex("37\\|\\\\\":\\\\\"\\K\\d*");

    std::smatch m;

    regex_search(s, m, regex);
    std::cout << "match: " << m.str(1) << std::endl;

    return 0;
}

Why does it not match the value 4234235 ?

Testing the regex here: https://regex101.com/r/A2cg2P/1 It does match.

Answer 1

Your online regex test is wrong because your actual text is {"|1|":"A","|2|":"B","|37|":"4234235","|4|":"C"} , you may see that your regex does not match it .

Besides, you are using an ECMAScript regex flavor in std::regex , but your regex is PCRE compliant. Eg ECMAScript regex does not support \K match reset operator.

You need a "\|37\|":"(\d+) regex, see the regex demo . Details :

• "\|37\|":" - literal "|37|":" text
• (\d+) - Group 1: one or more digits.

See the C++ demo :

#include <iostream>
#include <regex>

int main() {
    std::string s = "{\"|1|\":\"A\",\"|2|\":\"B\",\"|37|\":\"4234235\",\"|4|\":\"C\"}";
    std::cout << s <<"\n";
    std::regex regex(R"(\|37\|":"(\d+))");
    std::smatch m;
    regex_search(s, m, regex);
    std::cout << "match: " << m.str(1) << std::endl;
    return 0;
}