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.

Attempting to parse JSON-formatted data in C++ using regular expressions will only end in tears. To do this correctly you should use a real JSON parser. — Sam Varshavchik
– Sam Varshavchik, Commented Feb 28, 2021 at 19:45
Just a side note: Representing regex patterns using raw string literals is way easier,to write and better to read. But @Sam is correct, it's pretty hoeless to get that right. Use a JSON parser library like nlohmann's. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Feb 28, 2021 at 19:48
Much like trying to use regex to parse HTML: blog.codinghorror.com/content/images/2014/Apr/… — Eljay
– Eljay, Commented Feb 28, 2021 at 19:49

Wiktor Stribiżew · Accepted Answer · 2021-02-28 19:50:50Z

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. E.g. 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;
}

answered Feb 28, 2021 at 19:50

Wiktor Stribiżew

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