c++regexregex-replace

Replace Character Only if it isn't Immediately Preceded By Specified Character

Given a multiple line string A;1;\nB;2\nC;3. It's needed to add a ; before those newline characters that are not immediately preceded by ;. So, the result should be A;1;\nB;2;\nC;3 (a single ; is added after 2). I've tried different regexes but without luck (one of them is presented below). I think the lookbehind assertion would do the job, but it isn't supported.

Question: What regex expression can solve this problem? If none, how to solve it using other methods using pure C++ only (I'm using C++20)?

std::string s{ "A;1;\nB;2\nC;3" };
const std::regex re("(?!;)\\n");  // Produces "A;1;;\nB;2;\nC;3" (redundant ";" after "1").
s = std::regex_replace(s, re, ";\n");
Solution

  • You can match and capture either a string start position or any character other than a ; char, and then capture a newline char, and replace the matches using backreferences:

    #include <iostream>
#include <regex>

int main() {
    std::string s{ "A;1;\nB;2\nC;3" };
    const std::regex re("([^;]|^)(\\n)");  // Produces "A;1;;\nB;2;\nC;3" (redundant ";" after "1").
    s = std::regex_replace(s, re, "$1;$2");
    std::cout << s;
    return 0;
}

    See the C++ demo and the regex demo.

    Output:

    A;1;
B;2;
C;3

    Pattern details: