#include <regex>
#include <string>
#include <iostream>
using namespace std;
 
std::string regexEscape(std::string str) {
	return std::regex_replace(str, std::regex(R"([.^$|()[\]{}*+?\\])"), R"(\$&)");
}
int main()
{
	std::cout << "Test escaped pattern: " << regexEscape("[da-d$\\]")  << std::endl; // = > \[da-d\$\\\]
	std::string key = "\\56";
	string input = "John\\56 Fred\\12";
	std::regex rx(R"((\w+))" + regexEscape(key));
	smatch m;
	if (std::regex_search(input, m, rx)) {
	    std::cout << "Who has \\56? - " << m[1].str() << std::endl;
	}
}

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