test.cpp

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

int main(){
    string s1 = "Gcc aasdlkf blah Gcc",
           s2 = "Gcc asldkfjae adf Gcc";
    
    regex rx ("Gcc");
    string replace = "gcc";

    regex_replace(s1,rx,replace);
    regex_replace(s2,rx,replace);

    cout << s1 << "\n";
    cout << s2;

    return 0;
}