void main()

{

    import std.regex;

    import std.stdio: write, writeln, writef, writefln;

    // insert comma as thousands delimiter

    auto re = regex(r"(?<=\d)(?=(\d\d\d)+\b)", "g");

    writeln(replaceAll("12000 + 42100 = 54100", re, ",")); // "12,000 + 42,100 = 54,100"

}