How to explain “$1,$2” in Javascript when using regular expression?

Question

A piece of Javascript code is as follows:

    num=\"11222333\";
    re = /(\\d+)(\\d{3})/;
    re.test(num);
    num.replace(re, \"$1,$2\");

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Answer 1

It's not a "variable" - it's a placeholder that is used in the .replace() call. $n represents the nth capture group of the regular expression.

var num = "11222333";

// This regex captures the last 3 digits as capture group #2
// and all preceding digits as capture group #1
var re = /(\d+)(\d{3})/;

console.log(re.test(num));

// This replace call replaces the match of the regex (which happens
// to match everything) with the first capture group ($1) followed by
// a comma, followed by the second capture group ($2)
console.log(num.replace(re, "$1,$2"));