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\");

<

Answer 1

It is known that in Javascript, the name of variables should begin with letter or _,

No, it's not. $1 is a perfectly valid variable. You have to assign to it first though:

$variable = "this is a test"

This is how jQuery users a variable called $ as an alias for the jQuery object.