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

You are misinterpreting that line of code. You should consider the string "$1,$2" a format specifier that is used internally by the replace function to know what to do. It uses the previously tested regular expression, which yielded 2 results (two parenthesized blocks), and reformats the results. $1 refers to the first match, $2 to the second one. The expected contents of the num string is thus 11222,333 after this bit of code.