Question

I can’t understand why I can’t use a variable for the regex string. See link: http://jsfiddle.net/nmWuw/1/

It works without the variable but not when a variable is used for the regex. I escaped my backslashes as well. Output should be ‘1,234,567,890’.

Answer 1

var regex = "/\\d(?=(?:\\d{3})+(?!\\d))/g, '$&,'";

That is a string. It is not a regexp object and a replacement string. When it is passed to replace, it is one parameter, not two. You need to pass both parameters separately:

var regex = /\d(?=(?:\d{3})+(?!\d))/g;
var replace = '$&,';
var num = 1234567890;

alert(String(num).replace(/\d(?=(?:\d{3})+(?!\d))/g, '$&,'));
alert(String(num).replace(regex, replace));
alert(regex);

http://jsfiddle.net/nmWuw/3/

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Note that you could define them in one call, if you really wanted to, using an array and apply:

var regex = [/\d(?=(?:\d{3})+(?!\d))/g, '$&,'];
alert(String.prototype.replace.apply(num, regex));

http://jsfiddle.net/nmWuw/6/

This is hardly a good idea, however — it’s much harder to read and much less intuitive.