How can I replace a regex substring match in Javascript?

Question

var str   = \'asd-0.testing\';
var regex = /asd-(\\d)\\.\\w+/;

str.replace(regex, 1);

That replaces the entire string str with

Answer 1

var str   = 'asd-0.testing';
var regex = /(asd-)\d(\.\w+)/;
str = str.replace(regex, "$11$2");
console.log(str);

Or if you're sure there won't be any other digits in the string:

var str   = 'asd-0.testing';
var regex = /\d/;
str = str.replace(regex, "1");
console.log(str);