How to replace only part of a string found by reg. expression?

Question

JavaScript

https://regex101.com/r/BZGwi8/1
is to replace ib.
But how to replace only part of the result found by the regular expression?

Answer 1

var str = "https://days.pravoslavie.ru/jpg/is847.jpg";
var regex = /(is)+([0-9]{1,})+([.])+([a-zA-Z]{2,})/;
console.log(str.replace(regex,str.match(regex)[0].replace(/is/,"ib")));
//https://days.pravoslavie.ru/jpg/ib847.jpg

Answer 2

https://regex101.com/r/BZGwi8/2