Join the string with same separator used to split

Question

I have a string that need to be split with a regular expression for applying some modifications.

eg:

const str = "Hello+Beautiful#World";
const splited = str.split(/[\+#]/)

// ["Hello", "Beautiful", "World"]

Now the string has been split with + or #. Now say after applying some modification to the items in the array, I have to join the array using the same separator that used to split, so the character + and # has to be in the same position as before.

eg: if i applied some modification string and joined. Then it should be.

Hello001+Beutiful002#World003

How can i do this?

Answer 1

When you place a pattern inside a capturing group, split will return the matched delimiters as even array items. So, all you need to do is modify the odd items:

var counter=1;
var str = "Hello+Beautiful#World";
console.log(
  str.split(/([+#])/).map(function(el, index){
    return el + (index % 2 === 0 ? (counter++ + "").padStart(3, '0') : '');
  }).join("")
);