[рони]

fori,

let num = 3;
let regexp = new RegExp(`(\\d)(?=(\\d{${num}})+$)`, `g`);
alert('123321'.replace(regexp, '$1 '));
let k = 4;
let reg = new RegExp(`\\d{${k}}(?=.)`, `g`);
alert('123321'.replace(reg, '$& '));