JSFiddle - React, Tailwind, and code Playground

JavaScript

function splitNumber(inputNumber){
 
    var firstPart = /^(\d{3})/;
    var secondPart = /^\d{3}(\d{2})/;
    var thirdPart = /^\d{5}(\d{3})/;
    
    var matches = [];
    
   	matches[0] = inputNumber.match(firstPart)[1];
    
       	matches[1] = inputNumber.match(secondPart)[1];
    
           	matches[2] = inputNumber.match(thirdPart)[1];

    return matches.join(".");
    
}

console.log(splitNumber("12345678"));