var myString = 'EXAMPLEstring';

var myNewString = myString.replace(/[A-Z]/g, '0');

console.log(myNewString);

​

​

function replaceFunc (a, b, c) {

  console.log(a, b, c);

  return a.toLowerCase();

}

​

var myOtherString = myString.replace(/[A-Z]/g, replaceFunc);

​

console.log(myOtherString);