incremental regex replace in nodejs

sort.js

var fs = require('fs');

var file = 'EoCApp.CT';
if (process.argv.length > 2)
  file = process.argv[2];

var f = fs.readFileSync(file, 'utf8');
var i = 0;
f = f.replace(/<ID>\d+<\/ID>/gm, function () {return '<ID>' + i++ + '</ID>'});

l = file.lastIndexOf('.');
var file2 = file.substring(0, l) + '2.' + file.substring(l + 1);
fs.writeFileSync(file2, f);
console.log(file2 + " written.");