the difference the regexp result - regex

Question

Why ghi's result is different with abc or def?
abc's result is abc: a-b-c-d-e-f
def's result is def: a-b-c-d-e-f
ghi's result is ghi: a-{1}-c-{3}-e-{5}
What is the reason?
function abc(){
var lang = "{0}-{1}-{2}-{3}-{4}-{5}";
var args = ["a","b","c","d","e","f"];
var exp = /\{(\d+)\}/;
var matches = exp.exec(lang);
while (matches) {
var index = parseInt(matches[1], 10);
lang = lang.replace(matches[0], args[index]);
matches = exp.exec(lang);
}
console.log('abc: ' + lang);
}
abc();
function def(){
var lang = "{0}-{1}-{2}-{3}-{4}-{5}";
var args = ["a","b","c","d","e","f"];
var exp = /\{(\d+)\}/g;
var matches = exp.exec(lang);
while (matches) {
var exp = /\{(\d+)\}/g;
var index = parseInt(matches[1], 10);
lang = lang.replace(matches[0], args[index]);
matches = exp.exec(lang);
}
console.log('def: ' + lang);
}
def();
function ghi(){
var lang = "{0}-{1}-{2}-{3}-{4}-{5}";
var args = ["a","b","c","d","e","f"];
var exp = /\{(\d+)\}/g;
var matches = exp.exec(lang);
while (matches) {
var index = parseInt(matches[1], 10);
lang = lang.replace(matches[0], args[index]);
matches = exp.exec(lang);
}
console.log('ghi: ' + lang);
}
ghi();

Answer 1

Function ghi() needs to reset RegExp object's lastIndex inside the loop:
function ghi(){
var lang = "{0}-{1}-{2}-{3}-{4}-{5}";
var args = ["a","b","c","d","e","f"];
var exp = /\{(\d+)\}/g;
while (matches = exp.exec(lang)) {
var index = parseInt(matches[1], 10);
lang = lang.replace(matches[0], args[index]);
exp.lastIndex = 0;
}
console.log('ghi: ' + lang);
}
Because of exp.lastIndex = 0; call now output will be same.
JavaScript's RegExp object is stateful. When global flag is used, and you call a method the same RegExp object, it will start on the next index from the end of the last match.