How does "g" in RegExp test method works alternatively?

Question

http://jsfiddle.net/bpt33/

var t = "";

var a = ["atom-required","atom-label","atom-data-type","atom-regex"];

var r = /atom\-(label|required|regex|data\-type|class|is\-valid|field\-value|error)/i;

function test(a, r){
    for(var i = 0; i<a.length; i++){
        t += a[i] + " => " + r.test(a[i]) + "<br/>";
    }
}

test(a, r);

t += "<br/>";

a = ["atom-required","atom-label","atom-data-type","atom-regex"];

var r = /atom\-(label|required|regex|data\-type|class|is\-valid|field\-value|error)/gi;

test(a, r);

$("#results").get(0).innerHTML = t;

When g is not specified, it works correctly,

atom-required => true
atom-label => true
atom-data-type => true
atom-regex => true

When g is specified, it works alternatively

atom-required => true
atom-label => false
atom-data-type => true
atom-regex => false

Answer 1

Because with the g modifier, the regexp becomes stateful, and resumes the next search at the index after the last match.

When no match is found, it resets itself.

You can observe the starting point by using the .lastIndex property.

r.lastIndex; // 0 or some higher index

You can reset it manually by setting that property to 0.

r.lastIndex = 0