RegExp.exec() returns NULL sporadically

I am seriously going crazy over this and I've already spent an unproportionate amount of time on trying to figure out what's going on here. So please give me a hand =)

I need to do some RegExp matching of strings in JavaScript. Unfortunately it behaves very strangely. This code:

var rx = /(cat|dog)/gi;
var w = new Array("I have a cat and a dog too.", "There once was a dog and a cat.", "I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.","I have a cat and a dog too.", "There once was a dog and a cat.");

for (var i in w) {
    var m = null;
    m = rx.exec(w[i]);
    if(m){
        document.writeln("<pre>" + i + "\nINPUT: " + w[i] + "\nMATCHES: " + m.slice(1) + "</pre>");
    }else{
        document.writeln("<pre>" + i + "\n'" + w[i] + "' FAILED.</pre>");
    }
}

Returns "cat" and "dog" for the first two elements, as it should be, but then some exec()-calls start returning null. I don't understand why.

I posted a Fiddle here, where you can run and edit the code.

And so far I've tried this in Chrome and Firefox.

Solution 1:

Oh, here it is. Because you're defining your regex global, it matches first cat, and on the second pass of the loop dog. So, basically you just need to reset your regex (it's internal pointer) as well. Cf. this:

var w = new Array("I have a cat and a dog too.", "I have a cat and a dog too.", "I have a cat and a dog too.", "I have a cat and a dog too.");

for (var i in w) {
    var rx = /(cat|dog)/gi;
    var m = null;
    m = rx.exec(w[i]);
    if(m){
        document.writeln("<p>" + i + "<br/>INPUT: " + w[i] + "<br/>MATCHES: " + w[i].length + "</p>");
    }else{
        document.writeln("<p><b>" + i + "<br/>'" + w[i] + "' FAILED.</b><br/>" + w[i].length + "</p>");
    }
    document.writeln(m);
}

Solution 2:

The regex object has a property lastIndex which is updated when you run exec. So when you exec the regex on e.g. "I have a cat and a dog too.", lastIndex is set to 12. The next time you run exec on the same regex object, it starts looking from index 12. So you have to reset the lastIndex property between each run.