Are Regular Expressions Stateful?

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I seem to have stumbled across a bug regarding regular expressions using the g modifier, where they seem to preserve internal state across calls to RegExp.prototype.test. I’m looking for confirmation or a “you’re stupid, this is what is going on” dismissal.

Apparently regular expressions with the g modifier are stateful. See for yourself.

<button id='bad'>Wait, WHAT?</button>
stateful = /button/ig;
bad.onclick = function (e) {

The button alternates between alerting true and false in Google Chrome 38.0.2125.122, and also on Firefox 31.

As it turns out, this is the “expected” behavior. I did not expect that. Regular expressions with the g modifier on them will keep track of the lastIndex where a match occurred, even across calls to .test.

You can check out the live example on CodePen, too.

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