isRegex changes lastIndex since node v6

[Xotic750]

Since node v6 (I believe), testing if an object is a regexp causes lastIndex to be reset to 0. I haven't had a chance to track it down, but my guess is executing call on it?

regexExec.call(value);

test('lastIndex', function (t) {
	var re1 = /a/;
	re1.lastIndex = 3;
	t.ok(re1.lastIndex === 3, 'lastIndex is 3 before isRegex');
	t.ok(isRegex(re1), 'is regex');
	t.ok(re1.lastIndex === 3, 'lastIndex is 3 after isRegex');
	t.end();
});

Running the following in node v6 and above confirms the problem for me.

var regexExec = RegExp.prototype.exec;
var re1 = /a/;
re1.lastIndex = 3;
console.log(re1.lastIndex);
regexExec.call(re1);
console.log(re1.lastIndex);

I tried with RegExp.prototype.test and from v6 onward I get the same result.

The following change appears to fix the problem with the tests that I have performed.

var tryRegexExecCall = function tryRegexExec(value) {
	try {
		var lastIndex = value.lastIndex;
		regexExec.call(value);
		value.lastIndex = lastIndex;
		return true;
	} catch (e) {
		return false;
	}
};

Although I've opened this issue here, I'm not sure that this wouldn't be more appropriate logged against an es-shim?

[ljharb]

It's certainly appropriate here; checking the type of it shouldn't cause it to be mutated.

Whether it's an es shim bug depends entirely on whether v8 made a change, and whether it was part of the spec to do so.

However, regardless, one should never rely on lastIndex of a regex you didn't create - my expectation after verifying that you had a regex would be that you'd do var copy = new RegExp(re.source, re.flags) so as to ensure not mutating re.

[ljharb]

It appears that this is not a problem in node 0.10 or 0.12, but it is a problem starting with io.js v1.

[ljharb]

Thanks for the report!