User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153 Safari/537.36 OPR/22.0.1471.70 Steps to reproduce: As per ECMAScript Spec 262 under paragraph 15.10.2.3, the expression /((a)|(ab))((c)|(bc))/.exec("abc") should return ["abc", "a", "a", undefined, "bc", undefined, "bc"] Actual results: In fact though, in Firefox it returns ["abc", "a", "a", "", "bc", "", "bc"] Expected results: All browsers except Firefox behave as expected.

What version of Firefox did you test? Your testcase produces the desired results in latest nightly, latest beta (31), latest release (30) and in the slightly older 28.

(In reply to :Gijs Kruitbosch from comment #1) > What version of Firefox did you test? Firefox 30.0 on OS X 10.9.4 > Your testcase produces the desired > results in latest nightly, latest beta (31), latest release (30) and in the > slightly older 28. In fact, in Windows I'm getting correct results, so the issue may be specific to OS X.

Can you please attach a short html testcase ?

(In reply to mdn from comment #2) > (In reply to :Gijs Kruitbosch from comment #1) > > What version of Firefox did you test? > > Firefox 30.0 on OS X 10.9.4 > > > Your testcase produces the desired > > results in latest nightly, latest beta (31), latest release (30) and in the > > slightly older 28. > > In fact, in Windows I'm getting correct results, so the issue may be > specific to OS X. I tested on OS X. It works for me. Can you try with a clean profile? ( https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles )

(In reply to Matthias Versen [:Matti] from comment #3) > Can you please attach a short html testcase ? It was a bad example, my wrong. But here's one that produces the unexpected empty string in FF: http://jsfiddle.net/QdBsn/ Hope you can reproduce this one.

(In reply to :Gijs Kruitbosch from comment #4) > (In reply to mdn from comment #2) > > (In reply to :Gijs Kruitbosch from comment #1) > > > What version of Firefox did you test? > > > > Firefox 30.0 on OS X 10.9.4 > > > > > Your testcase produces the desired > > > results in latest nightly, latest beta (31), latest release (30) and in the > > > slightly older 28. > > > > In fact, in Windows I'm getting correct results, so the issue may be > > specific to OS X. > > I tested on OS X. It works for me. Can you try with a clean profile? ( > https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox- > profiles ) I testes again with a clean profile and got the same results. Here's a correct fiddle demonstrating the issue: http://jsfiddle.net/QdBsn/

The testcase in the URL field shows "undefinedc" in Chrome, and "c" in Firefox. Source: <p> <script> document.write("ac".replace(/a(b)?/, function(s, m) { return m + "" })); </script> </p>

I'm not actually personally sure this is a bug per spec, despite reading the spec for String.prototype.replace and Regexp.prototype.exec 3 times - and if it is a bug, if we shouldn't change the spec because "" seems nicer as a function parameter than |undefined|, but I'll leave that for more enlightened minds to decide on. Kannan, do you know, and/or do you know someone who knows? :-)

Yes, this is a bug. Just like `"ac".match(/a(b)?/)` is supposed to return `[ "a", (void 0) ]`, the 1th capture in `"ac".replace(/a(b)?/, (s, m) => m + "")` is supposed to be `undefined`, which means the result string needs to be `"undefinedc"`. 15.5.4.11 String.prototype.replace: > If replaceValue is a function, then for each matched substring, call the function with the following > m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression, > the next m arguments are all of the captures in the MatchResult (see 15.10.2.1).