javascript - RegExp.exec() returns null when called repeatedly in Firefox? - Stack Overflow

I've encountered an issue using the regex function exec() in Firefox 10 and 11.

The function seems to behave erratic when it is called a lot. Among the correct result, it also returns null a lot. From Safari 5.1.3, Chrome 18 and the above mentioned Firefox versions, I can see the issue only within Firefox.

I've created a JSFiddle to demonstrate the problem: / , source:

var i, x = "";

for (i = 0; i < 10000; i++) {
    var matches = /foo/g.exec('sdkfjfooasdknal');
    x += matches + "<br>";
}

$('body').html(x);

In my two Firefox versions, this returns 40x foo, 1x null, 41x foo, and from then on it swaps between these two on every single call.

Have you encountered this?

I've encountered an issue using the regex function exec() in Firefox 10 and 11.

The function seems to behave erratic when it is called a lot. Among the correct result, it also returns null a lot. From Safari 5.1.3, Chrome 18 and the above mentioned Firefox versions, I can see the issue only within Firefox.

I've created a JSFiddle to demonstrate the problem: http://jsfiddle/KSH3S/ , source:

var i, x = "";

for (i = 0; i < 10000; i++) {
    var matches = /foo/g.exec('sdkfjfooasdknal');
    x += matches + "<br>";
}

$('body').html(x);

In my two Firefox versions, this returns 40x foo, 1x null, 41x foo, and from then on it swaps between these two on every single call.

Have you encountered this?

• javascript
• regex
• firefox

Share Improve this question Follow edited Apr 12, 2019 at 17:58 Bhargav Rao 52.3k2929 gold badges127127 silver badges141141 bronze badges asked Apr 16, 2012 at 0:11 snwflksnwflk 3,52744 gold badges2727 silver badges3838 bronze badges 12

• What happens if it is invoked using the same regex object? (e.g. var foo = /foo/g outside the loop) – user166390 Commented Apr 16, 2012 at 0:15
• 1 What problem are you actually trying to solve? Calling exec() repeatedly on a regular expression with the "g" flag accumulates some state from one call to the next to allow you to find multiple matches in the same source string. – jfriend00 Commented Apr 16, 2012 at 0:18
• @jfriend00 But it's a new regex object in this case, and even if that were the case, it should be a predictable pattern. The issue is that FF behaves erratically. – user166390 Commented Apr 16, 2012 at 0:19
• @pst - Are you sure it's a new regex object? JS has function scope, not block scope. – jfriend00 Commented Apr 16, 2012 at 0:20
• @jfriend00 variables are scoped to functions. objects are not. /foo/g constructs a new RegExp object, just as "foo" constructs a new string object. Unless of course there is some "funny caching" going on (perhaps after the 80th such RegExp construction?). In any case, still not an explanation for erratic behavior :-) – user166390 Commented Apr 16, 2012 at 0:21

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3 Answers 3

Sorted by: Reset to default 2

It's a bug in the JS engine in Firefox, like jfriend00 says. Should be fixed in Firefox 12. See https://bugzilla.mozilla/show_bug.cgi?id=728021

The problem has something to do with the implicit declaration of the regex inside the loop. My guess is that the browser is caching it somewhere or getting confused by that somehow.

If you explicitly create a new regex object each time through the for loop, then Firefox no longer has a problem with this:

var x = "";

for (var i=0; i<10000; i++) {
    var re = new RegExp("foo", "g");
    var matches = re.exec( 'sdkfjfooasdknal' );
    x += matches+"<br>";
}

$('body').html( x );

http://jsfiddle/jfriend00/F49db/

And, it doesn't matter which way you explicitly declare the regex as this method works also:

var x = "";

for (var i=0; i<10000; i++) {
    var re = /foo/g;
    var matches = re.exec( 'sdkfjfooasdknal' );
    x += matches+"<br>";
}

$('body').html( x );

This is only a partial explanation. If you try this fiddle it seems to work fine. So the problem or bug might be in the regex parsing of a literal? I'm not sure, but could it be something where the loop optimization treats the literal differently than the variable.

Here is the fiddle code:

var x = "";

for (var i=0; i<10000; i++) {
    var a='sdkfjfooasdknal';
    var b=/foo/g;
    var matches = b.exec(a);
    x += matches+"<br>";
}

$('body').html( x );​

UPDATE: Here is something I tried to force it out of a normal loop and the problem reappeared. Here is the fiddle http://jsfiddle/xXYdF/.

var x = "";
i=200;
function reg(){
    var matches = /foo/g.exec('sdkfjfooasdknal');
    x += matches+"<br>";
}
    function run(){
  setTimeout(function(){
    if(i--){
      reg();
      run();
      $('body').html( x );
    }
  },10);
}

run();