Javascript Regex literal with /g used multiple times

Question

Hello,

I have a weird issue working with the Javascript Regexp.exec function. When calling multiple time the function on new (I guess ...) regexp objects, it works one time every two. I don't get why at all!

Here is a little loop example but it does the same thing when used one time in a function and called multiple times.

for (var i = 0; i < 5; ++i) {
  console.log(i, (/(b)/g).exec('abc'));
}

> 0 ["b", "b"]
> 1 null
> 2 ["b", "b"]
> 3 null
> 4 ["b", "b"]

When removing the /g, it gets back to normal.

for (var i = 0; i < 5; ++i) {
  console.log(i, (/(b)/).exec('abc'));
}             /* no g ^ */

> 0 ["b", "b"]
> 1 ["b", "b"]
> 2 ["b", "b"]
> 3 ["b", "b"]
> 4 ["b", "b"]

I guess that there is an optimization, saving the regexp object, but it seems strange.

This behaviour is the same on Chrome 4 and Firefox 3.6, however it works as (I) expected in IE8. I believe that is intended but I can't find the logic in there, maybe you will be able to help me!

Thanks

Answer 1

/g is not intended to work for simple matching:

/g enables "global" matching. When using the replace() method, specify this modifier to replace all matches, rather than only the first one.

I'd imagine internally javascript holds the matching after the capture, so it would be able to resume matching and therefore null is returned since b occur only once in the subject. Compare:

for (var i = 0; i < 5; ++i) {
  console.log(i +'    ' + (/(b+)/g).exec("abbcb"));
}

returns:

0 bb,bb
1 b,b
2 null
3 bb,bb
4 b,b

Answer 2

If you're going to reuse the same regular expression anyway, take it out of the loop and explicitly reset it:

var pattern = /(b)/g;
for (var i = 0; i < 5; ++i) {
  pattern.lastIndex = 0;
  console.log(i + ' ' + pattern.exec("abc"));
}

Answer 3

Thanks :)

I found an interesting side effet, it's possible make a static variable (in sense of C, global but only visible from the function) without closure!

   function test () {
     var static = /a/g;
     if ('count' in static) {
       static.count++;
     } else {
       static.count = 1;
     }
     console.log(static.count);
   }

   for (var i = 0; i < 5; ++i) { test(); }
   1
   2
   3
   4
   5

(I'm making a new answer because we can't put code inside a comment)