Regular Expression literal with /g modifier and eval - weird behaviour in FireFox and Chrome

The optimizers in V8 and spidermonkey create a regex object when they see a regex literal and reuse it.

Per ECMA3, this is compliant behavior, but it will become non-compliant in ECMA5.

7.8.5 Regular Expression Literals

A regular expression literal is an input element that is converted to a RegExp object (section 15.10) when it is scanned. The object is created before evaluation of the containing program or function begins. Evaluation of the literal produces a reference to that object; it does not create a new object. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp (section 15.10.4) or calling the RegExp constructor as a function (section 15.10.3).

ECMAScript Language Specification Edition 3

Compare to:

7.8.5 Regular Expression Literals

A regular expression literal is an input element that is converted to a RegExp object (see 15.10) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp (see 15.10.4) or calling the RegExp constructor as a function (15.10.3).

ECMAScript Language Specification Edition 5

Here are some workarounds:

• Don't use the /g flag with exec.
• Create a RegExp from the RegExp consructor instead of from a regexp literal.

Doing either or both of these should, I think, make the problem go away.

This is somewhat mentioned in the Mozilla Javascript Reference:

If your regular expression uses the "g" flag, you can use the exec method multiple times to find successive matches in the same string. When you do so, the search starts at the substring of str specified by the regular expression's lastIndex property

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/RegExp/exec#Description

I don't understand though why the lastIndex property is kept after leaving MethodC

EDIT:I found this bug which seems to describe excatly what you experienced here: https://bugzilla.mozilla.org/show_bug.cgi?id=98409

The lastIndex property is acting as a static variable: in Chrome (&FF):

but not IE:

As to why this is happening, I'm not sure. You can work around this by using .match()

function MethodC(subject, iteration) {    var myRegexp = /5/g;    var matches = subject.match(myRegexp);    AddItem(matches ? "OK" : "no match", "listC");}