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Asked: 2026-05-16T20:17:27+00:00

I was experiencing some weird behaviour in some of my javascript code, but only

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I was experiencing some weird behaviour in some of my javascript code, but only in Firefox and Chrome. IE is fine.

I have isolated the problem and created a little page so you can see the behaviour yourself.

Essentially, it appears as if the Regular Expression object in MethodC is being reused across method calls to MethodC, even though it’s a local variable. Can someone explain this behaviour?

<html>
<head>
<script type="text/javascript">
    function RunDemo()
    {
        var subject = "01234 555 6789";

        for (var i = 1; i <= 10; i++) {
            MethodA(subject, i);
            MethodB(subject, i);
            MethodC(subject, i);
        }
    }

    // OK, OK, OK, OK, OK, OK, OK, OK, OK, OK
    function MethodA(subject, iteration)
    {
        var myRegexp = new RegExp("5", "g");
        var matches = myRegexp.exec(subject);
        AddItem(matches ? "OK" : "no match", "listA");
    }

    // OK, OK, OK, OK, OK, OK, OK, OK, OK, OK
    function MethodB(subject, iteration)
    {
        var myRegexp = /5/;
        var matches = myRegexp.exec(subject);
        AddItem(matches ? "OK" : "no match", "listB");
    }

    // OK, OK, OK, no match, OK, OK, OK, no match, OK, OK (in FireFox and Chrome, IE is fine)
    function MethodC(subject, iteration) {
        var myRegexp = /5/g;
        var matches = myRegexp.exec(subject);
        AddItem(matches ? "OK" : "no match", "listC");
    }

    function AddItem(itemText, listID) {
        var li = document.createElement("li");
        li.innerHTML = itemText;
        document.getElementById(listID).appendChild(li);
    }   

</script>
</head>
<body onload="RunDemo()">
    <h2>Method A</h2>    
    <ul id="listA"></ul>

    <h2>Method B</h2> 
    <ul id="listB"></ul>

    <h2>Method C</h2> 
    <ul id="listC"></ul>
</body>
</html>
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1 Answer

  1. Editorial Team

    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.

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