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XRegExp breaks jQuery Animations

Posted on Thursday, September 15, 2011, at 1:12:00 AM UTC

Update: This bug was fixed in XRegExp 1.5.1.
However, as far as I know, there are no released versions of SyntaxHighlighter that contain the fix.

XRegExp is an open source JavaScript library that provides an augmented, extensible, cross-browser implementation of regular expressions, including support for additional syntax, flags, and methods.

It’s used by the popular SyntaxHighlighter script, which is in turn used by many websites (including this blog) to display syntax-highlighted source code on the client.  Thus, XRegExp has a rather wide usage base.

However, XRegExp conflicts with jQuery.  In IE, any page that includes XRegExp and runs a numeric jQuery animation will result in “TypeError: Object doesn't support this property or method”.

Demo (only fails in IE)

This bug is caused by an XRegExp fix for an IE bug in which the exec method doesn't consistently return undefined for nonparticipating capturing groups.  The fix, on line 271, assumes that the parameter passed to exec is a string.  This behavior violates the ECMAScript standard (section, which states that the parameter to exec should be converted to a string before proceeding.

jQuery relies on this behavior in its animate method, which parses a number using a regex to get the decimal portion.  (source)

Thus, calling animate with a number after loading XRegExp will fail in IE when XRegExp tries to call slice on a number.

Fortunately, it is very simple to fix XRegExp to convert its argument to a string first:

if (XRegExp) {
    var xExec = RegExp.prototype.exec;
    RegExp.prototype.exec = function(str) {
        if (!str.slice) 
            str = String(str);
        return, str);
Here is an updated demo that uses this fix and works even in IE.

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