XRegExp 0.5.0 added the methods RegExp.prototype.apply/call. XRegExp 2.0.0-beta moved them to XRegExp.prototype, added XRegExp.install('methods') to copy them back to RegExp.prototype, and made XRegExp(regexp)/XRegExp.globalize augment copied regexes with apply and call methods.
I'm considering removing the regex apply and call methods altogether in XRegExp 2.0.0 final. Since the built-in array collection methods (such as Array.prototype.filter) don't use duck-typed apply or call, adding these methods to regexes just doesn't seem useful often enough to justify them.
Brendan Eich recently referenced XRegExp's callable regexes on es-discuss. That and various other comments / blogs across the web show there is some (small) interest in callable regexes.
(A scenario where I've used callable regexes in the past is validation methods that can be extended with functions and/or regexes that are passed in.)
I've moved the XRegExp.prototype.apply/call methods to the new XRegExp Prototype Methods addon.