XF 2.1 Using a slice or a parallel function

[canina]

How can I cut a string using a slice or similar function?

<xf:if is="{$xf.fullUri}.slice(0, 40) == 'https://www.domain.com/register/'">
    <div> abc </div>   
</xf:if>

(The question is that when the registration page is in Popup So the URI of PopUp also includes all the parameters are sent).

https://www.domail.com/register/?_x...f295eaaa77e8d2fe11eb9c0f&_xfResponseType=json