\documentclass[a4paper,12pt]{article}
\usepackage{xltxtra}
\usepackage{polyglossia,hijrical}
\usepackage{bidi}
\setdefaultlanguage{farsi}
\setromanfont{Yas}
\defaultfontfeatures{Scale=MatchLowercase}
\setmonofont{Yas}
\setsansfont{Yas}
\begin{document}
اگر $2^3$
\end{document}