Replace certain Unicode character with another in XeTeX

Question

I use XeLaTeX and XePersian to write Persian documents. Unfortunately, some frequently used fonts in Persian does support important characters like ی so I should replace them with Arabic counterparts like ي to get the expected result. Some others support it, so I should revert the changes.

I can not replace every single character of ی with ي, because in this way, my Persian macros and localised XePersian commands will fail, and if I do this manually, with a change of font I will have to do this again. For sure this can be fixed in the font-level, but I can not edit the font. :-/

My question is that is it possible to replace ی by ي in macro level?

I tried this, but it didn't work:

\documentclass[14pt,a4paper]{article}
\usepackage[localised]{xepersian}
\settextfont{Arial}

\defی{ي}

\begin{document}
می‌شود.

% added to show the problem with the localised commands
% even without it, it won't compile when placing replace commands
%before including xepersian
\سطرجدید %it means new line
\newline

\end{document}

Answer 1

Not essentially different from Tobi's answer, but the substitution is more robust.

\documentclass[a4paper]{article}
\usepackage{xepersian}
\usepackage{newunicodechar}
\settextfont{Arial}

\newunicodechar{ی}{ي}

\begin{document}
می‌شود.
\end{document}

(The thing appears to be reversed, I can't understand what the editor does when Arabic is involved.)

Answer 2

You can make the character that is written an active character by use of \catcode and then define it to be the other character:

\documentclass[14pt,a4paper]{article}
\usepackage{xepersian}
\settextfont{Arial Unicode MS}

\catcode`\ی=\active
\def ی{ي}

\begin{document}
می‌شود.
\end{document}

(I had to change the font on my system)