Xstring Problem with Ş

Question

I want to use xstring version 1.7. However ş and ç cause problems. For instance

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{xstring}

\begin{document}

\StrSubstitute{ş}{a}{b}

\end{document}

creates a long list of errors like this one:

! Illegal parameter number in definition of \@xs@arg@ii.

I am using the encoding package with the utf8 option and other characters like ö, ü and ğ are fine.

Edit: Here is the mwe spelled out in full.

Answer 1

Package xstring uses \edef to expand the arguments by default, then commands can break like the UTF8 character.

The expansion mode can be changed, e.g. \noexpandargs to prevent expansion at all. It can be used inside a group or the expansion mode can be saved and restored afterwards:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{xstring}

\begin{document}

\saveexpandmode
\noexpandarg
\StrSubstitute{ş}{a}{b}
\restoreexpandmode

\end{document}