Replacing a substring

\documentclass{article}
\usepackage{xstring}
\def\ReplaceStr#1{%
  \IfSubStr{#1}{XY}{%
    \StrSubstitute{#1}{XY}{$\to$}}{#1}}

\begin{document}

\ReplaceStr{aXYbXYc}

\ReplaceStr{abc}

\end{document}

\documentclass{article}
\usepackage{listofitems}
\newcommand\substr[3]{%
  \setsepchar{#2}%
  \readlist\parsedinput{#1}%
  \foreachitem\x\in\parsedinput{%
    \ifnum\xcnt=1\else#3\fi\x%
  }%
}
\begin{document}
\substr{aXYbXYc}{XY}{$\to$}
\end{document}