tex macro, gsub

subst.tex

\documentclass{article}
\makeatletter
\def\gsub#1#2#3{%
  \in@{#2}{#3}\ifin@
    \errhelp{`#3' contains `#2', which is not allowed.}%
    \errmessage{Nested Substitution}%
  \fi
  \in@{#2}{#1}\ifin@
    \def\gsubresult##1#2##2\par{##1#3##2}%
    \def\next{\expandafter\gsub\expandafter{\gsubresult#1\par}{#2}{#3}}%
  \else
    \def\next{\def\gsubresult{#1}}%
  \fi
  \next
}
\protected\def\segment#1{%
  \gsub{#1}{.}{dot}%
  \expandafter\gsub\expandafter{\gsubresult}{?}{QM}%
%  \expandafter\gsub\expandafter{\gsubresult}{O}{Oh!}%
  \expandafter\gsub\expandafter{\gsubresult}{!}{EXCL}%
  \gsubresult
}
\makeatother
\usepackage{trace}
\begin{document}
|\segment{OO \TeX20.9 ?}|
\end{document}