Replacing characters in the argument - even within other commands

Question

I need to replace all characters given as argument with other characters, doing some sort of transliteration.

This answer provides a solution that works nicely with XeLaTeX and Unicode. Unfortunately it cannot replace within arguments of commands that occur within the \myreplace:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\myreplace}{m}
 {
  \tl_set:Nn \l__maxd_argument_tl { #1 }
  \tl_replace_all:Nnn \l__maxd_argument_tl { e } { ẹ }
  \tl_replace_all:Nnn \l__maxd_argument_tl { E } { Ẹ }
  \tl_use:N \l__maxd_argument_tl
 }
\tl_new:N \l__maxd_argument_tl
\ExplSyntaxOff

\begin{document}

\myreplace{Ee} \myreplace{E\emph{e}}

\end{document}

A general solution for all text-commands would be fantastic, but solving it for \emph only would be fine for my purpose.

I'd also be interested to learn whether I could add some basic error-handling, e.g. by making the macro replace all characters other than the specified ones by something that's spotted easily, like "???".

Answer 1

regex_replace_all:nnN does work here:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\myreplace}{m}
 {
  \tl_set:Nn \l__maxd_argument_tl { #1 }
  \regex_replace_all:nnN { e } { x } \l__maxd_argument_tl
  \regex_replace_all:nnN { E } { X } \l__maxd_argument_tl
  \tl_use:N \l__maxd_argument_tl
 }
\tl_new:N \l__maxd_argument_tl
\ExplSyntaxOff

\begin{document}

\myreplace{Ee} \myreplace{E\emph{e}}

\end{document}

To make it work for your special characters with pdfLaTeX one has to specify the correct category codes for the replacement (don't use this with LuaLaTeX or XeLaTeX -- it will throw an error):

\documentclass{article}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\myreplace}{m}
 {
  \tl_set:Nn \l__maxd_argument_tl { #1 }
  \regex_replace_all:nnN { e } { \cA(ẹ) } \l__maxd_argument_tl
  \regex_replace_all:nnN { E } { \cA(Ẹ) } \l__maxd_argument_tl
  \tl_use:N \l__maxd_argument_tl
 }
\tl_new:N \l__maxd_argument_tl
\ExplSyntaxOff

\begin{document}

\myreplace{Ee} \myreplace{E\emph{e}}

\end{document}

Answer 2

Here's a LuaLaTeX-based solution. It can handle utf8-encoded characters in the text substitution operation, and it can handle all embedded macro names, including \emph. (The only assumption is that argument of \myreplace contains no macros whose names contain the characters ẹ or Ẹ to begin with. This shouldn't be a binding restriction, right?)

% !TEX TS-program = lualatex
\documentclass{article}

\usepackage{luacode}
\begin{luacode*}

function myreplace ( s )
  s = unicode.utf8.gsub ( s, "E", "Ẹ" )
  s = unicode.utf8.gsub ( s, "e", "ẹ" )
  s = unicode.utf8.gsub ( s , "\\[%aẸẹ]+" , 
        function ( x )
           x = unicode.utf8.gsub ( x , "ẹ" , "e" )
           x = unicode.utf8.gsub ( x , "Ẹ" , "E" )
           return ( x )
        end )
  tex.sprint ( s )
end

\end{luacode*}
\newcommand\myreplace[1]{\directlua{myreplace(\luastringN{#1})}}

\begin{document}
\myreplace{Ee} \myreplace{E\textit{e}\emph{e}\textbf{E}}
\end{document}