Using @namedef to store equations

Question

This is a modification of the question here: @namedef and math mode.

I am trying to define a macro called myLabel that 1) defines a standard label for an equation, and 2) saves the equation so that it can be easily reprinted with a second macro called displayEquation. The MWE is here:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xstring}

%strip any alignment characters and display as an inline equation
\newcommand{\cleanLabel}[1]{$  \StrSubstitute{#1}{&}{} $ }

\makeatletter
\newcommand\myLabel[2]{%
  \label{#1}%
  \global\@namedef{label@store@content@#1}{#2}%
  #2}
\newcommand{\displayEquation}[1]{
    \cleanLabel{%
        \@nameuse{label@store@content@#1}%
    }
    }  
\makeatother


\begin{document}

Define three equations
\begin{align}
  \myLabel{simpleEq}{ f(x) &= 5x  } \\
  \myLabel{harderEq}{ f(x) &= \ln{5x}  } \\
  \myLabel{anothEq}{ \bm{a} &= \ddot{\bm{x}}}   
\end{align}
Equation \ref{simpleEq} has tag simpleEq, and I can display it inline as: \displayEquation{simpleEq}
\\
Equation \ref{harderEq} has tag harderEq,  and I can try to display it inline as: %\displayEquation{harderEq}  %FAILS
\\
Equation \ref{anothEq} has tag anothEq,  and I can try to display it inline as: %\displayEquation{anothEq}   %FAILS
\end{document}

The macros work fine when I have 'simple' equations. However it seems to fail if I have any equation with a backslash character. Is there a way to modify my setup so it can work for more complex equations?

Answer 1

The problem is that \StrSubstitute fully expands its argument, which might not work out for every content. The following uses the expl3 function \tl_remove_all:Nn to remove the ampersands instead.

\documentclass{article}
\usepackage{amsmath}
\usepackage{bm}
\usepackage{xparse}

%strip any alignment characters and display as an inline equation
\ExplSyntaxOn
\tl_new:N \l_jpdomann_tmp_tl
\cs_new_protected:Npn \cleanLabel #1
  {
    $
    \tl_set:Nv \l_jpdomann_tmp_tl { #1 }
    \tl_remove_all:Nn \l_jpdomann_tmp_tl { & }
    \l_jpdomann_tmp_tl
    $
  }
\ExplSyntaxOff

\makeatletter
\newcommand\myLabel[2]{%
  \label{#1}%
  \global\@namedef{label@store@content@#1}{#2}%
  #2}
\newcommand{\displayEquation}[1]
  {%
    \cleanLabel{label@store@content@#1}%
  }  
\makeatother

\begin{document}

Define three equations
\begin{align}
  \myLabel{simpleEq}{ f(x) &= 5x  } \\
  \myLabel{harderEq}{ f(x) &= \ln{5x}  } \\
  \myLabel{anothEq}{ \bm{a} &= \ddot{\bm{x}}}   
\end{align}
Equation \ref{simpleEq} has tag simpleEq, and I can display it inline as: \displayEquation{simpleEq}
\\
Equation \ref{harderEq} has tag harderEq,  and I can try to display it inline
as: \displayEquation{harderEq}  %FAILS
\\
Equation \ref{anothEq} has tag anothEq,  and I can try to display it inline as: \displayEquation{anothEq}   %FAILS
\end{document}