My attempt to local redefine a command is failing and not sure why

Question

I want to decorate some text and then reprint it undecorated. This is the code that I was using to do this.

\documentclass{article}
\usepackage{xcolor}
\makeatletter
\newcommand{\@aeformatcontent}[1]{\parbox[t]{5in}{\emph{#1}}}
\newcommand{\aeformatcontent}[1]{%%'
  $\star$\hspace*{0.5em}\@aeformatcontent{#1}\par
  \bgroup
    \def\aeget#1{#1}%%'
    $\phantom{\star}$\hspace*{0.5em}\@aeformatcontent{#1}%%'
  \egroup
} 

\newcommand{\aeget}[1]{\csname aephrase #1\endcsname}
\newcommand{\aedefine}[1]{\expandafter\def\csname aephrase #1\endcsname{{\color{red}#1}}}

\makeatother

\begin{document}

  \aedefine{this is a trial}
  \aeget{this is a trial}

  \aeformatcontent{\aeget{this is a trial}}

\end{document}

When I try to compile this, I get the error:

! Use of \aeget doesn't match its definition.
<argument> \aeget {
                   this is a trial}
l.24 ... \aeformatcontent{\aeget{this is a trial}}

? 

If I replace \def\aeget#1{#1} with

\renewcommand{\aeget}[1]{#1}

I get stuck in an infinite loop until LaTeX's capacity is exceeded.

I don't understand why neither of these approaches works. Neither do I understand why I'm getting different error messages.

Answer 1

Your internal redefinition of \aeget needs a doubled use of the parameter text:

\def\aeget##1{##1}

This resets \aeget to a no-op, but since it is redefined inside \aeformatcontent, you have to double the #s. Also see Defining LaTeX commands within other commands.

\documentclass{article}
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\makeatletter
\newcommand{\@aeformatcontent}[1]{\parbox[t]{5in}{\emph{#1}}}
\newcommand{\aeformatcontent}[1]{%%'
  $\star$\hspace*{0.5em}\@aeformatcontent{#1}\par
  \bgroup
    \def\aeget##1{##1}%%'
    $\phantom{\star}$\hspace*{0.5em}\@aeformatcontent{#1}%%'
  \egroup
}
\newcommand{\aeget}[1]{\csname aephrase #1\endcsname}
\newcommand{\aedefine}[1]{\expandafter\def\csname aephrase #1\endcsname{{\color{red}#1}}}
\makeatother
\begin{document}
\aedefine{this is a trial}
  \aeget{this is a trial}
\aeformatcontent{\aeget{this is a trial}}
\end{document}