How can I append code to an item-label(-box)?

Question

For executing code after the end of a parameterless macro there are \ltx@GlobalAppendToMacro and \ltx@LocalAppendToMacro from the ltxcmds-package.

For executing code after the end of an environment there is \AfterEndEnvironment from the etoolbox-package, and without the e-TeX extension one can use \renewenvironment for this.

But how can one "append" code after a label of an \item? \item has one (optional) parameter, thus \ltx@GlobalAppendToMacro and \ltx@LocalAppendToMacro cannot be used. At \end{description} would be too late.

MWE:

\documentclass{article}
\begin{document}
\begin{description}
 \item[Label\ReplaceMe{\textbf{OK}}] Text.
 \item[Label\ReplaceMe{\textbf{abc}}] Text.
 \item[Label] Text.
 \item Item without label.
\end{description}
\end{document}

The

\item[Label\ReplaceMe{\textbf{OK}}] Text.

should "expand" to

\item[Label]\textbf{OK} Text.

and the others accordingly. \item itself (or macros called by it) as found in

File A: ltlists.dtx Date: 2002/10/28 Version v1.0s (source 2e, 2011/06/27)

contains \box, \hbox,... The label text is thus "boxed", and I want to "free" a part of it. With "boxed" I do not want to indicate that there would be any rectangular line around "Label".

Looking at the answer provided by Werner, would it be sufficient to use the following code?

\documentclass{article}
\usepackage{letltxmacro}
\makeatletter
\newcommand*{\ReplaceMe}[1]{\gdef\saved{#1}}
\ReplaceMe{}
\LetLtxMacro{\@olditem}{\@item}%
\newenvironment{desc}%
  {\renewcommand*{\@item}[1][]{% \begin{desc}
     \@olditem[##1]\saved\gdef\saved{}%
    }%
   \begin{description}}%
  {\end{description}}% \end{desc}
\makeatother

\begin{document}
\noindent Here is some text.
\begin{desc}
 \item[Label\ReplaceMe{\textbf{OK}}] Text.
 \item[Label\ReplaceMe{\textbf{abc}}] Text.
 \item[Label] Text.
 \item Item without label.
\end{desc}
Here is some more text.
\begin{description}
 \item[Label\ReplaceMe{\textbf{OK}}] Text.
 \item[Label\ReplaceMe{\textbf{abc}}] Text.
 \item[Label] Text.
 \item Item without label.
\end{description}
\end{document}

Answer 1

Here is one option that you could use. The \ReplaceMe macro is constructed to use a toggle (true/false boolean variable) and store the contents in a box that can be retrieved later. Additionally, the regular \item macro (and associated \@item) is modified to box the label (using \fbox) and process the saved contents of \ReplaceMe.

Heeding @egreg's suggestion, an entirely new description environment desc is defined to restrict the scope of the \@item redefinition and keep it local:

\documentclass{article}
\usepackage{letltxmacro}% http://ctan.org/pkg/letltxmacro
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\newsavebox{\ReplaceMeBox}% Store \item's \ReplaceMe
\newtoggle{ReplaceMe}% \ReplaceMe was used/not
\newcommand*{\ReplaceMe}[1]{%
  \global\toggletrue{ReplaceMe}% \ReplaceMe was used
  \global\sbox{\ReplaceMeBox}{#1}% Store contents in \ReplaceMeBox
}
\makeatletter
\LetLtxMacro{\@olditem}{\@item}%
\newenvironment{desc}%
  {\renewcommand*{\@item}[1][]{% \begin{desc}
   \@olditem[\fbox{##1}]\iftoggle{ReplaceMe}{\usebox{\ReplaceMeBox}\global\togglefalse{ReplaceMe}}{}}%
   \begin{description}}
  {\end{description}}% \end{desc}
\begin{document}
\noindent Here is some text.
\begin{desc}
 \item[Label\ReplaceMe{\textbf{OK}}] Text.
 \item[Label\ReplaceMe{\textbf{abc}}] Text.
 \item[Label] Text.
\end{desc}
Here is some more text.
\begin{description}
 \item[Label\ReplaceMe{\textbf{OK}}] Text.
 \item[Label\ReplaceMe{\textbf{abc}}] Text.
 \item[Label] Text.
\end{description}
\end{document}​

etoolbox provides the toggle mechanism, while letltxmacro was used as a precaution to handle optional argument copying across macros. To cross scope boundaries when switching toggles and saving boxes, \global was used.