LaTeX xstring \StrCut with macros

Question

I've been trying to give some formatted input to \StrCut from the xstring package in a very complex code, and I've been getting an error.

Here is a MWE :

\documentclass[a4paper]{article}
\usepackage{xstring}

\begin{document}

\newcommand{\test}{\emph{First} part:Second part}
\StrCut{\test}{:}\firstpart\secondpart%
\firstpart --- \secondpart%

\end{document}

I get this error :

! Use of \@xs@StrCut@@ doesn't match its definition.
\text@command #1->\def \reserved@a {
#1}\ifx \reserved@a \@empty \let \check@...
l.7 \StrCut{\test}{:}
\firstpart\secondpart%

Any tip ?

EDIT :

With this MWE :

\documentclass[a4paper]{article}

\usepackage{xstring}

\begin{document}

\newcommand{\test}{\emph{First} part:Second part}
\newcommand{\testbis}{\test}

\expandarg\StrCut{\testbis}{:}\firstpart\secondpart%

\firstpart --- \secondpart%

\end{document}

I don't get any error, but the output is not what I expect, since \test isn't cut.

Answer 1

You don't need \StrCut for this application. There are two problems with your code:

1. You seem to want an arbitrary layer of macros above macros
2. You want to use “dangerous” commands such as \emph, that don't survive full expansion.

You can use \protected@edef in order to overcome the two issues:

\documentclass{article}

\makeatletter
\newcommand{\splitatcolon}[3]{%
  \protected@edef\split@temp{#1}%
  \expandafter\split@colon\split@temp:\@nil{#2}{#3}%
}
\def\split@colon#1:#2\@nil#3#4{%
  \def#3{#1}%
  \if\relax\detokenize{#2}\relax
    % no colon
    \let#4\@empty
  \else
    \split@eatcolon#2\@nil{#4}%
  \fi
}
\def\split@eatcolon#1:\@nil#2{\def#2{#1}}
\makeatother

\begin{document}

\newcommand{\test}{\emph{First} part:Second part}
\newcommand{\testbis}{\test}

\splitatcolon{\emph{First} part:Second part}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\test}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\testbis}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\emph{First} part:Second part:Third part}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\secondpart}\secondpart\thirdpart

\firstpart\ --- \secondpart\ --- \thirdpart

\end{document}

The same idea with xstring:

\documentclass{article}
\usepackage{xstring}

\makeatletter
\newcommand{\splitatcolon}[3]{%
  \protected@edef\split@temp{#1}%
  \saveexpandmode
  \expandarg\StrCut{\split@temp}{:}#2#3%
  \restoreexpandmode
}
\makeatother

\begin{document}

\newcommand{\test}{\emph{First} part:Second part}
\newcommand{\testbis}{\test}

\splitatcolon{\emph{First} part:Second part}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\test}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\testbis}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\emph{First} part:Second part:Third part}\firstpart\secondpart

\firstpart\ --- \secondpart

\splitatcolon{\secondpart}\secondpart\thirdpart

\firstpart\ --- \secondpart\ --- \thirdpart

\end{document}

Answer 2

The cutting fails most likely due to \emph, so expand the arguments:

\documentclass[a4paper]{article}
\usepackage{xstring}

\begin{document}

\newcommand{\test}{\emph{First} part:Second part}
\expandarg\StrCut{\test}{:}{\firstpart}{\secondpart}%
\firstpart\ --- \secondpart%

\end{document}

Update

\documentclass[a4paper]{article}

%\usepackage{xstring}

\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\StrCutNew}{mmmm}{%
  \tl_set:Nx \l_tmpa_tl {#1}%
  \seq_set_split:NnV \l_tmpa_seq {#2} {\l_tmpa_tl}
  \expandafter\DeclareDocumentCommand\csname #3\endcsname{}{\seq_item:Nn \l_tmpa_seq{1}}
  \expandafter\DeclareDocumentCommand\csname #4\endcsname{}{\seq_item:Nn \l_tmpa_seq{2}}
}
\ExplSyntaxOff

\begin{document}

\newcommand{\test}{First part:Second part}
\newcommand{\testbis}{\test}
\StrCutNew{\test}{:}{firstpart}{secondpart}

\firstpart\ --- \secondpart

\StrCutNew{\test}{:}{firstpart}{secondpart}


\firstpart\ --- \secondpart

\end{document}

Answer 3

I found something which seems to be working well for the Edit :

\documentclass[a4paper]{article}

\usepackage{xstring}

\begin{document}

\newcommand{\test}{\emph{First} part:Second part}
\newcommand{\testbis}{\test}

\StrExpand[2]{\test}{\toto}
\StrExpand[2]{\testbis}{\totobis}

\expandarg\StrCut{\emph{First} part:Second part}{:}\firstpart\secondpart%
\firstpart --- \secondpart%

\expandarg\StrCut{\test}{:}\firstpart\secondpart%
\firstpart --- \secondpart%

\expandarg\StrCut{\testbis}{:}\firstpart\secondpart%
\firstpart --- \secondpart%

\expandarg\StrCut{\toto}{:}\firstpart\secondpart%
\firstpart --- \secondpart%

\expandarg\StrCut{\totobis}{:}\firstpart\secondpart%
\firstpart --- \secondpart%

\end{document}

It still doesn't solve the issue in my complex code, but I can't get a MWE to reproduce the issue. I'll come back :D

Thank you Christian Hupfer for your help.