Substitute string that contains a backslash

Question

I need to create a function that will transform a string such as:

Hello\textbackslash{}Bye

into this:

Hello \\ Bye

I have tried using StrSubstitute (from the xstring package) without success. I have attempted things along these lines:

\newcommand{\TRANSFORM}[1]{\StrSubstitute{#1}{\textbackslash{}}{ \noexpand\\ }}

\TRANSFORM{Hello\textbackslash{}Bye}

Answer 1

You could just temporarily redefine the meaning of \textbackslash, letting it to \\:

\documentclass{article}

\newcommand{\TRANSFORM}[1]{{\let\textbackslash\\#1}}
\setlength{\parindent}{0pt}% Just for this example

\begin{document}

Hello\textbackslash{}Bye

\TRANSFORM{Hello\textbackslash{}Bye}

\end{document}

Answer 2

You need to use the \noexpandarg mode of xstring, so it won't try expanding its arguments:

\documentclass{article}
\usepackage{xstring}

%\noexpandarg % set \noexpandarg globally

\newcommand{\TRANSFORM}[1]{%
  \saveexpandmode\noexpandarg % set \noexpandarg locally
  \StrSubstitute{#1}{\textbackslash{}}{\\}%
  \restoreexpandmode % restore the previous mode
}

\begin{document}

\TRANSFORM{Hello\textbackslash{}Bye}

\end{document}

Depending on your usage of xstring, you might also set \noexpandarg globally (by commenting/uncommenting the relevant lines).

Of course, in this case, redefining a macro is better:

\newcommand\TRANSFORM[1]{{% open a group
   \renewcommand{\textbackslash}[1]{\\}%
   #1%
}}

Why \renewcommand and not \let? Because this redefinition also swallows {}.