Generate labels using xstring and \Makelowercase

Question

I am trying to write a command that allows me to reference a subsection with a generated label.

The idea is that when I declare a step \step{Long Name With Many Uppercase Letters}, the generated label will be \label{step:long_name_with_many_uppercase_letters}.

I already found Generate labels based on macro argument, but I fear the answer based on using \@firstofone does not work for my case.

My current approach is the following:

\makeatletter
% Step command
\newcounter{step}[section]
\setcounter{step}{0}

\newcommand{\step}[1]{%
  \subsection{#1} 
  \refstepcounter{step} 
  \label{step:\MakeLowercase{\StrSubstitute[0]{#1}{ }{\_}}}
}
\makeatother

Unfortunately, using the label command with other command inside gives me the following errors:

Missing \endcsname inserted \step{First Step}
TeX capacity exceeded, sorry [input stack size=5000] \step{First Step}

If I remove the \label command, it works (but of course gives me warnings about undefined references).

A minimal example looks like this:

\documentclass{scrbook}

\usepackage{xstring}

\makeatletter
% Step command
\newcounter{stepctr}[section]
\setcounter{stepctr}{0}

\newcommand{\step}[1]{%
  \subsection{#1} 
  \refstepcounter{stepctr} 
  \label{step:\MakeLowercase{\StrSubstitute[0]{#1}{ }{\_}}}
}
\makeatother

\begin{document}

\chapter{My only chapter}

\section{A section where I reference steps}

I'd like to reference step number \ref{step:first_step} and \ref{step:third_step} here.

\section{A section where I define steps}

\step{First Step}

Foo.

\step{Second Step}

Bar.

\step{Third Step}

Whatever. 

\end{document}

Answer 1

You want to lowercase the argument of label, not have \label containing a lowercase command in its argument so invert the order of the expressions:

\documentclass{scrbook}

\usepackage{xstring}


% Step command
\newcounter{stepctr}[section]
\setcounter{stepctr}{0}

\newcommand{\step}[1]{%
  \subsection{#1} %
  \refstepcounter{stepctr}%
  {\lccode`\ `\_\lowercase{\label{step:#1}}}}


\begin{document}

\chapter{My only chapter}

\section{A section where I reference steps}

I'd like to reference step number \ref{step:first_step} and \ref{step:third_step} here.

\section{A section where I define steps}

\step{First Step}

Foo.

\step{Second Step}

Bar.

\step{Third Step}

Whatever. 

\end{document}