Problems with \ref when using soul for highlighting

Question

Here is a minimal example showing the problem.

\documentclass[10pt]{article}
\usepackage[usenames,dvipsnames]{color}
\usepackage{soul}
%\soulregister{\ref}{1}
\begin{document}

\section*{x}\label{x}

\section*{y}
\hl{ Some text.
  Section~\ref{x}.
}
\end{document}

I'm getting the error

LaTeX Warning: Reference `{x}' on page 0 undefined on input line 12.

! Argument of \ref has an extra }.

The problem is with \ref. Commenting out the line beginning with "Subsection" makes the error disappear. \soulregister is intended for font commands. I don't know if \ref is a font command, but I could not get \soulregister to fix the problem (see the commented line above).

Answer 1

Enclosing the \ref{x} in { } works. soul then takes it as one element instead of breaking it up, I guess. It seems to me that it reads both \ref and {x} in two steps and sets it together again while adding another set of braces, i.e. the result is \ref{{x}}. This doesn't matter much for font commands but \ref takes the inner braces as part of the label name. This causes the warning about the unknown label. The error is caused because \ref is then also expanded by soul inside a { } group, i.e. {\ref}. Don't ask me why.

Note that you can't label a \section* because it doesn't has a number.

\documentclass[10pt]{article}
\usepackage[usenames,dvipsnames]{color}
\usepackage{soul}
%\soulregister{\ref}{1}
\begin{document}

\section{x}\label{x}

\section{y}
\hl{ Some text.
   Section~{\ref{x}}.
}
\end{document}