Highlight asterisk in lstlisting

Question

I would like to emphasise the asterisk used in \begin{equation*} in lstlisting. I have tried using emph, also including the alsoletter macro, but it doesn't work. Why not?

\documentclass{article}

\usepackage{listings,xcolor}

\begin{document}
\begin{lstlisting}[alsoletter={*},emph={*},emphstyle={\color{red}}]
% \usepackage{mhchem}

\begin{equation*}
  \label{eq:chem}
  \ce{CaCO3(s) <=>[H2O] 
  Ca2+(aq) + CO3^{2-}(aq)}
\end{equation*}
%
Some text
\end{lstlisting}
\end{document}

Answer 1

There is no really robust way of doing that in listings. Declaring * as a letter will break highlighting of keywords that are immediately preceded or followed by * (e.g. int*). Here, I would abuse the literate key; see below.

For information, the test in my \processAsterisk macro is a hacky way of highlighting asterisks only in listings' "normal" mode, not within strings, comments, and other delimited stuff.

\documentclass{article}

\usepackage{listings}
\usepackage{xcolor}

\makeatletter
\newcommand\processAsterisk{%
  \ifnum\lst@mode=\lst@Pmode\relax%
    \textcolor{red}{*}%
  \else
    *%
  \fi
}
\makeatother

\lstset{
  basicstyle=\ttfamily,
  literate={*}{\processAsterisk}1,
  morecomment=[l]\%,
}

\begin{document}
\begin{lstlisting}
% \usepackage{mhchem}

\begin{equation*}
  \label{eq:chem}
  \ce{CaCO3(s) <=>[H2O] 
  Ca2+(aq) + CO3^{2-}(aq)} % 2 * 2 == 4
\end{equation*}
%
Some text
\end{lstlisting}
\end{document}

Answer 2

Another approach could be with moredelim key:

\documentclass{article}

\usepackage{listings,color}
\lstset{basicstyle=\ttfamily}
\lstdefinestyle{star}{%
  moredelim=[is][\itshape\color{red}]{|}{|}}

\begin{document}

\begin{lstlisting}[style=star]
% \usepackage{mhchem}

\begin{equation|*|}
  \label{eq:chem}
  \ce{CaCO3(s) <=>[H2O] 
  Ca2+(aq) + CO3^{2-}(aq)}
\end{equation|*|}
%
Some text
\end{lstlisting}

\end{document}