Underscore should not force mathmode in macro

Question

When trying to color words in arabic (transliteration), LaTeX wants me to be in mathmode (Missing $ inserted.), as there is an underscore in the transliteration. This is of course completely wrong as it is not math rather than arabxetex’s convention for some arabic letters.

The problem only occurs if I try to color it using a macro, the version using the \color command directly works without any problems. However, to me the two are seemingly identical, yet to LaTeX they obviously are not.

If I force the following code (ignoring all errors), I get this output:

The first line is “Try No. 1” and the second one “Try No. 2” and how it should actually look. I suspect the problem might lie in my macro but I fail to see where the error is.

% !TEX TS-program = xelatex
% !TEX encoding = UTF-8 Unicode

\documentclass{scrartcl}

\usepackage{xcolor}

\usepackage{fontspec,xltxtra,xunicode}
\setromanfont[Mapping=tex-text]{Linux Libertine O}
\setsansfont[Mapping=tex-text]{Linux Biolinum O}
\newfontfamily\arabicfont[Script=Arabic,Scale=1.5]{Scheherazade}

\usepackage{arabxetex}
\SetTranslitConvention{dmg}

\newcommand*{\place}[1]{{\color{cyan} #1}}

\begin{document}

\place{\textarab[trans]{\UC ya_trib}}% Try No. 1
{\color{cyan} \textarab[trans]{\UC ya_trib}}% Try No. 2

\end{document}

Answer 1

The \textarab command changes internally the category code of _, so that it's not any more the subscript character for math formulas, but only in the argument of \textarab. However, this change is not any more possible if the argument has already been read (in this case as argument to \place).

You have two strategies available.

Strategy 1

\begin{document}
\catcode`_=12

If you have little or no math in your document it is the simplest one; for an occasional subscript in math you can use \sb instead of _.

Strategy 2

\DeclareRobustCommand{\place}{{\color{cyan}\scantokens{#1\noexpand}}}

Here \place will make XeTeX reread the argument so that the change in the category code is performed correctly.