Partial number parsing in siunitx

Question

I would like the second and third examples to be printed like the first ($\times$ instead of x, spacing, repeated unit). However, in order to make siunitx accept \sqrt{2} or 2^n, I need to turn off number parsing, which turns off formatting of the number part completely.

\documentclass{article}
\usepackage{siunitx}
\begin{document}
    \SI{2 x 2}{\meter}

    \SI[input-symbols=n,parse-numbers=false]{2^n x 2^n}{\meter}

    \SI[parse-numbers=false]{\sqrt{2} x \sqrt{2}}{\meter}
\end{document}

Is there a way to partially parse the number part, so that spacing etc. is as intended and \sqrt{2} or 2^n still accepted?

Answer 1

This works:

\documentclass{article}
\usepackage{siunitx}
\begin{document}
    \SI{2 x 2}{\meter}

    % in some cases, you can use the full expression as an "input symbol"; 
    % however, this looks wrong:
    \SI[input-symbols=2^n]{2^n x 2^n}{\meter}

    % so to fix this, one may try:
    \newcommand{\twon}{2^\mathit{n}}
    \SI[input-symbols=\twon]{\noexpand\twon x \noexpand\twon}{\meter}

    $2^n$ (for comparison)

    % the same trick works here:
    \newcommand{\sqrtwo}{\sqrt{2}}
    \SI[input-symbols=\sqrtwo]{\noexpand\sqrtwo x \noexpand\sqrtwo}{\meter}

    % and this is even a little simpler;
    % although maybe less readable if your expression is longer:
    \SI[input-symbols=\sqrt{2}]{\noexpand\sqrt{2} x \noexpand\sqrt{2}}{\meter}

    $\sqrt{2}$ (for comparison)

\end{document}

Note that the simple fix (such as \SI[input-symbols=2^n]) does not even compile with \usepackage[detect-mode]{siunitx} (or \usepackage[detect-all]{siunitx}).

Also note that with detect-mode, one can use \newcommand{\twon}{2^n} instead of \newcommand{\twon}{2^\mathit{n}} outside math mode; but this may print wrongly in math mode:

\documentclass{article}
\usepackage[detect-mode]{siunitx}
\newcommand{\twon}{2^n}
\begin{document}
    \num[input-symbols=\twon]{\noexpand\twon}

    $\num[input-symbols=\twon]{\noexpand\twon}$
\end{document}