I would like to detect that an argument contains something else than a letter. Here is a basic example of use (in reality this would be to produce the expansion of 2x2 determinant).

```
\documentclass[12pt]{article}

\newcommand\doubleit[1]{%
    2\,#1%
}

\begin{document}

\doubleit{x}    % --> 2 x       [OUTPUT EXPECTED]

\doubleit{-x}   % --> 2(-x)     [OUTPUT EXPECTED]

\doubleit{x-3}  % --> 2(x - 3)  [OUTPUT EXPECTED]

\end{document}

```