\documentclass{article}
\usepackage{amsmath}
\input{macros.sty}
\begin{document}

$$\mat{1,2,3;4,5,6}$$

$$ \begin{bmatrix} \mathbf{x} & 0 & 0 \\ 0 & 1 & 2 \end{bmatrix} $$

% $$\mat{\mathbf{x},0,0;0,1,2}$$
\end{document}
