How to make a usual latex installation compile the following non-standard array-syntax?

Question

Once I seem to have worked on a system where, for some reason, latex code containing

     $\array{a & b \\ c & d }$

seems to have worked. (Note that, in particular, the "alignment" options of the {rcrcr} kind, standard in the usual \begin{align} \end{align} construction, is missing here. Now I need to compile this on what is a more or less standard latex installation, and it does not compile as is. Of course, one option would be to edit everything into the standard \begin{align} \end{align}.

Question: can you recommend an alternative, so as to be able to compile such code with as little editing overhead as possible? Some package or option in the preamble?

Answer 1

If you don't want to change the syntax at all, just [re]define \array, but I would not do that, as array is a widely used beginning to the array environment of LaTeX. I would rename it as \myarray, which can be defined to be an aligned TABstack.

\documentclass[12pt]{article}
\usepackage{tabstackengine}
\newcommand\myarray{\alignCenterstack}
\TABstackMath
\setstackgap{L}{1.2\normalbaselineskip}% inter-row baselineskip
\setstackaligngap{1em}% horizontal gap between align groups
\begin{document}
$\myarray{a =& b \\ c =& d }$\quad
$\myarray{y =& mx + b \\ E =& mc^2 }$ or this
$\myarray{a =& b & y =& mx + b\\ c =& d& E =& mc^2 }$
\end{document}

Answer 2

As @egreg has already pointed out in a comment, you may be thinking of the \matrix command. For sure, the following code, which sets up a macro called \myarray, compiles properly.

\documentclass{article}
\usepackage{amsmath} % for 'matrix' environment
\newcommand\myarray[1]{\begin{matrix}#1\end{matrix}}
\begin{document}
$\myarray{a & b \\ c & d }$
\end{document}