new environment for tikz matrix of nodes

Question

I am trying to write a new environment that takes a matrix and draws it as a matrix of nodes in tikz. This is a minimal example:

\documentclass[a4aper,pagesize]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usetikzlibrary{matrix}

\newenvironment{diagram}{
    \begin{tikzpicture}
        \matrix (m) [matrix of math nodes]
}{
\end{tikzpicture}
}

\begin{document}
    \begin{diagram}
        {
         A & B & C \\
         D & E & F \\
        };
    \end{diagram}
\end{document}

However I'd like to be able to write the diagram environment as one would write a tabular and dismiss the { ...}; used inside the environment.

I have tried using a \begingroup ... \endgroup;, but i think tikz really wants the curly braces and I do not know how to put those in the newenvironment definition without messing up my syntax.

I guess another alternative would be to use a \newcommand, but then I will have to use an ampersand replacement character, which I do not want either.

Here is a typical picture that should be generated:

Answer 1

You can grab the environment body but arrange & has the right catcode.

\documentclass[a4aper,pagesize]{scrartcl}
%\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usetikzlibrary{matrix}

\NewDocumentEnvironment{diagram}{b}{%
    \begin{tikzpicture}%
     \matrix (m) [matrix of math nodes]{#1};%
     \end{tikzpicture}%
}{}

\AddToHook{env/diagram/begin}{\catcode`\&\active}
\begin{document}
    \begin{diagram}
         AA & B & C \\
         D & E & F \\
    \end{diagram}
\end{document}

---

The above uses a current LaTeX, if you have an old LaTeX similar functionality is available in packages. This is tested with an Overleaf texlive 2017 image

\documentclass[a4aper,pagesize]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}

\usetikzlibrary{matrix}

\usepackage{environ}
\usepackage{etoolbox}

\NewEnviron{diagram}{%
    \begin{tikzpicture}%
     \matrix (m) [matrix of math nodes]{\BODY};%
     \end{tikzpicture}%
}

\AtBeginEnvironment{diagram}{\catcode`\&\active}
\begin{document}
    \begin{diagram}
         AA & B & C \\
         D & E & F \\
    \end{diagram}
\end{document}