Alignment for nested matrices

Question

I want to get a matrix of arrays, and I cannot make it aligned. I tried blkarray and bmatrix, but none of them works for me. Could somebody help me out? Code below is what I wrote:

a = \left[ 
\begin{array}{c@{}c@{}c}
    \left[\begin{array}{ccc}
      255 & 0 & 0 \\
    \end{array}\right] & \left[\begin{array}{ccc}
    0 & 94.5 & 94.5 \\
    \end{array}\right] \\
    \left[\begin{array}{ccc}
    0 & 99 & 0 \\
    \end{array}\right] & \left[\begin{array}{ccc}
    0 & 0 & 9 \\
    \end{array}\right] \\
\end{array}\right]
\end{equation*}

And it turn out to be misaligned:

Answer 1

With nicematrix package:

\documentclass[margin=3mm, varwidth]{standalone}
\usepackage{nicematrix}

\begin{document}
    \[
a = 
\begin{bNiceArray}{c@{}ccc@{}c c@{}ccc@{}c}
      [ & 255 & 0  & 0 &]   &   [& 0 & 94.5 & 94.5 &]   \\
      [ &   0 & 99 & 0 &]   &   [& 0 & 0    & 9    &]   \\
\end{bNiceArray}
\]
\end{document}

Addendum: By use of \CodeAfter and \SubMatrix (you need nicematrix version 5.10 or newer):

\documentclass[margin=3mm, varwidth]{standalone}
\usepackage{nicematrix}

\begin{document}
\[
a =
\begin{bNiceArray}{ccc@{\hspace{1em}}ccc}[margin]
      255 & 0  & 0   &   0 & 94.5 & 94.5 \\[1ex]
        0 & 99 & 0   &   0 & 0    & 9    \\
\CodeAfter
\SubMatrix[{1-1}{1-3}] \SubMatrix[{1-4}{1-6}]
\SubMatrix[{2-1}{2-3}] \SubMatrix[{2-4}{2-6}]
\end{bNiceArray}
\]
\end{document}

Answer 2

Just for the sake of variety, here's a solution that employs the machinery of the siunitx package -- specifically, the S column type -- to format the inner array environments. Observe that the numbers are not centered but are aligned on the (explicit or implicit) decimal markers.

\documentclass{article} % or some other suitable document class
\usepackage{amsmath} % for 'bmatrix' env.
\usepackage{siunitx} % for 'S' column type
\newcolumntype{T}[1]{S[table-format=#1]}

\begin{document}
\[
a = \begin{bmatrix} 
    \left[\begin{array}{@{} T{3.0} T{2.0} c @{}} % array #1
       255 & 0 & 0 \\
    \end{array}\right] & 
    \left[\begin{array}{@{} c T{2.1} T{2.1} @{}} % array #2
       0 & 94.5 & 94.5 \\
    \end{array}\right] \\[1.5\jot]
    \left[\begin{array}{@{} T{3.0} T{2.0} c @{}} % same format as array #1
       0 & 99 & 0 \\
    \end{array}\right] & 
    \left[\begin{array}{@{} c T{2.1} T{2.1} @{}} % same format as array #2
       0 & 0 & 9 \\
    \end{array}\right]
\end{bmatrix}
\]
\end{document}

Answer 3

There is another solution without nicematrix, only with plain TeX tools:

$$
  \def\[{[&\kern-.9em } \def\]{\kern-.9em&]}
  \left[\matrix{
      \[ 255 & 0  & 0 \]   &   \[ 0 & 94.5 & 94.5 \]   \cr
      \[   0 & 99 & 0 \]   &   \[ 0 & 0    & 9    \]   \cr
  }\right]
$$

Answer 4

With spalign package with several...adjustments.....

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{spalign}

\begin{document}
\[ a= \spaligndelims{[}{]} 
\spalignmat[c]{\spalignmat[c]{255 0 0} \spalignmat[c]{0 94.5 94.5}\\[-1em]; \spalignmat[c]{\phantom{0}0 99 0} \spalignmat[c]{0\phantom{\mkern1mu 0} 0\phantom{00} \phantom{0}9\phantom{0}}\\[.3em]}
\]

\end{document}