\documentclass{minimal}
\usepackage{amsmath}
\usepackage[active,tightpage]{preview}
\setlength\PreviewBorder{1pt}
\usepackage{pgf}
\usepackage{tikz}
\usepgfmodule{plot}
\usepgflibrary{plothandlers}
\usetikzlibrary{shapes.geometric}
\usetikzlibrary{shadings}
\begin{document}
\begin{preview}
%tikz
\begin{tikzpicture}[every node/.style={draw,scale=1pt,minimum width=20pt,inner sep=3pt,line width=0pt,draw=black!20}]
\matrix[row sep=2pt,column sep=2pt]
{\node[fill=black!50]{31};&
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\node[fill=red]{1};&
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\end{tikzpicture}
\end{preview}
\end{document}