\documentclass{article}
\usepackage{tabularx}
\usepackage{array}

\newcolumntype\vcenter[1]{>{$\vcenter\bgroup\hbox\bgroup}#1<{\egroup\egroup$}}
\newcolumntype{d}[1]{>{\rightdots{#1}}r<{\endrightdots}}

\begin{document}

\begin{tabular}{*{3}{>{$}c<{$}}}
  \hline
  Hello world !\tabularnewline
  \hline
\end{tabular}

\begin{tabular}{l*{4}{m{1cm}}}
  \hline
  Hello world !\tabularnewline
  \hline
\end{tabular}

% Array specific math separators
\begin{tabular}[ht]{|@{.}>{\centering$}m{1cm}<{$}|}
  \hline
  Hello world !\tabularnewline
  \hline
\end{tabular}

% Array environment also supports similar constructs
\[
  \mathrm{a} =
  \begin{array}{l*{4}{m{1cm}}r}
    \mathrm{b} & bar & baz & a & b & 2\\
    foo & bar & baz & a & b & 2\\
  \end{array}
\]

\end{document}