\documentclass{book}
\pagestyle{empty}

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~\a~\b~\c~\d~\e~\f~\g~\h~\i~\j\aftergroup\E\xdef\B{}\xdef\E{}}
\def~#1#2{\mathcode`#1"8000\lccode`~`#1\lowercase{\def~{\xdef#2{#2#1}}}}
~0\a~1\b~2\c~3\d~4\e~5\f~6\g~7\h~8\i~9\j\mathcode`-"8000\lccode`~`-
\lowercase{\def~{\xdef\B{\beginR}\xdef\E{\endR}}}

\begin{document}
  $52146729$

  $-52146729$ % reversed ordering

  $277981468517523153773703292073191466142223027369188752287219582183$
\end{document}