Switching a square-bracket-paired-delimiter command to \DeclaredPairedDelimiter

Question

I'm trying to use \DeclarePairedDelimiter instead of my old roll-your-own commands to the same effect. Well, this:

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\sparen{\left[}{\right]}
\begin{document}
Hello, world.
$\sparen{123}$.
\end{document}

doesn't compile:

! Extra }, or forgotten \right.
\MT_delim_sparen_nostar_wrapper:nnn ...thopen {#1}
                                                  #2\mathclose {#3}
l.6 $\sparen{123}
                 $.
? X

Why?

Answer 1

Remove the \left and \right. \sparen* will do the resizing.

Code:

\documentclass{article}
\usepackage{mathtools}
%\DeclarePairedDelimiter\sparen{\left[}{\right]}
\DeclarePairedDelimiter\sparen{[}{]}
\begin{document}
Hello, world.
$\sparen{12\displaystyle\frac{1}{2}3}$
$\sparen*{12\displaystyle\frac{1}{2}3}$.
\end{document}