%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 04-11-5 auf Seite 65.
%%  Copyright (C) 2018 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[10pt]{screxa}
\pagestyle{empty}
\setlength\textwidth{146.40707pt}
\makeatletter
\def\@bracketheight{2pt}
\def\underbracket{%
  \@ifnextchar[{\@underbracket}{\@underbracket[\@bracketheight]}}
\def\@underbracket[#1]{%
  \@ifnextchar[{\@under@bracket[#1]}{\@under@bracket[#1][0.4em]}}
\def\@under@bracket[#1][#2]#3{%\message {Underbracket: #1,#2,#3}
  \mathop{\vtop{\m@th\ialign{##\crcr $\hfil \displaystyle {#3}\hfil $%
  \crcr\noalign{\kern 3\p@ \nointerlineskip }\upbracketfill {#1}{#2}
  \crcr\noalign{\kern 3\p@ }}}}\limits}
\def\upbracketfill#1#2{$\m@th \setbox \z@ \hbox {$\braceld$}
  \edef\@bracketheight{\the\ht\z@}\bracketend{#1}{#2}
  \leaders \vrule \@height #1 \@depth \z@ \hfill
  \leaders \vrule \@height #1 \@depth \z@ \hfill \bracketend{#1}{#2}$}
\def\bracketend#1#2{\vrule height #2 width #1\relax} %$
\makeatother
\AtBeginDocument{\setlength\parindent{0pt}}
%StartShownPreambleCommands

%StopShownPreambleCommands
\begin{document}
$\underbracket{\mbox{foo~bar}}$
$\underbracket[0.1pt]{\mbox{foo~bar}}$
$\underbracket[2pt][1em]{\mbox{foo~bar}}$
\end{document}