How did these below the numbers do it?

Question

I saw this format in a paper. This is obviously not a picture. These numbers can be copied and pasted like other text. How is this done? Thanks.

Answer 1

An alternative with fully customizable bracket thickness \brwd, depth \brdp, and sidebearing \brsb, given by \ubracket[<offset>]{<content>}.

\documentclass{article}
\usepackage{stackengine} 
\newcommand\ubracket[2][.5pt]{%
  \sbox0{$#2$}\stackengine{-\brwd}{\stackengine{#1}{$#2$%
  }{\rule{\brwd}{\brdp}\kern\dimexpr\wd0-2\brwd-2\brsb\relax
  \rule{\brwd}{\brdp}}{U}{c}{F}{F}{S}}{%
  \rule{\dimexpr\wd0-2\brsb}{\brwd}}{U}{c}{F}{F}{S}
}
\newlength\brwd% bracket thickness
\brwd=.8pt\relax
\newlength\brdp% bracket depth
\brdp=2.5pt\relax
\newlength\brsb% bracket sidebearing
\brsb=0.5pt\relax
\begin{document}
Inline construction ($\ubracket{10}
\ubracket{01}
\ubracket{01}
\ubracket{00}
\ubracket{01}
\ubracket{10}
\ubracket{01}
\ubracket{10}
\ubracket{11}
$)
\end{document}