[Tex/LaTex] Enumerate list – numbers with prefix

#enumeratelists

How to make enumerate list format numbers like this:

REG/001 (content)

REG/002 (content)

REG/003 (content)

I already managed to do the following:

\begin{enumerate}[label=\textbf{REG/\arabic*},leftmargin=*]
\item content
\end{enumerate}

using

\usepackage{enumitem}

but I don't know how to add leading zeros.

Best Answer

\documentclass{article}
\usepackage{enumitem}

\def\threedigits#1{%
  \ifnum#1<100 0\fi
  \ifnum#1<10 0\fi
  \number#1}

\begin{document}

\begin{enumerate}[label={\textbf{REG/\protect\threedigits{\theenumi}}},leftmargin=*]
\item content
\item content
\item content
\end{enumerate}

\end{document}

enter image description here

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