[Tex/LaTex] howto convert roman into arabic numeral

Question

I know that there is a command \romannumeral that converts numbers in roman numeral

\romannumeral3 

gives iii

Is there a converse like

\arabicnumeral{x}

that should produce 10?

Answer 1

The etoolbox package has a \rmntonum command:

\documentclass[12pt]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{etoolbox}

\begin{document}

\rmntonum{MMXV}

\rmntonum{mmxvi}

\rmntonum{mMxViI}

\end{document}