mplib utf8 substring

utf8substr.tex

\documentclass{article}
\usepackage{luamplib}
\begin{document}
\directlua{
  function luamplib.utf8substr (t,b,e)
    local ob = utf8.offset(t,b+1)
    local oe = utf8.offset(t,e)
    local ms = utf8.char(utf8.codepoint(t,ob,oe)):gsub('"','"&ditto&"')
    return string.format('"\csstring\%s"', ms)
  end
}
\mplibtextextlabel{true}
\begin{mplibcode}
  def utfoolength primary t =
    runscript("return utf8.len[==["&t&"]==]")
  enddef;
  def utfoosubstring expr p of t =
    runscript("return luamplib.utf8substr([==["&t&"]==],"&decimal xpart p&","&decimal ypart p&")")
  enddef;
  beginfig(1);
    string Test;
    Test="AaeéB";
    for k=0 upto utfoolength(Test)-2:
      label.top(utfoosubstring (k,k+2) of Test, (20k,0));
    endfor;
  endfig;
\end{mplibcode}
\end{document}