2012/09/17 の解答（！！？）

length.tex

%#! lualatex
\documentclass[a4paper]{article}
\makeatletter
\def\len@begin{\directlua{
    local len_count = 0
    local len_flag = true
    while len_flag do
      local t = token.get_next()
      % print(token.csname_name(t))
      if token.csname_name(t) == 'len@end' then
        len_flag = false
        for i = 1, len_count do
          tex.write('*')
        end
      end
      len_count = len_count + 1
    end
}}
\def\len@end{done!}
\def\length#1{\len@begin#1\len@end}
\makeatother
\begin{document}
\def\check{\ifx\result\checkstr OK\else NG\fi}

\edef\result{\length{ \a {b\cd ef{g} }\h }}
\def\checkstr{*************}
\check

\edef\result{\length{{ }{ {}} {}}}
\def\checkstr{***********}
\check

\escapechar=-1
\edef\result{\length{\expandafter\expandafter\expandafter}}
\escapechar=`\\
\def\checkstr{***}
\check

\edef\result{\expandafter\length\expandafter{\csname ?TeX\endcsname}}
\def\checkstr{*}
\check
\end{document}

http://d.hatena.ne.jp/zrbabbler/20120917/1347856569