2012/09/17 の解答（！？）

length.tex

%#! lualatex
\documentclass[a4paper]{article}
\def\length#1{%
  \directlua{\unexpanded{
    require('lualibs-unicode')
    local input = [[#1]]
    local state = 'r'
    % print(input)
    for i = 1, utf.len(input) do
      local r = utf.byte(utf.sub(input, i, i))
      if state == 'r' then
        tex.write('*')
        if tex.getcatcode(r) == 0 then state = 'csb' end
      elseif state == 'csb' then
        if tex.getcatcode(r) == 11 then state = 'cs' else state = 'r' end
      elseif state == 'cs' then
        if tex.getcatcode(r) == 10 then
          state = 'r'
        elseif tex.getcatcode(r) ~= 11 then
          tex.write('*')
          state = 'r'
        end
      end
    end
  }}}
\def\starxiii{*************}
\def\starxi{***********}
\begin{document}
\edef\result{\length{ \a {b\cd ef{g} }\h }}
\ifx\result\starxiii OK\else NG\fi

\edef\result{\length{{ }{ {}} {}}}
\ifx\result\starxi OK\else NG\fi
\end{document}

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