\documentstyle{article}
% modify index file with control sequences and special characters
\begin{document}
\def \filename {arabdoc}

{\catcode `\|=0 \catcode `\\=12 |gdef |B{\}}

\def \do #1{\catcode`#1=12}

\def \indexentry {% build new index entry
\begingroup \dospecials \catcode`\{=1 \catcode`\}=2 \idxentry }

\def \idxentry #1#2{% build new index entry
\immediate \write \newfile
{\string \indexentry {\process #1!?}{#2}}\endgroup }

\def \process #1#2!#3?{% control sequences verbatim
\if #1\B.#2@\string \verb/#1#2/\else
\ifx #1-#2@\string \verb/#1#2/\else
\if #1_#2@#1#2\else
\ifx #1'#2@#1#2\else
\ifx #1`#2@#1#2\else
\ifx #1,#2@#1#2\else
\if #1\noexpand ~#2@#1#2\else
\ifx #1.\processcs #2!#3?\else
#1#2%
\fi\fi\fi\fi\fi\fi\fi\fi
\ifx \relax #3\relax \else !\process #3?\fi }

\def \processcs #1#2!#3?{% control sequence after dot
\if #1\B#2@#1#2\else .#1#2\fi }

\newwrite \newfile
\immediate \openout \newfile =\filename.idn\relax
\typeout {Writing new indexfile \filename.idn}

\input \filename.idx\relax

\end{document}