How to redefine commands and environments, and make them acceptable for `\write`

Question

My general problem is the following. I want to "convert" many .tex files into some other format (xml-like). I don't want to modify the tex files, I can only build a documentclass. The texfiles are organized in blocs ( i will call them content-blocs), through commands and environment, for example, roughly:

\begin{document}
\content{blablabla}
\titre{joli titre}
\explanation{i muss explain that...}
\begin{definition}
   here some nice text
\end{definition}
....
\end{document}

My present method, is to capture the content of the commands or environments and use \write to write them in a file:

\newwrite\tempfile
\immediate\openout\tempfile="test.xml"

For example (the following is defined in the documentclass used by the tex file):

\newcommand{\content}[1]{
  \immediate\write\tempfile{<content>}
  \immediate\write\tempfile{\content}
  \immediate\write\tempfile{</content>}
}

The complication comes from the fact, that the commands and environments in \content may contain more than text, e.g other macros. For example :

\content{
  some blablabla  \textbf{important} etc etc 
}

\begin{definition}
   consider the following points :
   \begin{quote}
      quote quote quote 
   \end{quote}
   and so on, and so on...
\end{definition}

In this form, this gives a compilation error, because \write can't write this into the file. On the other hand, I know the exhaustive list of commands/environments that can appear in the "content-blocs". I though about capturing them, through a redefinition. For example :

\renewcommand{\textbf}[1]{<bold>#1</bold>}

works, the command \content will produces in the file "test.xml" a line

some blablabla  <bold>important</bold> etc etc

Problem 1: This strategy does not work for environments. My attempt

\renewenvironment{quote}{<quote>}{</quote>}

fails miserably.

Problem 2: I would need this in particular for the itemize environment (instead of quote). I know that redefining \item may yield some problems

Problem 3: In the content-blocks, I have sometimes commands with optional arguments, eg :

\content{bla bla \com[opt]{AAA} bla bla}

Here again, it fails, because commands with optional arguments are not fully expandable. As workaround, i can redefine the command \com with \DeclareExpandableDocumentCommand from the package xparse.

\DeclareExpandableDocumentCommand\expandCom{ o m} {<com : #1>#2</com>}

It seems to solve my problem for commands with optional arguments, but I would need the same for environments with optional arguments.

Problem 4: I need to treat the content of the content-blocs, in order to extract information. For this i use in particular \edef, and i need that the macros (\com, \texbf, and so on...) expand correctly in a edef. But i guess that it is exactely the same problem like with \write. For both \edef and \write one needs fully expandable macros inside, isn't it ?

EDIT1 Concerning Bruno's question. I think that David's solution is somehow secure, because it does not redefine the existing commands/environments, but it rather creates new ones (\mwx\begin, etc...).

Nevertheless, following Bruno's suggestion, i try to simply redefine the command \begin and \end for the environments i need (quote and definition, in this example). My attempt fails nevertheless. It might be because of \ifthenelse, but on the other hand, i never feel comfortable with \ifx :

\documentclass{article}
\usepackage{ifthen}

\let\originalbegin\begin
\let\originalend\end
\def\begin#1{%
  \ifthenelse{\equal{#1}{quote}}{<quote:special def for quote>}{
    \ifthenelse{\equal{#1}{definition}}{<DEF: another definition>}{\originalbegin{#1}
    }
  } 
}
\def\end#1{%
  \ifthenelse{\equal{#1}{quote}}{</quote>}{
    \ifthenelse{\equal{#1}{definition}}{</DEF>}{\originalend{#1}
    }
  }
}
\newwrite\tempfile
\immediate\openout\tempfile="nico.xml"
\begin{document}

\def\content{
some blablabla  etc etc 
  \begin{definition}
     consider the following points :
      \begin{quote}
        quote quote quote 
      \end{quote}
     and so on, and so on...
  \end{definition}
}
\immediate\write\tempfile{\content}%
\immediate\closeout\tempfile
\end{document}

Answer 1

Something like this perhaps (plain TeX writing to the terminal, but could be adapted)

\long\def\mywrite{%
\begingroup
\obeyspaces\mywritey}

\long\def\mywritey#1{%
\immediate\write20{\mywritex#1\endmywritex}%
\endgroup}

\long\def\mywritex#1{%
  \expandafter\ifx\csname mwx\string#1\endcsname\relax
    \string#1%
   \expandafter\mywritex
  \else
  \csname mwx\string#1\expandafter\endcsname
  \fi}

\newlinechar=`^^J
\long\def\mywritedef#1{\expandafter\def\csname mwx\string#1\endcsname}
\mywritedef\endmywritex{}
\mywritedef\par{^^J<p/>^^J\mywritex}
\mywritedef\begin#1{<\mywritex{#1}\endmywritex>\mywritex}
\mywritedef\end#1{</\mywritex{#1}\endmywritex>\mywritex}
\mywritedef\textbf#1{<bold>\mywritex#1\endmywritex</bold>\mywritex}

\mywritedef{definition}{DEF\mywritex}

\mywrite{

  some blablabla  \textbf{important} etc etc 

\begin{definition}
   consider the following points :
   \begin{quote}
      quote quote quote 
   \end{quote}
   and so on, and so on...
\end{definition}


}

\bye

It writes out:

$ tex write.tex
This is TeX, Version 3.1415926 (TeX Live 2011/Cygwin)
(./write.tex

<p/>
  some blablabla  <bold>important</bold> etc etc
<p/>
<DEF>   consider the following points :   <quote>      quote quote quote   </qu
ote>   and so on, and so on...</DEF>
<p/>

<p/>

 )
No pages of output.

Answer 2

Pandoc can translate TeX into html. Presumably with a little work, you could use something like this to do the translation for you? I really think this would be a better solution that trying to have TeX write the files for you.