Block comments with *% and %*

Question

I know the package comment which allows to use :

\begin{comment}
Lines
to
comment
\end{comment}

This is not very friendly to use. I would like to have the following possibility:

*%
Lines
to
comment
%*

This is similar to the multiline version of comment in JavaScript, C or C++.

This kind of syntax is very useful because I could then use...

Comment

*%
Lines
to
comment
%*

No comment

%*%
Lines
to
comment
%*

Is there a way to use *% and %* instead of \begin{comment} and \end{comment} respectively?

Answer 1

It is possible without LuaTeX, but not in a robust way. I used ~% and %~ instead of * because making * active would break commands which expect * to be of category code other (e.g., \section*{...} would not work anymore).

\begingroup
    \catcode `\@ = 11
    \catcode `\~ = 13
    \catcode `\% = 12
    \protected\long\gdef\cmt@remove#1%~{\endgroup}
    \ifdefined~
        \global\let\cmt@old~
    \else
        \global\let\cmt@old\relax
    \fi
    \protected\gdef~{\begingroup\catcode`%=12
        \futurelet\next\cmt@}
    \protected\gdef\cmt@
      {\ifx%\next
           \expandafter\cmt@remove
       \else
           \endgroup\expandafter\cmt@old
       \fi}
\endgroup

Answer 2

In ConTeXt MkIV, you can do this with the normal comment environment (which is called hiding). The environment stop mark need not be on a line of its own. So, you can use:

\starttext
Normal
\starthiding                                                                                                                              
  Hidden
%\stophiding
\stoptext

If you comment the line starting with \starthiding, the hidden content will be shown.

But if you want to include and exclude environments, it is better to define a new comment environment and use \includecomment and \excludecomment to change its behavior.