Define a command so that it is only active within the document environment

Question

I would like to configure a custom command so that it only affects the document environment. Because the command changes a number of catcodes, it has to be included after the begin{document} tag in order to work properly. However, I would prefer to set it up so that I can include it in its own .sty file. Is there some way to define a command so that it is only active within the document environment?

The type of code that I am trying to use is similar to the following:

\AtBeginDocument{
    \catcode`\_=13  \def_#1_{\emph{#1}}
}

When I typeset the document with the \AtBeginDocument command, the console stops at the \begin{document} line and notes the following:

Missing control sequence inserted.
<inserted text>
    \inaccessible

If I include the code (without the \AtBeginDocument{} rule) after the \begin{document} command, however, the document will process correctly.

score 7 · Answer 1 · answered Oct 23 '12 at 07:47

The code you propose

\AtBeginDocument{
    \catcode`\_=13  \def_#1_{\emph{#1}}
}

can't work, because the _ is read as an argument to \AtBeginDocument and so its category code is frozen.

You can do it in an indirect way:

\AtBeginDocument{
  \begingroup\lccode`~=`_
  \lowercase{\endgroup\def~#1~}{\emph{#1}}%
  \catcode`_=\active
}

This works because ~ is active, so the \lowercase instruction will produce an active _. The \endgroup will revert the \lccode assignment, but when the change to the lowercase counterpart has already been performed.

score 6 · Accepted Answer · edited Jun 10 '20 at 12:32

You can use \AtBeginDocument to execute code at the start of the \document environment. If you attempt to use \SomeNewCommand before \begin{document} you get an error.

References:

My second question on this site How to automatically add text immediately after \begin{document} has some explanation.

Code:

\documentclass{article}
\AtBeginDocument{%
    \newcommand*{\SomeNewCommand}{The command SomeNewCommand is now defined.}%
}
%\SomeNewCommand% Undefined
\begin{document}
    \SomeNewCommand
\end{document}

Define a command so that it is only active within the document environment

2 Answers2

References:

Code:

Linked