How to make a real backslash (escape) character?

Question

I just want a good way to make a backslash character (or whatever character(s) any given format uses as escape characters), suitable to \write to a file or pass to a pdfTeX primitive.

Let me emphasize that getting it to typeset is not the point of the question. In fact, if using cmr10/11/12, what I want would not be usable to typeset a backslash, since these fonts use that slot for something else for some reason.

An acceptable answer will say how to do this in general, not just in LaTeX, though format-specific techniques are also of interest (and will be upvoted).

Bonus points for explaining how to do this for all characters with special catcodes. (Not literal bonus points, only figurative ones. At least, not from me; I only have 296 rep at the moment!)

Answer 1

Another way to do this kind of thing is to use a \lccode trick:

\begingroup\lccode`!=`\\\lowercase{\endgroup\def\@backslashchar{!}}

This avoids any need to change catcodes thanks to the special properties of \lowercase. Compared to the \expandafter\@gobble\string method, it's a bit more flexible as you can choose the catcode of the resulting character inside your macro. Compared to a change of catcodes, you don't have to use \global because of how \lowercase interacts with \endgroup.

Here's the detail of the code. The \begingroup is here to keep the lccode changes local. The

\lccode`!=`\\

means that when lowercasing ! you will get the backslash \ (the choice of ! is arbitrary, you just need a normal character which will not appear elsewhere inside the \lowercase). The important thing is that the lowercase version of ! wille be a normal character since \lowercase doesn't change catcodes.

The code \lowercase{\endgroup\def\@backslashchar{!}} is thus equivalent to \endgroup\def\@backslashchar{\} but with \ not being special (so \ followed by } won't be interpreted as \} but as two separate entities).

At the end, the result is that you have defined \@backslashchar to be a \ with normal catcode. It can thus be used inside a \write command without causing the same problems as \ does.

This lccode trick works with all other special characters. For example if you want a space character (of course, for spaces, the macro \space works fine):

\begingroup\lccode`!=`\ \lowercase{\endgroup\def\@spacechar{!}}

If ever you break the code on two lines, just be careful with the end of lines and put a % after the \:

\begingroup\lccode`!=`\ %
\lowercase{\endgroup\def\@spacechar{!}}

Answer 2

There are many ways to do it depending on what you want to do. You don't always need to change the catcode. Some examples:

\textbackslash will simply typeset \

For writing a macro to a file use \string\macro. This can also be used to just typeset it, also.

You can also define it by using:

\newcommand{\bs}{\symbol{'134}} % a backslash in tt type in OT1/T1

The \lccode trick and changing \catcode has been explained by the other posters and I haven't added it.

And a minimal with these:

\documentclass[11pt]{article} 
\begin{document}
\newcommand{\bs}{\symbol{'134}}% a backslash in tt type in OT1/T1
\newcommand{\BS}{\texttt{\symbol{`\\}}}

\BS, \texttt{\string\test}, \texttt{\bs}
\end{document}

Answer 3

Here's one possible way with changing catcodes (just saw that TH. proposed almost the same in this answer):

\begingroup
\catcode`\@=0
\catcode`\\=11
@global@def@backslash{\}
@endgroup

Note that I first give @ catcode 0 so that I can use it in place of \. For other special characters it's in fact easier; you don't need the @.

As Philippe pointed out, the drawback of the above approach is the use of \global. Here's another nice trick that avoids this:

\begingroup
\catcode`\@=0
\catcode`\\=11
@def@makeliteralbackslash{@def@backslash{\}}
@expandafter@endgroup@makeliteralbackslash

The macro @makeliteralbackslash is expanded before the end of the group; one expansion happens while the catcode changes are still active. The definition of \backslash itself then is executed outside the group. I'm not claiming that this is the best possible approach: It's just to give an example of interesting TeX core tricks.

Answer 4

LaTeX core appears to do this:

\catcode`\@=11
...
\long\def\@gobble#1{}
\edef\@backslashchar{\expandafter\@gobble\string\\}

I don't properly understand how this works, but it does seem to do what you want in \write.

Answer 5

expl3 solutions, for future references in case anyone come across this.

\iow_char:N \\

Semantically, it makes sense to use an iow function for the purpose of writing; although there's no guarantee on the number of expansion steps etc. or that you actually get a catcode-other backslash token.

(i.e. technically catcode letter/alignment/math superscript/math subscript works for \write as well, although it would be a rather weird choice to do that)

\cs_to_str:N \\

In practice, (at least in my version) \iow_char:N is identical to \cs_to_str:N. This expands to the result in 2 expansion steps.

\char_generate:nn {`\\} {12}

Again, guarantees to expand to a catcode-other backslash token in 2 expansion steps.