[Tex/LaTex] How to make a real backslash (escape) character


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!)

Best Answer

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


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


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`!=`\ %
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