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I am not very familiar with TeX's category codes. If I use

\catcode`\|=13 \renewcommand{|}{hello world}

then every use of | in my document will be replaced by hello world. However, how would I be able to still print the | sign? Of course, the circular reference

\catcode`\|=13 \renewcommand{|}{hello|world}

does not work (exceeding TeX's majestic capacity), although I would like all instances of | to print as hello|world. And, if I'm correct, the above would just hold for regular text mode. How would the above code change if | was to be used in math mode?

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1 Answer 1

up vote 11 down vote accepted

There are usually two approaches taken to solve this problem.


Save the character with original catcode in a macro before its catcode is changed. This works because catcodes are assigned at input time.


\renewcommand{|}{hello\mypipe world}





Force the catcode of | to 12 with \string:






Math mode

A character can also be only active in math mode, that is, its mathcode can be "8000. To get the original meaning in this case, one can define a control sequence to be a synonym for the previous mathcode of the character:


Look at the implementation of the icomma package for an example.

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