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From: marcel@cs.caltech.edu (Marcel van der Goot)
Newsgroups: comp.text.tex
Subject: Re: how to make "_" non-special?
Summary: make it active
Keywords: \active, \_, underscore, subscript
Message-ID: <1991Jan16.220735.17896@nntp-server.caltech.edu>
Date: 16 Jan 91 22:07:35 GMT
References: <8763@star.cs.vu.nl>
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Organization: California Institute of Technology (CS dept)
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In <8763@star.cs.vu.nl> Fred Douglis (douglis@cs.vu.nl) asks

> Can anyone tell me what magic might be necessary to redefine "_" to
> have its normal meaning?

If you always want to use an _ as just an _, you can say

	\catcode`\_=13
	\let_=\_

(actually, the second line is not necessary, it is already done
in plain.tex --- not documented in the TeXbook)

If you also want to write subscripts now and then, you can make " into
a subscript character by saying

	\catcode`\"=13

Then you can write $x"0$. It disables other usage of " though.

A slightly more sophisticated solution is to forget the above
and instead write

	\catcode`\_=13
	{\catcode`*=8
	 \gdef_{\ifmmode\let\next=*\else\let\next=\_\fi\next}
	}

This makes _ into a normal underscore when used in normal text, but
when you use it in mathmode it is still the underscore character. So
you can write
	some_identifier $= x_0$
but not
	$$some_identifier = x_0$$

Which solution is most suitable depends on your application.

					Marcel van der Goot
					marcel@vlsi.cs.caltech.edu