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2

I use _ character often in text mode, so I changed category of underscore: \catcode\_=12, but that applies also to mathmode and now I'm unable to make subscripts with underscore in mathmode. Is there a way to make latex interpret underscore in text mode as the underscore character, and in mathmode as subscript?

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7  
Check the underscore package. – Ulrike Fischer Jul 9 '12 at 11:23

3 Answers

up vote 12 down vote accepted

You can make _ math active:

\AtBeginDocument{
  \catcode`_=12
  \begingroup\lccode`~=`_
  \lowercase{\endgroup\let~}\sb
  \mathcode`_="8000
}

MWE:

\documentclass{article}
\usepackage[T1]{fontenc}

\AtBeginDocument{
  \catcode`_=12
  \begingroup\lccode`~=`_
  \lowercase{\endgroup\let~}\sb
  \mathcode`_="8000
}

\begin{document}
a_b $a_b$
\end{document}

enter image description here

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What are the advantages of this code over my more simplistic solution (below)? – JLDiaz Jul 9 '12 at 11:29
2  
This code is in general more robust: in \write operations the underscore will be written as is. Active characters may do surprising things, at times; math active characters, on the contrary, assume the "active aspect" only when typesetting math, so well down TeX's digestive process. – egreg Jul 9 '12 at 11:34
@egreg: I think you have a spurious \def\mathunderscore in your first code snippet. – Bruno Le Floch Jul 9 '12 at 12:10
@JLDiaz: I don't know where to paste your answer. This solution worked after pasting after usepackage but before begindocument – qwe Jul 16 '12 at 7:29
up vote 6 down vote 
\catcode`\_=\active
\def_{\relax\ifmmode\sb \else \_\fi}
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Where do I put this? Just after \catcode\_=12 ? – qwe Jul 9 '12 at 11:08
1  
Better to do it instead of making it catcode 12. – David Carlisle Jul 9 '12 at 11:10
up vote 7 down vote

FWIW, in ConTeXt the special characters _ and ^ do not need to be escaped in text mode and work correctly in math mode.

ConTeXt table and math alignment macros do not use & as an alignment marker, so & also does not need to be escaped in text mode.

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