[texhax] What's wrong with \catcode`\_=12

[Yue Wu]

On Thu, Oct 06, 2011 at 12:06:08PM +0200, Heiko Oberdiek wrote:
> On Thu, Oct 06, 2011 at 10:52:43AM +0800, Yue Wu wrote:
> 
> > On Thu, Oct 06, 2011 at 02:53:00AM +0200, Heiko Oberdiek wrote:
> > > On Thu, Oct 06, 2011 at 08:29:21AM +0800, Yue Wu wrote:
> > > 
> > > > I want to define a macro to let _ can be directly used in it but failed,
> > > > The minimum failed example:
> > > > 
> > > > \def\mytest#1{{\chardef\_=95 \catcode`\_=12 #1}}
> > > > \mytest{foo_bar}
> 
> > > What's the purpose of "\chardef\_=95"?
> > 
> > 
> > I want the char `_' to produce a literal `_' as it is, but with the
> > macros following, it will produce a `??'.
> > 
> > 
> > > That "\_" prints as "_"?
> > > 
> > > \def\mytest{%
> > >   \begingroup
> > >   \catcode`\_=12 %
> > >   \mytesti
> > > }
> > > \def\mytesti#1{%
> > >   \chardef\_=95 %
> > >   #1%
> > >   \endgroup
> > > }
> > > \mytest{foo_bar\_bla}
> 
> The `_' and `\_' both print the character 95 of the current font
> in the current font encoding. Font encoding T1 works:
> 
>   \font\eclmr=ec-lmr10
>   \eclmr
> 
> In OT1 is different, because OT1 is not consistent.
> Typewriter (cmtt) does have the underscore in that position:
> 
>   \tt
> 
> LaTeX users can use \textunderscore:
> 
> \documentclass{article}
> 
> \begingroup
>   \catcode`\_=\active
>   \makeatletter
> \@firstofone{%
>   \endgroup
>   \newcommand*{\mytest}{%
>     \begingroup
>     \catcode`\_=\active
>     \mytest at i
>   }%
>   \newcommand*{\mytest at i}[1]{%
>     \def_{\textunderscore}%
>     \def\_{\textunderscore}%
>     #1%
>     \endgroup
>   }%
> }
> 
> \begin{document}
> \mytest{foo_bar\_bla}
> \end{document}
> 

Thank you for clear explanation, I got it :)

-- 
Regards,
Yue Wu

Key Laboratory of Modern Chinese Medicines
Department of Traditional Chinese Medicine
China Pharmaceutical University(中國藥科大學)
No.24, Tongjia Xiang Street, Nanjing 210009, China