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Date:         Mon, 7 Dec 2009 09:37:53 +0000
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Rescanning an argument with a special catcode for the colon
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

On 03/12/2009 18:02, Julien RIVAUD (_FrnchFrgg_) wrote:
> Note that those characters would be set active only in the call of the
> function; outside of it the catcode is preserved (or so I think). In
> fact, in my first version of the \chemin macro, I did some kind of
> replace, but without expl3 that meant an ugly thing with a lot of ad-hoc
> looping code. Won't search/replace be slower than setting . and : to
> macros ? Anyway, the question here is more to play with expl3 than
> anything else, getting rid of chemin as it is now isn't a priority at
> all... :)

As Will said, there will be a speed hit but not hopefully too much. 
\tl_replace_all_in does not loop over every token in the input, so it's 
not actually too bad. More importantly, I found with siunitx that it is 
still possible to get a "race" condition even under the circumstances 
you describe. If you try siunitx v1 with htlatex, things go wrong with 
superscripts. This is fixed in v2 by not using active characters.

> I think I have tried such solutions, wouldn't TeX complain about
> nonexistent \char_make_active because a colon cannot anymore be in the
> name of a macro ?

Was forgetting about category code of ":". Fixed examples:

\group_begin:
   \char_set_lccode:nn { `\@ } { `\: }
\tl_to_lowercase:n {
   \group_end:
   \cs_set:Nn \my_int_function:n #1 {
     \tl_set:Nn \l_my_tmp_tl {#1}
     \tl_replace_all_in:Nnn \l_my_tmp_tl { . } { \cheminsommet }
     \tl_replace_all_in:Nnn \l_my_tmp_tl { @ } { \cheminface   }
    % Do more stuff with the input
   }

\group_begin:
   \char_set_lccode:nn { `\@ } { `\: }
   \char_make_active:N \@
   \char_make_active:N \.
\tl_to_lowercase:n {
   \group_end:
   \cs_set:Npn \my_int_function:n #1 {
     \tl_set_rescan:Nnx \l_my_tmp_tl
       {
         \char_make_active:N \@
         \char_make_active:N \.
         \cs_set_eq:NN . \cheminsommet
         \cs_set_eq:NN @ \cheminface
       }
       {#1}
     % Stuff with \l_my_tmp_tl
   }
--
Joseph Wright