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Date:         Tue, 25 Aug 2015 07:05:51 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Catcode changes
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Envelope-To: <rainer.schoepf@GMX.NET>
Status: R

On 26/08/2015 00:44, aparsloe wrote:
> (1) I wanted to use \tl_replace_all:Nnn on a token list that might
> contain braced groups. Using \tl_set_rescan:Nnn to change the category
> codes of { and } seemed the most direct way of proceeding. (Then
> resetting the category codes after replacement with an empty setup.)

I see what you mean (I have a similar case in siunitx, for historical
reasons). We'll explore what we can do with this: there are
mapping-based approaches one can imagine that will enter brace groups.
(We do that for \tl_lower_case:n, for example). However, for a general
'search and replace' I'd be slightly wary as the TeX convention that
stuff in brace groups is 'special' is quite prevalent. (Depending on
performance requirements, you might also look at l3regex if you are
doing the replacement only very rarely.)

> (2) Using \fp_to_scientific:n on the result of an l3fp calculation
> produces, say, 6.023e23. I want to write this as 6.023 \times 10^{23},
> but the "e" of 6.023e23 doesn't have its "usual" catcode so
> \tl_replace_once:Nnn doesn't find the "e". (I presume "e" has catcode
> "other" -- I haven't checked.) Hence I rescan 6.023e23 with an empty
> setup and then use \tl_replace_once:Nnn (which now does find the "e").

I see that we don't mention the catcode of "e" here: we should do
(Bruno?). For this particular use case, I'd set up your search using a
detokenized e

    \cs_generate_variant:Nn \tl_replace_all:Nnn { Nx }
    \tl_replace_all:Nnn <input> { \tl_to_str:n { e } } { \times }

but probably with a dedicated function

    \cs_new:Npn \__mymodule_format:n #1
      { \__mymodule_format:w #1 \q_stop }
    \use:x
     {
       \cs_new:Npn \exp_not:N \__mymodule_format:w
          ##1 \tl_to_str:n { e } ##2 \exp_not:N \q_stop
     }
     { \exp_not:n { #1 \times 10 ^ {##2} } }

which can be made more 'robust' if you need to allow for cases where
there is no "e".
--
Joseph Wright