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Message-ID:  <CANQYN6wzPW4aeO0WftqthfZ_6szd6-E60OzGhZcZ1g6JM6rseg@mail.gmail.com>
Date:         Wed, 26 Aug 2015 13:34:42 +0200
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From: Bruno Le Floch <blflatex@GMAIL.COM>
Subject: Re: Catcode changes
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <55DDFD0A.4000901@clear.net.nz>
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Envelope-To: <rainer.schoepf@GMX.NET>
Status: R

On 8/26/15, aparsloe <aparsloe@clear.net.nz> wrote:
> On 25/08/2015 1:35 a.m., Joseph Wright wrote:
>> On 25/08/2015 07:05, Joseph Wright wrote:
>>> 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.)
>> It would be useful here to see what your real use case is, as that's
>> often illustrative.
>> --
>> Joseph Wright
> Thank you for the responses. I was remiss. I didn't make it clear that
> looking inside braced groups was not the concern, rather to replace (or
> remove) a list of tokens that might include braced groups. I've been
> reworking the code of a package that turns latex expressions into
> expressions that can be digested by l3fp. In fact, this particular issue
> has now been reworked away, but the problem had been to remove a varying
> expression, e.g. \sqrt{123}  or \frac{1-x}{1+x}, from the initial
> segment of a formula. Rescanning { and } to "other" allowed me to use
> \tl_remove:Nn successfully. Although I can think of other means of doing
> this, they seem cumbersome by comparison.
>
> Andrew

\tl_set:No \l_result_tl { \use_none:nn \sqrt{123} + 123 }

could be a better approach in many cases, no?

Bruno