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Date:         Wed, 13 Aug 2014 11:35:18 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: l3regex suggestion
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

On 13/08/2014 10:42, aparsloe wrote:
> I have been using l3regex and tripped on the following circumstance.
> Suppose
> 
> \tl_set:Nn \l_tmpa_tl { abcdef }
> 
> Then
> 
> \regex_replace_all:nnN { cd } { 12 } \l_tmpa_tl
> 
> results in \l_tmpa_tl containing ab12ef, but there is no built-in
> function for the analogous match or extract function. This was
> disconcerting for a moment, to discover that I could replace text but
> couldn't match it. Of course I can, but it means resorting to
> \exp_args:Nno as in
> 
> \exp_args:Nno \regex_match:nnTF { cd } \l_tmpa_tl { true } { false }
> 
> It would be consistent with the presence of \regex_replace_all:nnN and
> \regex_replace_all:NnN if there were corresponding functions,
> \regex_match:nN and \regex_match:NN, and similarly for extract.
> 
> In the context in which I was working, the token list variable
> (analogous to \l_tmpa_tl above) is set to a mathematical formula
> elsewhere in the code. The formula is most naturally handled in this way
> as a token list variable rather than as a token list.
> 
> Andrew

There's a difference here between the two situations: in the replace
case, you need to have an N-type argument as you've got to have a tl to
set at the end of the process. On the other hand, in the case of
matching it's unimportant if the tokens are given in a tl or are given
explicitly. So

    \cs_generate_variant:Nn \regex_match:nnTF { nV }
    \regex_match:nVTF { cd } \l_tmpa_tl { true } { false }

would be my preference here. (We don't claim to define all possible
variants up-front, but there is nothing stopping you adding ones which
are useful.)
--
Joseph Wright