How do I insert a tie (~, non-breaking space, tilde) under expl3 syntax?

Question

I need to define a token list of the form

\tl_new:N  \g_silly_tl
\tl_set:Nn \g_silly_tl { Text ~ and ~ a ~ tie:?? }

How do I do this?

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More generally, is there a reference where one can see how to input characters 'normally' under expl3 syntax?

Answer 1

This question has two distinct parts, one of which is easier than the other!

The specific issue here is (presumably) not inserting a tilde but rather inserting a non-breaking space in some 'fixed' text stores as within a LaTeX3 tl variable. While a tilde is a convenient shortcut for this at the document level, here I think I would simply use the macro \nobreakspace.

\tl_set:Nn \l_silly_tl { Text ~ and ~ a \nobreakspace tie:?? }

(Note that at present there is no expl3 code-level 'non-breaking space' function.)

On the wider issue of 'inserting non-standard catcode characters', the expandable function \char_generate:nn is available to create tokens of (almost) all reasonable category codes

\tl_set:Nx \l_silly_tl
  {
    \char_generate:nn { `\a } { 4 }
  }

There are a few cases that are not covered by this approach. For example, with older expl3 releases we couldn't do active characters and you'd need to use the regex engine:

\RequirePackage{expl3}
\ExplSyntaxOn
\tl_clear:N \l_tmpa_tl
\regex_replace_all:nnN { } { \cA\~ } \l_tmpa_tl 

More recent versions of expl3 can do this using \char_generate:nn, so it's more of a historical note.

The second option is stick to the 'traditional' \lowercase trick, using expl3 naming of course

\group_begin:
\char_set_catcode_active:n { `\@ }
\char_set_lccode:nn { `\@ } { `\~ }
\tex_lowercase:D
   {
     \group_end:
     \tl_set:Nn \l_silly_tl { Text ~ and ~ a @ tie:?? }
   }

(Notice that there is no public interface to \lowercase beyond the saved-primitive.)

Answer 2

I recently made a package to do this. (now on CTAN.)

(remark: clearly everything the package can do l3regex and lowercase can also do, but this package provides a more convenient syntax...)

With the package, you can write

\documentclass{article}
\usepackage{precattl}    % ← this package!
\begin{document}
\ExplSyntaxOn
\precattl_exec:n {
    \tl_new:N  \g_silly_tl
    \tl_set:Nn \g_silly_tl { Text ~ and ~ a \cA~ tie:?? }
    % ↑ l3regex-replacement-like syntax, but not exactly identical.
    % Note that you can embed it directly into the code itself
}
\ExplSyntaxOff
\end{document}