How does one extract substrings from a non-explicit token list?
\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\tl_new:N \l_my_tl
\seq_new:N \l_my_seq
\tl_set:Nn \l_my_tl { foot }
\regex_extract_once:nnNTF { foo } { foot }
\l_my_seq
{ true }
{ false }
\par
\regex_extract_once:nnNTF { foo } { \l_my_tl }
\l_my_seq
{ true }
{ false }
\end{document}
The first extraction command has an explicit 'foot' and succeeds; I hoped the
second would too, expanding \t_my_tl
, but that is not what happened.
Output:
true
false