How to trim the spaces of the head of token list after using \tl_tail:N?

Question

Here is a MWE:

\documentclass{article}
\begin{document}
\ExplSyntaxOn
  \tl_new:N \l_tmpc_tl
  \tl_set:Nn \l_tmpa_tl {*~123}
  \tl_set:Nx \l_tmpb_tl
    {
      \tl_trim_spaces:n { \tl_tail:N \l_tmpa_tl }
    }
  \tl_set:Nn \l_tmpc_tl { \tl_tail:N \l_tmpa_tl }
  test \tl_use:N \l_tmpb_tl \par
  test \tl_use:N \l_tmpc_tl
\ExplSyntaxOff
\end{document}

I found that \tl_trim_spaces:n didn't work. What I want is test123. And if I didn't use the \tl_tail:N then it worked.

I don't know how to solve this problem. Thanks for any help!

Answer 1

I suggest in-place trimming with \tl_trim_spaces:N.

\documentclass{article}
\begin{document}
\ExplSyntaxOn
\tl_set:Nn \l_tmpa_tl {*~123}
\tl_set:Nx \l_tmpb_tl { \tl_tail:N \l_tmpa_tl }
\tl_trim_spaces:N \l_tmpb_tl
% \tl_show:N \l_tmpb_tl
test \tl_use:N \l_tmpb_tl
\ExplSyntaxOff
\end{document}