expl3: lowercase a token list inside a tikz node

Question

This question ask for a variant of the answer to expl3: lowercase a token list given by Joseph Wright

I want to convert to lowercase a token list that is composed by letter, digits and the control sequence \\ . The method used by Joseph Wright (which is to make robust \\) works fine in normal text, but fails with a compilation error when used inside a TikZ node.

\documentclass{article}

\usepackage{xparse}
\usepackage{tikz}

\ExplSyntaxOn
\NewDocumentCommand \LowerCase { m }
  {
    \group_begin:
      \cs_set_protected:Npx \\ { \exp_not:o \\ }
      \tl_lower_case:n {#1}
    \group_end:
  }

\tl_new:N \g_metadata_title_tl

\NewDocumentCommand \mytext {m}
  {
    \tl_gset:Nn \g_metadata_title_tl {#1}
  }

\NewDocumentCommand \thetext {}
  {
    \tl_use:N \g_metadata_title_tl
  }

\ExplSyntaxOff


\begin{document}
\mytext{This is my\\ Text}

\noindent\LowerCase{\thetext}

\begin{tikzpicture}[overlay, remember picture]
  \node[align=left] (text1) {\thetext};

  \node[align=left, below of = text1] (text2) {\LowerCase{\thetext}};
\end{tikzpicture}

\end{document}

Note: The text to lowercase is should be taken from a token list variable.

Answer 1

The issue here that you can't have \begingroup ... \endgroup around the content of the node if it's aligned: this can be seen with a simple example

\documentclass{article}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
  \node[align=left] {\begingroup Hello\\World\endgroup};
\end{tikzpicture}
\end{document}

Thus we need to save and restore the meaning of \\ without a group. Assuming no nesting is required:

\documentclass{article}

\usepackage{xparse}
\usepackage{tikz}

\ExplSyntaxOn
\NewDocumentCommand \LowerCase { m }
  {
    \cs_set_eq:NN \__texnik_newline: \\
    \cs_set_protected:Npx \\ { \exp_not:o \\ }
    \tl_lower_case:n {#1}
    \cs_set_eq:NN \\ \__texnik_newline:
  }

\tl_new:N \g_metadata_title_tl

\NewDocumentCommand \mytext {m}
  {
    \tl_gset:Nn \g_metadata_title_tl {#1}
  }

\DeclareExpandableDocumentCommand \thetext {}
  {
    \tl_use:N \g_metadata_title_tl
  }

\ExplSyntaxOff


\begin{document}
\mytext{This is my\\ Text}

\noindent\LowerCase{\thetext}

\begin{tikzpicture}[overlay, remember picture]
  \node[align=left] (text1) {\thetext};

  \node[align=left, below of = text1] (text2) {\LowerCase{\thetext}};
\end{tikzpicture}

\end{document}