Use expl3, and all the\XX_new:N command families and their nuances enter the picture too. :)

Expl3 ismuch easier to code in.

Same (philosophical) answer at base as the linked question, though.

MWE

% arara: lualatex\documentclass{article}\ExplSyntaxOn\tl_new:N \g_ds_doctitle_tl\tl_new:N \g_ds_docsubtitle_tl\tex_def:D \mymeta #1 {    \textsf{\textbf{ #1 }}}\tex_def:D \commandbuild #1#2#3 {        \c_backslash_str        #1        \c_underscore_str        #2        \c_colon_str        #3}\tex_def:D \texdef {        \begin{center}        The ~ difference ~ between ~ X ~ and ~ Y        \end{center}        \begin{center}        \mymeta { \commandbuild {tex}{def}{D} } ~ and ~ all ~ the ~        \mymeta { \commandbuild {XX}{new}{N} } ~ commands        \end{center}        }\NewDocumentCommand { \mytitle } { o +m } {    \tl_gset:Nx \g_ds_doctitle_tl { #2 }    \IfValueT { #1 }    {            \tl_gset:Nx \g_ds_docsubtitle_tl { #1 }            \regex_replace_all:nnN                    { (subtitle)+ }                    { \c{underline} \cB\{ \0 \cE\} }                    \g_ds_docsubtitle_tl            \tl_gput_left:Nn \g_ds_docsubtitle_tl             {                     \vspace { 2\tex_baselineskip:D }                    \begin{center}            }            \tl_gput_right:Nn \g_ds_docsubtitle_tl             {                     \end{center}            }                                \tl_gput_right:Nn \g_ds_doctitle_tl             {                     \large \upshape             }            \tl_gput_right:NV \g_ds_doctitle_tl \g_ds_docsubtitle_tl    }        \tl_gput_left:Nn \g_ds_doctitle_tl { \Large }    \tl_gput_right:Nn \g_ds_doctitle_tl { \texdef }            }\NewDocumentCommand { \printtitle } { } {        \tl_if_empty:NF \g_ds_doctitle_tl        {        \group_begin:        \center        \itshape        \tl_use:N \g_ds_doctitle_tl        \group_end:        }}\ExplSyntaxOff\mytitle[My subtitle A]{MultiparagraphMy Title A}%============  document \begin{document}\printtitle\end{document}

• 2
  Computer scientists keep telling me that new languages are easier to use. I guess it's true if you think more like a computer sicentist an less like a computer. I find assembly language easier to use.

  John Kormylo

  – John Kormylo

  2022-12-06 16:41:01 +00:00

  CommentedDec 6, 2022 at 16:41
• 2
  Expl3 is much easier to code in, but only in the sense that web pages with pop-up ads are much easier to read.

  user287367

  – user287367

  2023-05-05 18:22:26 +00:00

  CommentedMay 5, 2023 at 18:22

• macros
• def