Peek ahead and process characters

Question

I want to replace unicode character pairs in XeTeX similar to Country flags unicode char. I don't want to specify every option (every country code) and want to be flexible for other emoji (skin color variations, other non-country-code flags etc.). The problem with the code in the mentioned answer is, that it doesn't allow single pairable characters (even if I modify the False-statement accordingly).

After days of fiddling I found a solution (see below) which works for almost all cases I could imagine.

My questions:

• How can I check the last cases (9: following ~ and 21: following alignment character) and why is it not working if I put a letter between my character and the alignment character (case 22) but works with a seperating space (case 21)?
• Have I forgotten any cases?
• Is there a more elegant way? My solution seems very crude to me.

My solution including example-list with mentioned cases (not working cases [last two items] are commented out):

\documentclass[varwidth,border=10pt]{standalone}
\usepackage{expl3}
\usepackage{newunicodechar}
\renewcommand{\familydefault}{\sfdefault}   % math is more obvious
\ExplSyntaxOn

\cs_new_protected:Npn \single_uni:n #1 {
    \int_to_Hex:n{`#1}
}

% method is very specific, am I forgetting something? Not possible with ^^7e(~), ...
\cs_new_protected:Nn \dual_uni:n {
    \peek_catcode:NTF \c_space_token {
        \int_to_Hex:n{`#1}
    }{
        \peek_catcode:NTF \c_other_token {
            \int_to_Hex:n{`#1}
        }{
            \peek_charcode:NTF ^^24 {   % math, \c_math_toggle_token not working
                \int_to_Hex:n{`#1}
            }{              
                \peek_charcode:NTF ^^5e {   % math sub, \c_math_subscript_token not working
                    \int_to_Hex:n{`#1}
                }{                  
                    \peek_charcode:NTF ^^5f {   % math super, \c_math_superscript_token not working
                        \int_to_Hex:n{`#1}
                    }{
                        \peek_catcode:NTF \c_group_end_token {  % end group
                            \int_to_Hex:n{`#1}
                        }{
                            \peek_catcode:NTF \c_group_begin_token {    % begin group
                                \int_to_Hex:n{`#1}
                            }{
                                \peek_catcode:NTF \c_alignment_token {  % has no effect?!
                                    \int_to_Hex:n{`#1}
                                }{
                                    \dual_uni_cont:nn{#1}
                                }
                            }   
                        }   
                    }   
                }
            }
        }
    }
}

\cs_new_protected:Nn \dual_uni_cont:nn{
    \textbf{\int_to_Hex:n{`#1}}\textit{\int_to_Hex:n{`#2}}
}

\newunicodechar{➀}{\single_uni:n{➀}}    % single_uni
\newunicodechar{➁}{\dual_uni:n{➁}}  % dual_uni
% ➂ undeclared newunicodechar
\ExplSyntaxOff
\begin{document}
\begin{enumerate}
\item[] expected \quad -- \quad result
\item ➂ \quad -- \quad ➂ % comparsion undeclared uni (expected blank, because not in font)
\item 2780 \quad -- \quad ➀ % comparsion single_uni
\item \textbf{2781}\textit{78} \quad -- \quad ➁x % following letter
\item \textbf{2781}\textit{21} \quad -- \quad ➁! % following other
\item \textbf{2781}\textit{2782} \quad -- \quad ➁➂ % following unicode (other)
\item \textbf{2781}\textit{2781} \quad -- \quad ➁➀ % following "newunicode" as \single_uni (active?!)
\item \textbf{2781}\textit{2780}x \quad -- \quad ➁➁x % following "newunicode" as \dual_uni (active?!), following letter
\item 2781 x \quad -- \quad ➁ x % following space
\item 2781~x \quad -- \quad ➁~x % following active
\item 2781 \quad -- \quad ➁\\   % following newline
\phantom{nothing} % nothing for a new line
\item $\textbf{2781}\textit{78}$ \quad -- \quad $➁x$ % in math (following letter)
\item $2781^x$ \quad -- \quad $➁^x$ % following math superscript
\item $2781_x$ \quad -- \quad $➁_x$ % following math subscript
\item $2781$ \quad -- \quad $➁$ % in math (following math toggle)
\item 2781\$ \quad -- \quad ➁\$ % following \$
\item 2781\textbullet \quad -- \quad ➁\textbullet % following command
\item 2781\footnote{x} \quad -- \quad ➁\footnote{x} % following command
\item {2781} \quad -- \quad {➁} % following group end
\item 2781{x} \quad -- \quad ➁{x} % following group begin
\item 2781 \quad -- \quad ➁% %x following comment
\item \begin{tabular}{llllll} % in tabular, following space
2781&x &\quad -- \quad& ➁ x& x\\
\end{tabular}
\item \begin{tabular}{llllll} % in tabular, following letter, alignment
2781&x &\quad -- \quad& %➁x& x\\
\end{tabular}
\item \begin{tabular}{llllll} % in tabular, following alignment, space, letter
2781&x &\quad -- \quad& %➁& x\\
\end{tabular}
\end{enumerate}
\end{document}

The result looks like this at the moment (the last two items have no result): If characters, which are defined as potential double-characters find a following "partner", the result is the original character in bold font and the following in italics. I the character is nou double-character or hasn't found a "partner", it is printed normally:

Update: I tried another approach with different problems: Compare macro names instead of meaning. But this question still needs answering...

Answer 1

I think this is what I would do. You create a list with all the options, check if you can grab safely the token, and in case you can, you grab it, check if it's in the list and if it's the list then use the dual option, otherwise use the single option and leave the token.

\documentclass{article}

\usepackage{expl3}
\usepackage{newunicodechar}

\ExplSyntaxOn

\str_new:N \g_fjs_duals_list_str
\str_gset:Nn \g_fjs_duals_list_str { ➀➁ }

\cs_new:Nn \fjs_uni:N { [ \int_to_Hex:n { `#1 } ] }
\cs_new:Nn \fjs_uni:NN { [ \int_to_Hex:n { `#1 } ; \int_to_Hex:n { `#2 } ] }

\cs_new_protected:Nn \fjs_checkdual:N
 {
  \peek_N_type:TF { \fjs_checkdual_grab:NN #1 } { \fjs_uni:N #1 }
 }
\cs_new_protected:Nn \fjs_checkdual_grab:NN
 {
  \tl_if_in:NoTF \g_fjs_duals_list_str { \token_to_str:N #2 }
   { \fjs_uni:NN #1 #2 }
   { \fjs_uni:N #1 #2 }
 }
\cs_generate_variant:Nn \tl_if_in:NnTF { No }

\newunicodechar{➀}{ \fjs_checkdual:N ➀ }
\newunicodechar{➁}{ \fjs_checkdual:N ➁ }

\ExplSyntaxOff

\begin{document}

➁➀
➁➁
➀➁
➀➀

\end{document}

I don't know what would be the ideal approach, because here I'm mixing str functions with tl, but it's needed (at least in this current approach) because otherwise the symbols are made active and have different catcode so they are not in the original list.