Split string in characters and merge with formatting

Question

How can I create a macro

\split{ABC}

that outputs

where the formatting of the characters are made with another macro, i.e.

\formatchar{A}

where

\def\formatchar#1{{\color{red}#1}}

for any string length?

I tried to modify the code in Split a character string n by n

\documentclass{article}
\usepackage{xstring}
\def\split#1#2{%
    \StrSplit{#2}{1}\tempa\tempb
    x%
    \tempa\let\tempa\empty
    \unless\ifx\tempb\empty\def\tempa{|\split{1}\tempb}\fi
    y%
    \tempa
}
\begin{document}
\split{1}{ABCD}
\end{document}

And I get x and y before and after (since I failed limiting with a command and }) but I cannot get it to format properly. Also I would like to avoid the {1} in call, and use

\split{ABCD}

only. It is always 1-charater splits.

Any help is much appreciated!

Answer 1

If your input is not guaranteed to be “ASCII-only” an approach with \seq_set_split:Nnn may not work.

I changed the name to \splitchars because \split is taken up by amsmath.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{xcolor}
\NewDocumentCommand{\formatchar}{m}{%
  \textcolor{red!90!yellow}{#1}% <--- or whatever you like
}
\ExplSyntaxOn
\NewDocumentCommand{\splitchars}{O{|}m}
 {% #1 = optional separator, default |
  % #2 = text to split
  \mflxvii_split:nn { #1 } { #2 }
 }
\seq_new:N \l__mflxvii_split_items_seq
\cs_new_protected:Nn \mflxvii_split:nn
 {
  \seq_clear:N \l__mflxvii_split_items_seq
  \text_map_inline:nn { #2 }
   {
    \seq_put_right:Nn \l__mflxvii_split_items_seq { \formatchar{##1} }
   }
  \seq_use:Nn \l__mflxvii_split_items_seq { #1 }
 }
\ExplSyntaxOff
\begin{document}
\splitchars{ABC}
\splitchars[--]{ABC}
\splitchars{ábç}
\splitchars{ÅÄÖ}
\end{document}

In order to get a list-like output, here's an extension that allows a different separator between the last two items.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{xcolor}
\NewDocumentCommand{\formatchar}{m}{%
  \textcolor{red!90!yellow}{#1}% <--- or whatever you like
}
\ExplSyntaxOn
\NewDocumentCommand{\splitchars}{O{|}O{#1}m}
 {% #1 = optional separator, default |
  % #2 = optional separator between last two
  % #3 = text to split
  \mflxvii_split:nnn { #1 } { #2 } { #3 }
 }
\seq_new:N \l__mflxvii_split_items_seq
\cs_new_protected:Nn \mflxvii_split:nnn
 {
  \seq_clear:N \l__mflxvii_split_items_seq
  \text_map_inline:nn { #3 }
   {
    \seq_put_right:Nn \l__mflxvii_split_items_seq { \formatchar{##1} }
   }
  \seq_use:Nnnn \l__mflxvii_split_items_seq { #2 } { #1 } { #2 }
 }
\ExplSyntaxOff
\begin{document}
\splitchars{ABC}
\splitchars[--]{ABC}
\splitchars{ábç}
\splitchars{ÅÄÖ}
\splitchars[, ][ and ]{ÅÄÖ}
\splitchars[, ][ and ]{ÅÄ}
\end{document}

Answer 2

Here's a LuaLaTeX-based solution, which makes use of Lua's gsub and sub built-in functions. Actually, the code uses versions of the gsub and sub functions that can handle utf8-encoded characters directly.

The main user-level macro is called \splitstring rather than \split, because the amsmath package defines an environment called split along with macros called \split and \endsplit.

Observe that (a) the argument of \splitstring can contain whitespace and (b) formatchar is implemented as a Lua function rather than as a LaTeX macro.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{xcolor}  % for '\textcolor' macro
% Lua-side code
\directlua{

  function formatchar ( c )
     return ( '\\textcolor{red}{\\textbf{' .. c .. '}}|' )
  end
  function split ( s )
     s = unicode.utf8.gsub ( s , '.' , formatchar )
     return ( unicode.utf8.sub ( s , 1 , -2 ) ) % delete final '|'
  end

}
% LaTeX-side code
\newcommand\splitstring[1]{\directlua{tex.sprint(split('#1'))}}

\begin{document}
\obeylines % just for this example
\splitstring{ABC}
\splitstring{123abc}
\splitstring{Hello World}
\end{document}

Answer 3

Here is my solution attempt using ExplSyntax:

\documentclass{article}
\usepackage{xcolor}
\ExplSyntaxOn
\seq_new:N \l__my_split_seq
\NewDocumentCommand { \split } { m } {  __my_split:n { #1 }  }
\cs_new_protected:Npn __my_split:n #1
{
  \seq_set_split:Nnn \l__my_split_seq {} { #1 }
  \seq_set_map:NNn \l__my_split_seq \l__my_split_seq
  {  __my_split_fn:n { ##1 }  }
  % \seq_log:N \l__my_split_seq
  \seq_use:Nn \l__my_split_seq { | }
}
\cs_new_protected:Npn __my_split_fn:n #1 {   {  \color { red } #1  }   }
\ExplSyntaxOff
\begin{document}
\split{ABCD}
\split{1234}
\end{document}

Answer 4

Nobody actually tried to use the OPs code with xstring, which is arguably the easiest :) Here a recursive definition of \split, using \StrSplit and \IfStrEq. It works also for accented letters.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{xstring}
\usepackage{xcolor}
\def\formatchar#1{{\color{red}#1}}
\def\split#1{%
    \StrSplit{#1}{1}{\tempa}{\tempb}% split into first character \tempa and the rest \tempb
    \formatchar{\tempa}%            % print first character in red
    \IfStrEq{\tempb}{}{}{%          % if rest is empty (first {}) do nothing (second {}), which will stop the recursive loop.
    |%                              % else: print a bar, and
    \split{\tempb}%                 % call the function again for the rest,
    }%                              % which will split the first character of the rest etc.
}
\begin{document}
\split{ABCDÅÄÖ}
\end{document}

Answer 5

Since there are no LaTeX2e solutions offered, I give one here. I use a token cycle to grab characters one-by-one from the argument. This solution assumes the argument has no macros and no groups. I omitted those, because no guidance was given on how they should be handled, though they can be handled, as needed.

If unicode characters need to be handled, compilation must be performed under xelatex or lualatex.

\documentclass{article}
\usepackage{tokcycle,xcolor}
\newcommand\formatchar[1]{\textcolor{red}{#1}}
\def\split#1{%
  \tokencycle{\formatchar{##1}\splitchar}
  {}{}{##1\splitchar}#1\endtokencycle}
\newcommand\splitchar{\tcpeek\z\ifx\empty\z\else$\mid$\fi}
\begin{document}
\split{ABCD}
\split{Å ÄÖ}
\end{document}