How can I reverse the order of letters/tokens?

Question

I want a simple way to have:

\reverseit{abc defg \mycommand}

yield

\mycommand gfed cba

?

(Yes, the original motivation is related to Hebrew and RTL languages, but that can sort of be solved using \beginR ... \endR/\beginL ... \endL where necessary without changing the language. So I'm asking the general question.)

Edit: The solution in this question is acceptable as a non-LaTeX-3 solution...

Related: Most efficient way to reverse a token list (tail recursion). Please check if it is a duplicate. — Martin Scharrer, Jan 06 '12 at 10:19
@MartinScharrer While that question discusses the problem, it was oriented in finding the "most efficient" way. For short token lists the solution I provided (courtesy of the LaTeX3 team) is the shortest. — egreg, Jan 06 '12 at 10:49
Side note the "yield" part isn't equal to the reverse of the original as the space token originally before \mycommand is lost. — user202729, Jun 27 '22 at 08:23
Don't understand your comment. There are two space tokens in both sequences. — einpoklum, Jun 27 '22 at 08:54
Not that it matters, but back then I wanted to say that the tokenization of \mycommand gfed cba consist of \mycommand gfed␣cba (only one explicit space token) instead of the intended \mycommand ␣gfed␣cba. — user202729, Aug 04 '22 at 16:33

score 11 · Accepted Answer · edited Jan 06 '12 at 12:18

11

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand\reverseit { m }
  { \tl_reverse:n { #1 } }
\ExplSyntaxOff

\begin{document}
\reverseit{abc defg \LaTeX}
\end{document}

This also honors spaces.

edited Jan 06 '12 at 12:18

Azoun

2,317

answered Jan 06 '12 at 10:18

egreg

1,121,712

For completeness, mention \tl_reverse_items:n, which does not honor spaces, and puts braces around every item, but is much faster (100 times?). --- Sorry, didn't see that there were spaces in Eyal's example. – Bruno Le Floch Jan 06 '12 at 12:25

How can I reverse the order of letters/tokens?

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