percent signs in luacode* environment (LuaLaTeX)

Question

Can someone explain what happens here:

\documentclass{article}
\usepackage{luacode}
\begin{document}

\begin{luacode*}
tex.print("xx\%a","yy\\%b","zz\\\%c")
\end{luacode*}

\end{document}

The result is xxyy%b zz%c. (The xx,yy,zz is clear.) The \%a is "ignored", the \\%b and the \\\%c results in the "same" %b (or c). What is the preferred way to get a % in the PDF output? Why is the \%a ignored? What does TeX see after returning from the lua call?

Answer 1

I assume that luacode in LaTeX works the same way as \startluacode...\stopluacode in ConTeXt. luacode expands its contents before passing control to lua. So,

• xx\%a => xx%a (because \% = %)
• yy\\%b => yy\%b (because \\ = \)
• zz\\\%c => zz\%c (because \\ = \ and \% = %)

Thus, (as Paŭlo Ebermann) mentioned, lua writes the following to TeX input stream:

xx%a
yy\%b
zz\%c

which explains the output that you get. Perhaps the following test file will make this clear (Compare the console output from pdf output).

\def\%{?}
\def\?{!}
\starttext
\startluacode
tex.print("xx\%a","yy\\%b","zz\\\%c")
print("xx\%a","yy\\%b","zz\\\%c")
\stopluacode
\stoptext

Answer 2

I think the single \ serves to escape the next letter in Lua, letting itself dissappear. So, TeX gets to see these three lines:

xx%a
yy\%b
zz\%c

And obviously the first one causes the %a (+ the following line-break) to be ignored, while the second and third write out a %.

It is a bit annoying that all (most) programming languages use the same \ as an escape character, so when embedding one in another you need multiple-escaping.