[Tex/LaTex] How to manipulate a multi-line string in luatex

lualuatex

I want to write a Lua function that processes some multi-line string.

Thus, I created a new file test.lua which contains

userdata = userdata or {}
function userdata.surroundwithdashes(str)
  str = str:gsub("\n", " (linebreak)\\\\ ")
  tex.print("--" .. str .. "--")
end

and added \directlua{dofile('test.lua')} to my latex preamble. To call that function, I created a macro \newcommand{\dash}[1]{\directlua{userdata.surroundwithdashes([[#1]])}}.

This works well for regular text like

\dash{This is some test text}

However, It does not recognize line breaks

\dash{This is some test text
      with more than one line}

And throws errors (undefined control sequence) if I insert them manually

\dash{This is some test text \\
      with more than one line}

Also, this produces an additional "m" after the "TeX" for 

\dash{This is some \TeX text}

How can I parse the input string reliably and detect newlines in it?

Best Answer

The \\ gets expanded before it is passed to the Lua function userdata.surroundwithbraces. Because the expansion is an expansion like \edef and not LaTeX's \protected@edef, macro \\ breaks.

Expansion can be prevented by e-TeX's \detokenize.

Also I prefer \luaescapestring (or LuaLaTeX's \luatexluaescapestring) for passing string arguments, because the primitive takes care of the necessary escapings:

\RequirePackage{filecontents}
\begin{filecontents*}{dash.lua}
userdata = userdata or {}
function userdata.surroundwithdashes(str)  
  str = str:gsub("\n", " (linebreak)\\\\ ")
  tex.print("--" .. str .. "--")
end
\end{filecontents*}
\directlua{dofile('dash.lua')}

\documentclass{article}

\newcommand*{\dash}[1]{%
  \directlua{userdata.surroundwithdashes(%
    "\luatexluaescapestring{\detokenize{#1}}")}%
}

\begin{document}
  \dash{This is some test text}

  \dash{This is some test text \\
    with more than one line}
\end{document}

Result

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