How to break a word at a given length (without a hyphen)?

Question

The problem is to get

The question is justified. The answer is right flush. The dimensions of the rectangle are 5cm and 4cm. The length of the question is unknown.

Manually

\documentclass[11pt]{scrartcl}
\usepackage{tikz}

\begin{document}

  \def\quest{1/(4.2+5.4)-(3.5+4.7)\\/9} 
  \def\ans{-0.806944444} 

\begin{tikzpicture}[font=\Large] 
 \draw (0,0) rectangle (5,-4);
\node[align=justify,text width =5cm-1ex,
      anchor=north west,inner sep=1pt] (qu) {\quest};
\node[align=right,text width =5cm-1ex,
      anchor=north west,inner sep=1pt] (an) at (qu.south west) {\ans}; 
\end{tikzpicture}

\end{document}

and I get

But how to get this automatically ?

Update-information

The same question was asked on the list pgf/tikz. Mark Wibrow (who worked on pgfmath, and other parts of pgf) gave a beautiful solution :

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

\def\showsum#1{\ifx#1=\par\hfill\else#1\hskip0.1ex 
minus0.5ex\expandafter\showsum\fi}
\tikzset{calculator screen/.style={
    inner sep=2pt,
    fill=blue!20,
    font=\huge\tt,
    execute at begin node={\parindent=0pt\hsize=5cm\vbox to4cm\bgroup},
    execute at end node=\vfill\egroup
}}

\tikz\node[calculator screen]
    {\showsum 1/(4.2+5.4)-(3.5+4.7)/9 +3+2+1= -.80694444};

\end{document} 

Very impressive

Answer 1

I'd simply add a zero skip between any two tokens and set ragged right the question:

\documentclass{article}
\usepackage{tikz,xparse}
\ExplSyntaxOn
\NewDocumentCommand{\calcinput}{m}
 {
  \tl_map_inline:nn { #1 } { ##1 \hspace{0pt} }
 }
\ExplSyntaxOff



\begin{document}

  \def\quest{\calcinput{1/(4.2+5.4)-(3.5+4.7)/9}}
  \def\ans{\calcinput{-0.806944444}}

\begin{tikzpicture}[font=\Large\ttfamily]
 \draw (0,0) rectangle (5,-4);
\node[align=flush left,text width =5cm,
      anchor=north west,inner sep=1pt] (qu) {\quest};
\node[align=right,text width =5cm,
      anchor=north west,inner sep=1pt] (an) at (qu.south west) {\ans};
\end{tikzpicture}

\begin{tikzpicture}[font=\Large\ttfamily]
 \draw (0,0) rectangle (4,-4);
\node[align=flush left,text width =4cm,
      anchor=north west,inner sep=1pt] (qu) {\quest};
\node[align=right,text width =4cm,
      anchor=north west,inner sep=1pt] (an) at (qu.south west) {\ans};
\end{tikzpicture}

\end{document}

The window width should be an integer multiple of .5em in the monospaced font, in my opinion.