Is there a package in either PSTricks or Tikz to draw the following diagram?

that are so-called railroad diagrams for the syntax and can be set with simple nodes and loops:

\documentclass{article}
\usepackage{pst-node}
\def\CBox#1{\pscirclebox[fillcolor=green!20,fillstyle=solid]{\bfseries#1}}
\def\PBox#1{\psframebox[fillcolor=green!20,fillstyle=solid]{\bfseries\shortstack{#1}}}
\begin{document}

\begin{pspicture}(0,-5)(13,1)
\psset{linewidth=1.5pt,linearc=0.3}\psline{|-|}(13,0)
\psforeach{\iA}{0,1,..,13}{\pnode(\iA,0){O\iA}}
\ncloop[angleB=180,loopsize=-1]{O0}{O2}\ncput{\CBox{--}}
\rput(3,0){\CBox{0}}
\ncloop[angleB=180,loopsize=-1.75]{O2}{O6}\ncput[npos=2.3]{\PBox{digit\\1-9}}
\rput(5.5,-2.5){\rnode{A}{\PBox{digit}}}\ncloop[angleB=180,loopsize=0.75]{A}{A}
\ncloop[angleB=180,loopsize=0.75]{O6}{O9}
\end{pspicture}

\end{document}

No package but a tutorial: Diagrams as Simple Graphs in the pgfmanual page 60. You only need to change (modify) some styles

I am sure you can find an example on www.texample.net

that you can modify to suit your needs. I am not aware of a package that does precesily the job.