How to add newline within node using TikZ?

Question

I want to break the input of a path in order to draw pushdown automaton, so I tried to use the break line symbol \\ and even $$ $$, but it still doesn't break the lines.
For example, the input should be
0, 1, 2
3, 4, 5

Any idea? Thank you.

Code sample:

\documentclass[10pt,letterpaper]{article}
\usepackage[latin1]{inputenc}
\usepackage[left=1in,right=1in,top=1in,bottom=1in]{geometry} 
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{automata,positioning}

\begin{document}
\setlength{\parindent}{0pt}
\setlength{\parskip}{1ex}

\textbf{PDA:}\\
    \begin{tikzpicture}[shorten >=1pt,node distance=5cm,on grid,auto] 
        \node[state,initial]    (q_0)                   {$q_0$}; 
        \node[state,accepting]  (q_1)   [right=of q_0]  {$q_1$}; 
        \node[state]            (q_2)   [right=of q_1]  {$q_2$}; 
        \node[state]            (q_3)   [below=of q_1]  {$q_3$};
    
        \path[->]
        (q_0)   edge                    node {0,1}            (q_1)
                edge    [loop above]    node {0,1,2,3,4,5}            (q_0)
          
        (q_1)   edge                    node {0,1}            (q_2)
    
        (q_2)   edge    [loop right]    node {1}              (q_2)  
 
        ; %end path 
    \end{tikzpicture}
\\
\end{document}  

Answer 1

One simple method is to specify text characteristics within the node : text width, etc. This will let you do exactly what you want, without any extra package. For example,

\documentclass[]{minimal}

\usepackage{amsmath,amsfonts,amssymb}
\usepackage{tikz}
\usetikzlibrary{automata,positioning}

\begin{document}

\begin{tikzpicture}[shorten >=1pt,node distance=5cm,on grid,auto] 

\node[state,initial] (q_0) {$q_0$}; 
\path[->] (q_0) edge[loop above] node[text width=1cm,align=center] {0,1,2\\3,4,5} (q_0); 

\end{tikzpicture}
\end{document}  

The result is

Answer 2

Based on Frédérics answer and Chans comment you can just do:

\documentclass[]{minimal}

\usepackage{amsmath,amsfonts,amssymb}
\usepackage{tikz}
\usetikzlibrary{automata,positioning}

\begin{document}

\begin{tikzpicture}[shorten >=1pt,node distance=5cm,on grid,auto] 

\node[state,initial] (q_0) {$q_0$}; 
\path[->] (q_0) edge[loop above] node[align=center] {0,1,2\\3,4,5} (q_0); 

\end{tikzpicture}
\end{document}  

You don't need the text width option, just the align

Answer 3

You could use the makecell package. As stated in the package documentation, it provides

\makecell[<vertical or/and horizontal alignment>]{<cell text>}

that aids in the creation of (small-scale) multi-lined tabular cell. In that regard, consider the following alteration to your code:

...
\usepackage{makecell}%
...
\path[->]
  (q_0) edge node {0,1} (q_1)
        edge [loop above] node {\makecell[l]{0,1,2,\\3,4,5}} (q_0)
  (q_1) edge node {0,1} (q_2)
  (q_2) edge [loop right] node {1} (q_2)
 ; %end path

Answer 4

You could use \shortstack[alignment]{text1 \\ text1 \\ text1 etc. }
Where alignment can take:
r - right aligned
l - left aligned
c - centered (default)

\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows}

\tikzstyle{box} = [rectangle, rounded corners, minimum width=2cm, minimum height=1cm,text centered, draw=black, ]

\begin{tikzpicture}[node distance=2cm]
\node (box1) [box] {\shortstack{text1 \\ text2} };
\node (box2) [box, right of=box1, xshift=1cm] {Process 2b};
\end{tikzpicture}