What are the restrictions of node ID in GraphViz?

Question

For example, here are the results if I use the IDs 1a and 2.2.2 for nodes:

How does this work? What are other restrictions on the node name? I look at Attributes | Graphviz but it doesn't explain.

Can I do something like this?

0 -> {1a, 1b, 2, 3} 
1b -> 2
1b.1 -> {1b.1.1}
1b.1.1 -> {1b.1.1.1, 1b.1.1.3}
1b.2 -> {1b.2.1, 1b.2.2}
1b.2.2 -> {1b.2.2.1, 1b.2.2.2}
2 -> 2.1
3