Starboard Notebooks are stored on disk as a plaintext file in utf-8 encoding. The mimetype is application/vnd.starboard.nb, the file extension is either .nb or .sbnb.
The format is similar to Jupytext's percent format.
A notebook file can start with an optional YAML header which is delineated by ---. For example:
---
starboard:
some_metadata: 1234
version: "1.2.3"
---Notebook cells start with a cell delineator, which starts with a line comment (# or //) followed by at least two % characters. A regex for this is ^(#|\/\/)\s*%{2,}-*.
On the same line there is also the cell type identifier in square brackets (e.g. [javascript]). A cell type identifier should always be present.
A cell delimiter either contains metadata or doesn't, in case it does the % characters are followed by at least 3 hypens (-). In the case there is metadata, the next lines are interpreted as YAML until a line containing # ---%% or // ---%% or is read. The YAML lines start with a line comment which should be ignored when deserializing the YAML.
After this cell delineator the cell's content is present in plaintext.
# %% [python]
print("A simple python cell")
# %% [markdown]
**Markdown** is also wrapped in a cell
# %%--- [javascript]
# yaml_cell_header: "across multiple lines, delineated by special %%--- and ---%%"
# which: "should be compatible with Spyder, VS Code, etc. Although not understood of course"
# indented:
# looks_like: "this"
# ---%%
console.log("Hello!");
const x = 3;
x;