A short reference on regular expressions — what a pattern actually describes, the building blocks (character classes, quantifiers, anchors, groups, lookarounds), the flags, how flavours differ across languages, and the catastrophic-backtracking footgun to avoid.

A short reference on JSON Web Tokens — the three-part structure, the algorithms that sign them, what they are good for, what they are bad for, and why you should not roll your own auth on top.

A short reference on Unix timestamps — what the epoch is, why time is counted from 1970, the seconds vs milliseconds confusion, what happens in 2038, and why leap seconds quietly do not exist in Unix time.

A short reference on Jupyter notebooks — the two-process architecture, the five ZeroMQ sockets between frontend and kernel, the .ipynb file format, and the hidden-state pitfall that catches almost everyone.

A short reference on Cross-Origin Resource Sharing — what counts as a cross-origin request, the simple vs preflight distinction, the headers that matter, and why CORS is a browser-side restriction rather than a server-side one.