Compile a regex into an NFA one operator at a time, stamping a two-state fragment for each literal and ε-wired forks, loops, and seams for the operators, and predict the graph each one adds.

You type `a(b|c)*` into a regex tester and hit go. Before the engine reads a single character of your input, it has already thrown your pattern away and replaced it with a little map of states and arrows.

Run a string through an NFA by carrying the whole set of states the engine could occupy, expanding it for free across ε-arrows on every step, and deciding the match from whether the accept state lands in the final set.

You hand the engine the pattern `ab|ac` and the string `ac`, expecting one marker to walk the graph state by state. Instead it lights up several states at once and advances every one of them on each character.