If you want a . . . (well, I almost said "fun" but that's probably not the right word :) an interesting rabbit hole to go down check out Nondeterministic finite automaton and Deterministic finite automaton to get a better picture of what's going on under the hood in a regex engine. And/or find an undergraduate course in automata theory to audit. If you use the Regexp::Debugger that's already been mentioned above it'll let you watch and you can almost picture the state machine hopping along from node to node until it gets to a valid terminal state.