The Interactive RegEx Railroad & DFA/NFA Stepper shows what a regular expression actually is: a machine. Type a JavaScript pattern and it is parsed into an explicit syntax tree — no black box — then rendered three ways: as a classic railroad (syntax) diagram you can export as vector SVG, as the NFA produced by Thompson's construction, and as the DFA derived from that NFA by subset construction, with ε-edges drawn dashed and accepting states ringed.

The step debugger is where intuition gets built. A faithful reimplementation of a backtracking regex engine runs your pattern against your test text and logs every instruction: characters consumed, alternatives pushed, assertions checked, and — the part browsers never show you — every unwind. Play it, pause it, scrub it, or step one instruction at a time while the input caret, live backtrack counter and engine log update together.

Because backtracking is also a security story, the tool grades every run for ReDoS risk: a static analysis flags nested quantifiers with overlapping starts like (a+)+ and ambiguous alternation like (a|a)*, while a dynamic signal measures steps executed per input character. The simulation is hard-budgeted, so a catastrophic pattern truncates with a warning instead of freezing the tab — and the simplified-pattern export (a{1,} → a+, [0-9\d] → \d, redundant groups removed) gives you the safe rewrite to copy.

Everything is computed locally: the parser, compiler, automata and VM are pure TypeScript with no RegExp delegation and no network calls, so patterns and test text never leave the page.