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Regex to NFA Visualizer — Thompson's Construction, Built From Scratch

This is a real regex engine, not a JavaScript RegExp call with a nicer UI. Your pattern is parsed into an AST and compiled into an actual NFA (nondeterministic finite automaton) using Thompson's construction — the textbook algorithm regex engines are built on — then matched against your test string character by character, with every currently-active NFA state visible via epsilon-closure at each step.

How to use Regex to NFA Visualizer

  1. Enter a pattern — literals, ., character classes, |, (), *, +, ? are all supported.
  2. Enter a test string.
  3. Watch the active NFA state set update after each character.
  4. Try a preset to see alternation, classes and repetition compile differently.

Frequently asked questions

What is Thompson's construction?
A method for compiling a regular expression directly into an NFA, piece by piece — a literal becomes a two-state fragment, alternation adds branching epsilon transitions, star/plus/optional add looping or bypass epsilon transitions — the same construction used to explain how regex engines work in computer science courses.
What does "epsilon-closure" mean in the state trace?
A real NFA can have multiple valid paths active simultaneously — epsilon-closure is the full set of states reachable via free (non-character-consuming) transitions after each real match, which is exactly what is displayed after every character.
Why does the match fail as soon as the active set is empty?
An empty active state set means no path through the NFA survived that character — since there is nothing left to continue from, the match is dead immediately rather than only being checked at the end.
Does it support the same syntax as JavaScript RegExp?
It supports a real, useful core subset — literals, ., character classes [a-z]/[^0-9], alternation, grouping, and */+/? — built from scratch for this visualizer, not the full RegExp feature set (no backreferences or lookaround).

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