How to use it

Write a pattern, pick your flags, paste the text. Matches are highlighted in alternating colours, and every group — numbered or named — is listed with its value.

The flags

The tool forces g internally whatever you pick: without it, exec restarts from zero on every call and a search for all matches loops on the first one.

The lastIndex trap

A regex with g is mutable: it remembers its position between calls.

const regex = /\d+/g;
 
regex.test("42"); // true
regex.test("42"); // false — lastIndex is 2, the search restarts from the end

That is why a regex with g must never be declared as a shared constant and then reused with test or exec. Either recreate it, reset lastIndex to zero, or use matchAll, which works on a copy.

Empty matches

const regex = /a*/g;
let match;
 
// infinite loop: an empty match does not advance lastIndex
while ((match = regex.exec("bbb")) !== null) {
  console.log(match.index);
}

You have to increment lastIndex by hand when the match is empty. That is exactly what this tool does, and why a* against bbb returns four empty matches here instead of freezing the tab.

Named groups

const { groups } = /(?<year>\d{4})-(?<month>\d{2})/.exec("2026-08");
groups.year; // "2026"

In a replacement, $<name> has a surprising behaviour worth knowing:

// the pattern HAS named groups: an unknown reference becomes empty
"2026".replace(/(?<year>\d{4})/, "[$<unknown>]"); // "[]"
 
// the pattern has NO named group: the reference is copied literally
"2026".replace(/\d{4}/, "$<year>"); // "$<year>"

Neither case throws. A typo in a group name therefore goes completely unnoticed.

Catastrophic backtracking

// avoid: exponential time in the length of the input
/(a+)+b/.test("a".repeat(30));

A nested quantifier over an alternative that fails forces the engine to try every possible split. JavaScript offers no way to interrupt a running regex: the only protection in the browser is to bound the input size, which is what this tool does.

For patterns coming from a user, the real answer is a finite-automaton engine (RE2) running server-side with a time limit.