Test a regular expression against sample text, with every match highlighted and each capture group listed.
Patterns run in a background worker with a 2-second limit, so a pattern that backtracks catastrophically stops instead of freezing this tab. JavaScript regex syntax, which differs from PCRE in places — no lookbehind in older Safari, no atomic groups.
Format, validate and minify JSON in your browser — with the line, column and a plain-English reason when it will not parse.
Read the header and payload of a JSON Web Token, with its times explained. Decodes only — it does not verify the signature, and says so.
Generate UUID v4, v7 or v1, or Nano IDs — one or hundreds at a time, from your browser’s cryptographic random source.
Explain a cron expression in plain English and see when it will actually run next.