Regex Tester with Explanations & Groups

Q: How do I test advanced regex patterns?

Enter your regex pattern with flags (g, i, m, s, u) and a test string. The tool highlights all matches with capture groups shown in distinct colours. It also explains the pattern and validates regex syntax.

Q: How is this different from the basic Regex Tester?

The advanced version adds pattern explanation (shows what each part of the regex does), match indices, lookbehind/lookahead visualisation, and substitution preview — useful for complex patterns.

Q: Is my data sent to a server?

No. Every tool on jsonversal runs entirely in your browser using the Web Crypto API, File API, and Clipboard API. Nothing is ever uploaded — not even the JSON you paste in.

Q: Do I need an account or API key?

No signup, no email, no API key. All tools are free to use with no signup required.

Q: Can I use these tools offline?

Yes. Once the page loads, all tools work offline. There are no external API calls or CDN dependencies at runtime.

Test JavaScript and Python regex with live match highlighting. See groups, indices, captured values and match counts, plus common pattern presets.

Regex Flags

Presets:

Test Text

Matches

How it works

Type or paste a regex pattern and flags — the tool uses JavaScript's native RegExp, so any valid JS regex works.
Click Test to highlight every match in the test text and list each capture group with its start and end indices.
Click Explain to get a human-readable breakdown of what each token in your regex does.
Pick a preset to instantly load a common pattern like email, URL, or IP address matching.

Groups, captures and what backreferences match

A capture group records the substring it matched so it can be referenced later — in the replacement string, in a conditional, or through a backreference such as `\1`. Group numbering follows the order of opening parentheses, including groups inside lookarounds.

Named groups (`(?<name>...)` and `\k<name>`) survive refactoring in a way numbered groups do not, and they make an expression self-documenting. They are supported in JavaScript and Python; support elsewhere varies by flavour.

Lookarounds

A lookahead `(?=...)` asserts what follows without consuming it, a lookbehind `(?<=...)` asserts what precedes. This is how you match a word boundary that is not whitespace, or require quotes without consuming them, which keeps the match available to the surrounding pattern.

Lookbehind must be fixed-width in JavaScript, which rules out `(?<=a*)`. A character-class alternative such as `(?<=[^,])` is usually the workaround.

Frequently Asked Questions

How do I test advanced regex patterns?

Enter your regex pattern with flags (g, i, m, s, u) and a test string. The tool highlights all matches with capture groups shown in distinct colours. It also explains the pattern and validates regex syntax.

How is this different from the basic Regex Tester?

The advanced version adds pattern explanation (shows what each part of the regex does), match indices, lookbehind/lookahead visualisation, and substitution preview — useful for complex patterns.

Is my data sent to a server?

No. Every tool on jsonversal runs entirely in your browser using the Web Crypto API, File API, and Clipboard API. Nothing is ever uploaded — not even the JSON you paste in.

Do I need an account or API key?

No signup, no email, no API key. All tools are free to use with no signup required.

Can I use these tools offline?

Yes. Once the page loads, all tools work offline. There are no external API calls or CDN dependencies at runtime.

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Your data never leaves your device

All processing happens in your browser. No uploads, no tracking, no servers. Your API keys, configs, and private payloads stay private — 100% client-side.