# Regex tester: run it free, or from your agent

Live regex matching with highlighted matches, capture groups, flags and a replace preview. JavaScript syntax.

## Prompt

Use looot to read [web page URL] and pull every [email address, phone number or price] out of it. 1. Set up https://looot.ai/skill.md. 2. Search the catalog for an endpoint that returns a page as markdown and check its price before you call. 3. Run it. 4. Apply a regular expression for the pattern, drop duplicates and return the matches with a count and the regex you used.

## What it needs

A regular expression, flags and test text

## What comes back

- highlighted matches
- match count
- numbered and named capture groups
- match positions
- replace preview

## Endpoints and providers

None. The tool on the web page runs in the browser and calls no endpoint.

## What it costs

Nothing. The tool on the web page runs in the visitor's browser: no account, no API call and no limit per hour. The prompt above runs the same job from your own looot account with current data, billed per call.

## Extract data

Pull emails, ids or dates out of logs and scraped pages.

## Validate input

Check a form pattern against good and bad examples before you ship it.

## Bulk edits

Preview a find and replace with groups before you run it on real files.

## Check agent regex

Test the pattern an agent wrote before it runs over thousands of rows.

## Which regex flavor does this tester use?

JavaScript, through the browser's own engine, the same one Node.js uses. Python, PCRE, Go and Java differ in places such as lookbehind rules, possessive quantifiers and named group syntax.

## How do I use capture groups?

Wrap part of the pattern in parentheses and it becomes group 1, 2 and so on. Write (?<name>...) for a named group. The match table lists every group of every match.

## What do the flags g, i, m, s, u, y, d and v do?

g finds every match, i ignores case, m makes ^ and the end anchor work per line, s lets the dot match line breaks, u turns on Unicode mode, y matches only at the last index, d records group positions and v enables Unicode set notation.

## Why does my regex only find one match?

Without the g flag JavaScript stops at the first match. Turn g on to list them all.

## Why did the tester stop my pattern?

Patterns with nested repeats, such as (a+)+, can take exponential time on some text. The tester runs the pattern in a background worker and stops it after 1.5 seconds so the page stays usable.

## How do I reference groups in the replacement?

Use a dollar sign and the group number for a numbered group, a dollar sign and the name in angle brackets for a named group, and a dollar sign followed by an ampersand for the whole match. The preview updates as you type.

## Is my test text sent anywhere?

No. Matching runs in your browser, so logs and customer data you paste stay on your machine.

## Related tools

- /free-tools/json-formatter
- /free-tools/json-to-csv
- /free-tools/cron-expression-explainer

## Last reviewed

2026-10-14.