How do I use capture groups?

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

Regex testerValidators

Regex tester. See every match as you type.

Type a pattern and some text. Every match is highlighted, each capture group is listed, and a replace preview shows the result. It uses the JavaScript regex engine, so flags and syntax match Node.js and the browser.

Cost to you

Free

Runs in your browser. No signup, no API call, no limit per hour.

browser
runs in: none
API calls: none
account

Use it now

Use the Regex tester here

It runs in your browser. Nothing you type is sent anywhere, there is no signup and no limit.

Pattern

Regular expression (JavaScript syntax)

Common patterns

Flags

Test text

Matches

Order 4471 shipped on 2026-10-14 to ada@example.com. Refund for order 4398 sent 2026-09-30 to grace.hopper+billing@example.org. Server 192.168.1.20 answered with color #1f6feb at https://status.example.com/run?id=7.

Replace

Show a replace preview

Replacement

Use $& for the whole match, $1 for group 1 and $<name> for a named group.

Runs in your browser with the JavaScript regex engine, the same one Node.js and Chrome use. Python, PCRE and Go differ in places such as lookbehind and possessive quantifiers.

Run it from your agent

Copy one prompt, give it to your agent.

Fill in the placeholder, paste it, and the agent runs the same job with current data on your looot account and shows the cost.

You give it: a regular expression, flags and test text.
Billed per call, nothing while your agent is idle.
The agent shows the price before it calls.

Prompt: Regex tester

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.

Give this to your agent

Setup command

set up looot - https://looot.ai/skill.md

What comes back

What your agent gets from regex tester

Field names as the providers return them. The exact set varies by endpoint, and your agent reads it from the catalog.

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

Use it for

What people do with regex tester

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.

Set up once

Three steps. Your agent does the rest.

Paste one prompt into your agent

Set up looot

Done, looot is connected.

Copy one prompt from /connect into Claude Code, Cursor, or any MCP agent. No SDK, no dashboard click-through first.

Your agent finds the right operation

Hunter$0.0265

Exa$0.0090

Diffbot$0.0032

The agent finds the operation it needs and sees the price first. It calls nothing blind.

You watch what it cost

Hunter$0.0265

Exa$0.0090

Diffbot$0.0032

Total$0.0387

Every call settles for its quoted price and lands in the ledger. You always know what ran and what it cost.

Keep going

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Converts a JSON array to CSV in the browser, flattening nested keys into dotted columns. Preview, copy or download.
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Explains a five-field cron expression in plain English and lists the next five run times in UTC or local time.
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FAQ

Questions about regex tester

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.

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.

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.

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

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.

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.

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

For agents

tool: Regex tester
category: Validators
input: A regular expression, flags and test text
output: highlighted matches, match count, numbered and named capture groups, match positions, replace preview
price: per call, read live from the catalog on the web page
web tool: runs in the visitor's browser, no API call, free and unlimited
schedule: on demand
setup: https://looot.ai/skill.md
llms.txt: https://looot.ai/llms.txt
updated: 2026-10-14

markdown /free-tools/regex-tester.md

Run regex tester from your own agent.

Start your workspace, top up, and paste the prompt into the agent you already use. It runs the job with live data and shows the cost first.

Start your workspace See all free tools