A regex tester online lets you write a regular expression, paste your test text, and see matches highlighted in real time — without writing a single line of code. You get instant feedback on whether your pattern works and exactly what it captures.

Regex Visualizer is a free online regex tester that runs entirely in your browser. Nothing leaves your device. Paste your text, enter your pattern, and matches appear immediately as you type.

What does an online regex tester do?

An online regex tester takes a pattern and a piece of text and shows you:

Which parts of the text match the pattern (highlighted inline)
The content of each capturing group for every match
How many matches the pattern finds in total
Whether the pattern compiles without errors

Good tools also let you switch flags — global, case-insensitive, multiline — and see the results change immediately.

How to test a regex pattern online

Open Regex Visualizer.
Type or paste your regular expression into the pattern field. Do not include the surrounding slashes if the tool handles them for you.
Paste the text you want to search into the test area below the pattern.
Matches highlight as you type. Each match is outlined or colored so you can see exactly what the pattern found.
Click any match to inspect the group captures — useful when your pattern has multiple capturing groups.
Toggle flags like g (find all matches) or i (ignore case) from the flag controls and watch the results update.

Common reasons a regex does not match

When your pattern matches nothing, the problem usually falls into one of these categories:

Anchors in the wrong place: using ^ and $ makes the pattern match only the full string. Remove them if you want to find a match inside a longer text.
Missing global flag: without g, the engine stops after the first match. Enable global mode to find every occurrence.
Special characters not escaped: characters like ., (, [, and + have special meaning in regex. To match them literally, add a backslash: \., \(.
Case mismatch: by default, most engines are case-sensitive. hello does not match Hello unless you add the i flag.
Greedy quantifier consuming too much: .* matches everything to the end of the line. Use .*? for a lazy match that stops as early as possible.

Testing email patterns — a common use case

Finding email addresses in text is one of the most common regex tasks. A good starting pattern is:

[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}

Steps to test it:

Open the regex tester.
Paste the pattern above into the pattern field.
Set the g flag and the i flag.
Paste a paragraph of text that contains email addresses.
Every email address in the text highlights as a match. Check the results to make sure the pattern does not pick up anything it should not.

Testing date patterns

Date formats vary, so you usually need to test your pattern against real examples. To match dates in YYYY-MM-DD format:

\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])

Paste a few valid and invalid dates into the tester to confirm the pattern accepts the right values and rejects the wrong ones — such as month 13 or day 32.

Understanding group captures in the tester

Capturing groups, written as (pattern), store part of the match so you can use it separately. When the tester shows group results, each numbered group corresponds to a pair of parentheses in your pattern, from left to right.

Example pattern:

(\d{4})-(\d{2})-(\d{2})

For the input 2026-07-29, the tester reports:

Group 1: 2026
Group 2: 07
Group 3: 29

Named groups, written as (?<year>\d{4}), make the output easier to read in tools that support them.

Multiline mode explained

Without the multiline flag m, the anchors ^ and $ match only the very start and end of the whole string. Enable m and they match the start and end of each line. This changes how many results the tester returns for multi-line input.

Toggle the m flag in the tester while looking at a multi-line sample to see the difference right away.

Privacy when testing online

If your test text contains personal data — emails, names, user IDs — check that the tool runs locally in your browser and does not send anything to a server. Regex Visualizer processes everything on your device. No text you enter is ever uploaded.

FAQ

What is an online regex tester?

An online regex tester is a browser tool where you enter a regular expression and test text. The tool highlights matches in real time and shows group capture results, so you can verify a pattern without running code.

How do I test a regex for email addresses?

Paste your email regex pattern into the tester, set the g and i flags, then paste a text block that includes email addresses. Every email the pattern matches highlights. Check that it catches all valid addresses and does not match anything else.

Why does my regex match nothing?

The most common reasons are: anchors (^ and $) that prevent partial matching, a missing global flag, unescaped special characters like . or (, or a case mismatch with no i flag. Test edge cases one at a time in the tester to isolate the problem.

What is the difference between a capturing group and a non-capturing group?

(pattern) is a capturing group — the matched text is stored and shown in the group results. (?:pattern) is a non-capturing group — it groups the pattern for quantifiers or alternation but does not save the result.

Does the regex tester support all regex flavors?

Regex Visualizer uses the JavaScript regex engine built into your browser. Most patterns work the same in Python, Ruby, and other languages, but some advanced features — like lookbehind length limits — differ between engines.

For a full syntax reference, see the regex cheat sheet. More articles: Regex visualizer blog.

Regex Tester Online: Test Patterns Free