Regex Tester & Debugger — Free Online Tool

What Is Regex Tester & Debugger?

A free online regex tester and debugger. Write patterns with flags (global, case-insensitive, multiline, dotall, unicode, sticky), see live match highlighting, and inspect capture groups with match indices. All in your browser.

Regular expressions are one of the most powerful tools in a developer's arsenal — and one of the hardest to get right. A single misplaced character in a regex pattern can turn a working match into no match, or worse, match everything it should not. Regex Tester & Debugger provides instant feedback as you build your pattern, showing exactly what matches, where the capture groups land, and how each flag affects the result.

Type your pattern into the expression field and toggle any combination of flags: global (g), case-insensitive (i), multiline (m), dotall (s), unicode (u), and sticky (y). The test string area accepts any text, and as you type, matching portions are highlighted in real time. Below the matches, capture groups are displayed individually with their group numbers and matched text, making it easy to see which part of the pattern captured which content.

The tool uses JavaScript's native RegExp engine, so the behavior matches exactly what you will see in Node.js, browser JavaScript, and most modern JS frameworks. This means patterns you develop here work immediately in your code without surprises. The match count and index positions are displayed alongside the results, helping you understand not just what matched but where in the string each match occurred.

For debugging complex patterns, the capture group extraction is particularly valuable. Each group is listed with its group number, making it easy to verify that your capturing groups are capturing the intended content and that non-capturing groups (?:...) are not interfering. The sticky flag (y) behavior is also clearly demonstrated — matches start from the lastIndex position, which is useful for incremental parsing scenarios that are otherwise hard to debug inline.

Real-World Use Cases for Regex Tester & Debugger

Validating Email Address Format in a Form

You need a regex pattern that validates email addresses on a sign-up form. Write and test the pattern against a collection of valid emails (user@example.com, first.last@company.co.uk) and invalid strings (notanemail, @missingusername.com). The instant highlighting shows which test strings match, and you can adjust the pattern until the validation works correctly for your use case.

Extracting Data from Server Logs

Your application logs contain lines like "2024-03-15 14:30:00 ERROR User 12345: Connection timeout". Build a regex with capture groups for the timestamp, log level, user ID, and message. The capture group display shows each extracted piece individually, confirming your pattern correctly parses the log format before you deploy it.

Building a URL Slug Generator

Create a regex that converts article titles into URL-friendly slugs. Write a pattern that matches non-alphanumeric characters and replaces them with hyphens, then collapses multiple hyphens into one. Test it against sample titles with special characters, trailing spaces, and consecutive punctuation to ensure the slug output is clean.

Why Use Regex Tester & Debugger?

Live match highlighting in the test string
Capture group extraction with group numbers
All six regex flags supported (g/i/m/s/u/y)
Match index positions and count display

How to Use Regex Tester & Debugger — Step by Step

Enter your regex pattern in the pattern field.

Toggle flags (g, i, m, s, u, y) as needed.

Paste your test string into the input area.

View highlighted matches and capture groups below.

Who Is Regex Tester & Debugger Best For?

pattern validation
data extraction
log parsing
string matching

Pro Tips for Regex Tester & Debugger

1Start with the simplest pattern that works and add complexity incrementally. It is much easier to debug a two-character pattern than a 50-character one with nested groups and lookaheads.
2Use the global (g) flag when you need all matches, not just the first one. Without the g flag, the regex stops after the first match. This is the most common source of unexpected behavior.
3The dotall (s) flag makes the dot character match newlines. Without it, . matches everything except newline characters. Toggle this flag when your pattern needs to match text spanning multiple lines.
4Test your pattern against edge cases — empty strings, strings with only special characters, and very long strings — to ensure the regex handles extremes gracefully.

Regex Tester & Debugger — Frequently Asked Questions

What regex flavor is supported?

The tool uses JavaScript's native RegExp engine, which supports the standard ECMAScript regex syntax.

What does the sticky (y) flag do?

The sticky flag matches only at the regex lastIndex position, not just anywhere in the string. It is useful for incremental parsing.

Can I use these tools with sensitive/proprietary data?

Absolutely. All processing runs locally in your browser with zero server uploads. This makes these tools safe for proprietary code, internal API keys, and confidential data.

Do these tools work offline?

Yes, once the page has loaded, most developer tools continue functioning without an internet connection. This makes them ideal for air-gapped environments or travel.

What browsers are supported?

All modern browsers are supported: Chrome, Firefox, Safari, and Edge. Internet Explorer is not supported. The tools leverage modern JavaScript APIs like Web Crypto, TextEncoder, and BigInt.

Ready to Use Regex Tester & Debugger?

Free online tool — works in your browser, no sign-up required. Start using it right now.

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