Regex Tester
Regexx
Write it. Test it. Ship it.
Live regex matching in your browser — highlights, groups, replace, cheat sheet. No server, no sign-up, zero tracking.
Result
Quick patterns
Quick Reference
What is Regexx?
Regexx is a free, browser-native regular expression tester built for developers who want instant feedback without friction. Paste a pattern, paste a string, and see matches highlighted in real time — no page reload, no server round-trip, no account required. Everything runs inside a Web Worker thread in your own browser, so your data never leaves your device.
Regular expressions are one of the most powerful tools in a developer's toolkit. They can validate form inputs, extract structured data from messy text, transform strings at scale, and enforce naming conventions across a codebase. But they're notoriously hard to read and debug in isolation. Regexx gives you a focused environment to write, test, and understand your patterns before committing them to production code.
Why use an online regex tester?
Instant visual feedback
Seeing exactly which parts of your string match — and which don't — is orders of magnitude faster than running a script, reading output, and adjusting. Regexx highlights every match as you type, including capture group breakdowns, so you can iterate in seconds.
Understand before you commit
Shipping a regex you haven't fully verified is a common source of production bugs. An online tester lets you stress-test edge cases — empty strings, Unicode characters, multi-line inputs, pathological backtracking — before the pattern touches real data.
Learn by doing
The best way to internalize regex syntax is to experiment. Regexx's quick reference panel keeps every character class, quantifier, and anchor one scroll away, so you can look up \b or (?<name>...) and try it immediately without switching tabs.
Find & Replace preview
Replacing text with a regex is powerful and dangerous in equal measure. Regexx's replace panel shows you the full result before you apply it to a file or database, with full support for backreferences like $1 and named group substitutions.
Regexx Architecture
Regexx is engineered for raw performance, utilizing browser-native HPC principles to handle regex operations. By delegating all matching logic to dedicated Web Worker threads, the engine operates independently of the main UI thread. This architectural choice ensures that the interface remains completely responsive, effectively mitigating UI freezes even when processing massive inputs or complex patterns prone to heavy backtracking.
Our zero-backend approach guarantees total data sovereignty. Every operation—from pattern compilation to match rendering—executes entirely on your local hardware. No data is transmitted over the network; your input remains isolated within your browser’s memory and is instantly purged the moment you close the tab.
Frequently asked questions
Is Regexx free?
Yes, completely free. No subscription, no usage limits, no account. It's part of the RuntimeHub suite of free developer tools.
Which regex flavor does Regexx use?
Regexx uses the JavaScript RegExp engine built into your browser — the same engine used by Node.js and every modern browser. This means it supports ES2018+ features like named capture groups ((?<name>...)), lookbehind assertions, and the s (dotAll) flag. It does not support PCRE-specific syntax like \K or conditional patterns.
Can I use Regexx with multiline strings?
Yes. Paste any multi-line text into the test area. Enable the m flag to make ^ and $ match the start and end of each line instead of the whole string, and the s flag to make . match newline characters.
Does Regexx support named capture groups?
Yes. Write (?<groupName>pattern) in your regex and Regexx will display the named group values alongside each match in the match list below the test area.
How do I use the Replace feature?
Type your replacement string in the Replace field. You can reference capture groups with $1, $2, etc., or with $<name> for named groups. Click Apply and the full replaced string appears below. The original test string is not modified — you can copy the result separately.
Is my data private?
Completely. All processing happens in a Web Worker inside your browser. Your patterns and test strings are never sent to any server. Regexx has no backend, no analytics of its own, and sets no cookies. See the Privacy Policy for full details.
What are the quick pattern snippets?
The snippet buttons load pre-written patterns for common use cases — email addresses, URLs, IPv4 addresses, ISO dates, hex color codes, phone numbers, UUIDs, HTML tags, slugs, and more. Click any snippet to load it into the pattern field, then customise it to fit your needs.
Why do I get a syntax error on a valid-looking pattern?
JavaScript's regex engine is strict about certain syntax. Common causes: unescaped special characters (use \. instead of . to match a literal dot), lookbehind assertions require a fixed-length pattern in older engines, and some PCRE syntax is not supported. The error message shown in the status bar is the browser's native error — it usually points directly to the problem character or position.
Can I share a regex with someone?
Not via a permalink currently — Regexx is a stateless tool by design. The simplest way to share is to copy the pattern and paste it into a message or code comment. A share-via-URL feature may be added in a future update.