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Regex Tester

Test, debug, and explain regular expressions with live matching and capture groups.

Your files stay on your device - processed locally via WebAssembly, never uploaded

//g

All processing runs entirely in your browser. Nothing is sent to any server.

What is the best free regex tester online?

OptiPix Regex Tester tests, debugs, and explains regular expressions with live match highlighting, replace preview, and a pattern explainer, all running locally in your browser. Unlike regex101 or RegExr, the strings you test are never sent to a server, so production data and logs stay private.

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About Regex Tester

Last updated: May 2026

OptiPix Regex Tester is a full-featured regular expression playground that runs entirely in your browser. Paste or type your regex pattern and test string — matches highlight instantly with alternating colors so you can see boundaries clearly. Every capture group (numbered and named) is listed per match with index ranges. Switch to Replace mode to test substitution patterns with $1, $2, and named back-references. The built-in explanation panel parses your regex token-by-token and translates it into plain English. A collapsible cheatsheet covers character classes, quantifiers, anchors, groups, and lookarounds, and you can click any item to insert it at your cursor. The Common Patterns library loads battle-tested regexes for email, URL, phone, IP, date, hex color, username, and password validation with one click. Catastrophic backtracking protection aborts patterns that take too long. Uses JavaScript's native RegExp engine with full flag support (g, i, m, s, u, y). No accounts, no network calls — your data stays on your device.

How It Works

Constructs a JavaScript RegExp from your pattern and flags, runs matchAll() on the test string, and highlights results in real-time.

Use Cases

  • •Debug regex patterns before using in code
  • •Validate email/URL/phone patterns
  • •Learn regex with explanations
  • •Test find-and-replace patterns
  • •Extract data with capture groups

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OptiPix Regex Tester vs regex101 vs regexr.com vs RegExr alternatives

FeatureOptiPixregex101regexr.comRegExr alternatives
Test text sent to a serverNeverYes (saved/shared patterns)Client-side, account features uploadVaries
Live match highlightingYesYesYesVaries
EngineNative JavaScript (ECMAScript)PCRE2, JS, Python, Go, moreJavaScriptJavaScript
Capture group inspectionYes, named + numberedYesYesVaries
Works offlineYesNoNoNo

Competitor details reflect publicly listed free-tier features and may change.

Engine behavior and flag semantics

Patterns execute in the browser's actual ECMAScript regex engine, so behavior matches exactly what your JavaScript/TypeScript code will do in production - including lookbehind support, named capture groups ((?<name>...)), Unicode property escapes (\p{Letter} with the u flag), and the s flag making . match newlines.

All matches are highlighted live in the test text as you edit either the pattern or the input, with each capture group's contents listed per match. This makes off-by-one quantifier mistakes (greedy .* swallowing too much, forgetting ? for lazy matching) visible immediately rather than after a failed deploy.

Note the engine difference if you'll run the pattern elsewhere: Python's re and PCRE diverge from JavaScript on lookbehind variable lengths, possessive quantifiers, and \Z anchors. For JS, Node, Deno, or browser code, what you test here is authoritative.

Frequently Asked Questions

Which regex flavor?
JavaScript's native RegExp engine with full flag support.
Can I test replacements?
Yes. Toggle to Replace mode and use $1, $2, $&, or ${name} references.
Does it explain my regex?
Yes. Every token gets a human-readable description.
Is my data sent to a server?
No. Everything runs locally in your browser.
What about backtracking?
Patterns that take over 2 seconds are automatically aborted.
Why does my regex work here but not in Python?
JavaScript and Python use different regex engines. Common gotchas: Python spells named groups (?P<name>...) versus JS (?<name>...), JS needs the u flag for Unicode property escapes, and re.match anchors at string start while JS .match does not.
What do the g, i, m, s, u flags mean?
g finds all matches instead of stopping at the first; i ignores case; m makes ^ and $ match at line breaks; s lets . match newlines; u enables full Unicode mode. Flags combine freely - gim is the most common combination for log searching.
How do I match text between two words?
Use a lazy quantifier between anchors: start(.*?)end captures the shortest span between the words. With the s flag it will also cross line breaks. Greedy .* instead would run to the last 'end' in the text.
How do I test a regular expression online?
Paste your pattern and a sample string into OptiPix Regex Tester: matches highlight live as you edit, with capture groups listed separately. It supports flags, replace preview, and a plain-English pattern explainer. Everything runs locally in your browser, so production log data you test against is never uploaded.
What does my regex pattern mean?
OptiPix Regex Tester includes an explainer that breaks your pattern into parts and describes each token in plain English: quantifiers, character classes, anchors, groups, and lookarounds. This is the fastest way to understand inherited patterns in legacy code before modifying them, without reading the full regex specification.
What is the regex for validating an email address?
A practical pattern is ^[^\s@]+@[^\s@]+\.[^\s@]+$, which checks for one @ with non-space text around it and a dot in the domain. Fully RFC-compliant email regex is famously enormous and rarely worth it; validate format with simple regex, then confirm deliverability with a verification email.
Why does my regex work in one language but not another?
Regex flavors differ: JavaScript, PCRE (PHP), Python, and Go each support different features like lookbehind, named groups, or possessive quantifiers, and escape syntax varies. OptiPix Regex Tester uses the JavaScript engine, which since ES2018 supports lookbehind and named capture groups, covering most modern patterns.
Is it safe to test regex against production data online?
Only in a tester that runs locally. OptiPix Regex Tester executes patterns in your browser with no network request, so logs, customer records, and connection strings you paste never reach a server. With server-side testers, everything you test against is transmitted and potentially retained by a third party.

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