What regex flavour does this tester use?

It uses the JavaScript (ECMAScript) regex engine — the same one running in Chrome, Firefox, Safari, and Node.js. Patterns you test here work directly in your JS code.

Can I test regex with named capture groups?

Yes. Use (? pattern) syntax. The tool displays named groups in the match details panel alongside numbered groups.

How do I enable flags like case-insensitive?

Toggle flags (g, i, m, s, u) using the checkboxes next to the pattern input. You can also type them inline as /pattern/gi.

Does it explain what my regex does?

Yes. The tool provides a token-by-token plain-English explanation of your pattern, making it easier to understand complex expressions.

Can I test regex replacements?

Enable the 'Replace' mode to provide a replacement string with $1, $2 back-references. The output shows the replaced text in real time.

Is there a performance warning for catastrophic backtracking?

The tester runs a timeout guard. If your regex causes exponential backtracking (e.g., nested quantifiers on overlapping character classes), it aborts with a warning.

Can I share my regex with a colleague?

The pattern, flags, and test string are encoded in the URL. Copy the browser address bar to share the exact test case.

Does this work offline?

Once loaded, the regex tester works offline since all matching happens in your browser. Useful for testing patterns on flights or spotty connections.

Regex Tester

Dev Live

Test regular expressions with live highlighting and explanation.

About this tool

Regex Tester — Test regular expressions with live highlighting and explanation. Free, fast and private. Runs entirely in your browser at Quick Calci Tools.

Examples

Common scenarios for the Regex Tester.

Extract emails

regex: [\w.+-]+@[\w-]+\.[\w.-]+
text: Contact alice@x.com and bob@y.io

2 matches: alice@x.com, bob@y.io.

Replace whitespace

regex: \s+
flags: g
replacement

Collapses any run of whitespace into a single space.

Who uses the Regex Tester?

Common scenarios and audiences for this tool.

Backend developers

Build and validate input-validation patterns (email, phone, postal code) before embedding them in form validators or API schemas.

Data engineers

Write extraction patterns for log parsing, CSV field matching, and ETL pipeline transformations.

DevOps & SRE teams

Craft regex for log-alerting rules (Datadog, Splunk, Grafana Loki) and test them against sample log lines.

Technical writers

Find-and-replace patterns across documentation repos to update version numbers, URLs, or terminology consistently.

Students learning regex

Experiment with quantifiers, lookaheads, and character classes while seeing real-time visual feedback on what matches.

Frequently asked questions

Everything you need to know about the Regex Tester.