Regex Tester
Free online tool · runs in your browser
Test regex patterns with live matches.
Quick tutorial
- 1Enter a regex pattern.
- 2Enter test text and select flags.
- 3Click Test.
Free online regex tester
Test regular expressions against sample text with live matches and capture groups. Enter a pattern, choose JavaScript flags (g, i, m, s, u, y), and see what matched. This regex tester uses the browser’s RegExp engine — the same semantics as frontend JavaScript — and never sends your pattern or corpus to a server.
Documentation last reviewed .
JavaScript regex semantics
This tester uses your browser’s RegExp engine — the same flavor as frontend JavaScript. Patterns that work in PCRE or Python may differ (lookbehind support, POSIX classes, and some escape sequences).
Test against realistic sample text, enable the flags you need (g, i, m, s, u, y), and watch for catastrophic backtracking on large inputs. Patterns and corpora never leave your device.
How to use Regex Tester
- Enter a regex pattern.
- Enter test text and select flags.
- Click Test.
Limitations
- Complex patterns on huge text may be slow.
- Uses JavaScript RegExp semantics.
Example
Input
Pattern: \w+ Text: hello world
Output
Match 1: "hello" at index 0
FAQ
Frequently Asked Questions
Which regex flavor is this?
JavaScript RegExp. Patterns valid in PCRE or Python may differ (lookbehind, POSIX classes, etc.).
Why is my regex slow?
Catastrophic backtracking on large input can freeze the tab. Simplify the pattern or shorten the test text.
Which flags are supported?
g, i, m, s, u, y — standard JS flags.
Is Regex Tester free to use?
Yes. Regex Tester on ShadowReference is free, with no account, no watermarks, and no server-side processing.
Does this tool send my data to a server?
No. All processing happens locally in your browser. Your input never leaves your device.
Related guides
Deeper reading on private, browser-based workflows for regex tester and related tools.
- Why Private Browser Developer Tools Matter — Why ShadowReference runs every tool in your browser, what “no upload” really means, and how that helps day-to-day engineering work.
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