Regex Tester — Free Online Tool

A regex tester is a tool that runs regular expressions against sample text to show matches, capture groups, and substitution results in real time. SWEDevTools: Prism provides a free, local-first regex tester that runs entirely in your browser—no test strings are ever sent to a server—making it safe for testing patterns against production logs, PII data, and sensitive content.

Features

• Real-time match highlighting: See matches update instantly as you edit the pattern or test text—no need to click a button for each iteration.
• Capture group inspection: View numbered and named capture groups with their matched values to verify extraction logic before writing code.
• Flag support: Test with standard regex flags: global (g), case-insensitive (i), multiline (m), dotAll (s), and unicode (u).
• Works offline as a PWA: Install SWEDevTools: Prism and test regex patterns without an internet connection.
• No signup required: Test regular expressions instantly with zero account creation, no email verification, and no usage limits.
• Local-first privacy: All pattern matching happens in your browser. No test strings or patterns are ever uploaded to a server.
• Pipeline chaining support: Chain regex test results into other tools for multi-step text processing workflows.

Frequently Asked Questions

Is the regex tester free?

Yes, the SWEDevTools: Prism regex tester is completely free with no signup, no ads, and no usage limits.

Does the regex tester work offline?

Yes. SWEDevTools: Prism is a PWA that works without an internet connection once installed on your device.

Is my data safe when testing regex patterns?

Yes. All pattern matching happens locally in your browser. No test strings or regex patterns are ever uploaded to a server, making it safe for production logs, PII, and sensitive data.

Which regex flavor does this use?

The tester uses JavaScript's built-in RegExp engine. Always verify patterns in your target runtime (Python, Java, .NET, Go, PCRE) since features like lookbehind support and Unicode properties vary between engines.

Why does my regex work here but not in my code?

Common causes: different flags, double-escaping in string literals (\\d vs \d), or engine-specific features. JavaScript regex may support features not available in RE2 or POSIX, and vice versa.

What is catastrophic backtracking (ReDoS)?

Some regex patterns take exponential time on certain inputs, creating a denial-of-service vulnerability. Avoid nested quantifiers like (a+)+ and test with long adversarial strings. Consider using RE2-compatible patterns for safety.

Should I parse HTML with regex?

Generally no—use a proper HTML parser. Regex can work for small, constrained extraction tasks (e.g., pulling URLs from a known format) but is too brittle for general HTML processing.

Why does "." not match newlines?

By default, the dot metacharacter matches any character except newlines. Enable the dotAll flag (s) to make dot match newlines, or use [\\s\\S] as a cross-engine alternative.

Do named capture groups work everywhere?

Named groups (?<name>...) are supported in JavaScript ES2018+, Python, .NET, Java 7+, and PCRE. Older engines and some regex libraries may not support them.

How is this different from Regex101?

Regex101 is a hosted service that processes your test strings on their server. SWEDevTools: Prism runs entirely in your browser with zero network requests, provides pipeline chaining with other dev tools, and works offline as a PWA.

Is Prism a good alternative to regex101.com?

Yes. The biggest advantage is privacy: regex101.com processes your test strings on their servers, while SWEDevTools: Prism runs entirely in your browser with zero network requests. Prism also works offline as a PWA, requires no signup, and offers pipeline chaining to combine regex testing with other developer tools.

What happened to smalldev.tools?

smalldev.tools is no longer available. Prism by SWEDevTools offers the same developer tools and more, with offline support, pipeline chaining, and completely free usage — no signup required.