What flavor of regex does this tester use?

This regex tester uses the JavaScript RegExp engine built into your browser. That means the syntax is ECMAScript-compatible (similar to PCRE, but not identical). Features like lookaheads, lookbehinds (in modern browsers), named groups, and Unicode flags are supported where your browser supports them.

How do I add flags like global or case-insensitive?

Use the checkboxes above the regex input to enable flags: g for global, i for case-insensitive, m for multiline, s for dotAll, u for Unicode, and y for sticky. The tester will rebuild the RegExp with the selected flags and re-run the test on every change.

Can I see capturing groups and their contents?

Yes. Below the test area you will see a structured table of all matches. For each match, the tester lists the full match, each capturing group, and named groups (if any). In the highlighted text, different capturing groups are color-coded for quick visual debugging.

Why does my regex work on other sites but not here?

Different tools use different regex engines (PCRE, .NET, Java, RE2, etc.). This tester uses the JavaScript engine, which has slightly different syntax and supported features. For example, some advanced backtracking control verbs or atomic groups from PCRE are not available in JavaScript.

Is my test data sent to a server?

No. All regex evaluation happens entirely in your browser using JavaScript. Your pattern and test text are not uploaded to a server, which makes this tool suitable for testing sensitive snippets like logs or anonymized user data.

Online Regex Tester & Visual Debugger (JavaScript, PCRE, Flags)

How this regex tester works

This tool uses the built-in RegExp engine in your browser (ECMAScript/JavaScript flavor). Every time you change the pattern, flags, or test string, the tester:

Builds a new RegExp with the selected flags.
Runs it against the test string (using exec in a loop when g is set).
Highlights matches and capturing groups in the text.
Displays a structured table of matches, groups, and named groups.

Commonly used flags

g – global: find all matches instead of stopping after the first.
i – ignore case: make the pattern case-insensitive.
m – multiline: ^ and $ match line boundaries, not just the whole string.
s – dotAll: . also matches newline characters.
u – Unicode: enables full Unicode support and Unicode escapes like \p{L}.
y – sticky: matches must start exactly at lastIndex.

Examples you can try

Email-like strings:
/\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}\b/g
IPv4 addresses:
/\b(?:\d{1,3}\.){3}\d{1,3}\b/g
Capture date parts:
/(\d{4})-(\d{2})-(\d{2})/
Groups 1–3 will contain year, month, and day.

Tips for debugging regular expressions

1. Start simple

Begin with a small pattern that you know works, then gradually add more pieces. This makes it easier to see where things break.

2. Use capturing groups strategically

Capturing groups ( ... ) not only help you extract data, they also make the highlighted output easier to understand. Use non-capturing groups (?: ... ) when you only need grouping, not extraction.

3. Be careful with greediness

Quantifiers like +, *, and {m,n} are greedy by default. If your match “eats” too much, try the lazy versions: +?, *?, {m,n}?.

4. Remember engine differences

Many online tools use PCRE or .NET regex engines. This tester uses JavaScript’s engine, which is similar but not identical. Some advanced constructs (like atomic groups or backtracking control verbs) are not supported.

FAQ

Is this safe for sensitive data?

Yes. All processing happens in your browser; the pattern and test string are not sent to a server. Still, avoid pasting fully identifiable personal data when possible.

Can I test multiline logs?

Yes. Paste any multiline text into the test area. Use the m flag if you want ^ and $ to match the start and end of each line.

Online Regex Tester & Visual Debugger

Highlighted matches

Match details