Regex Tester
Regex Tester — runs in your browser. Inputs are not stored.
Regex tester
Enter values, then press Calculate.
Tests only in the browser with JavaScript RegExp. Nothing is sent to the server.
How to use
- In Pattern, enter the regex body only—no slashes. The sample uses email-like groups.
- Use the Flags field or checkboxes to turn on g (global), i (ignore case), m (multiline), s (dotAll), and u (unicode).
- Paste the text to inspect into Test string.
- Press Test to see the match count, highlights, and the Match list / groups.
- Edit the pattern and test again. Press Reset to clear the fields.
Key concepts
The regex tester shows matches with JavaScript RegExp in the browser only. Syntax and flags can differ from PCRE or Python, so a server-side pattern pasted as-is may fail. Capture groups appear in the list with their numbers. If a log contains personal data, try with fake values instead.
With g off, you may mainly see the first match. i ignores case, m makes ^ and $ work per line, s lets the dot match newlines, and u is needed for Unicode escapes. Invalid patterns show a status message. This screen does not run find-and-replace; use the text replace tool for that.
Example
Put an email-like group in the pattern, turn on g, and paste two addresses in the test box to get a match count of 2, highlights, and a group list. Turn on i to catch uppercase domains too. A missing parenthesis shows an error and hides the result. Because this is the JavaScript engine, some lookbehind support varies by browser. With the global flag off, only the first match is highlighted—turn on g to see the full list. Do not paste real personal data; use similar fake values. When you retry the same input, reset first so it does not mix with the previous output. On a shared computer, clear the fields when you finish so the original text is not left on screen. Closing the tab discards what you pasted; you do not need to send a delete request to a server. Button names and field labels match the tool above, so you can follow this page as you go.
Related: Text replace · Text compare
Frequently asked questions
Which engine does the regex tester use?
Browser JavaScript RegExp. Some syntax differs from PCRE and Python.
What happens if I turn off flag g?
It is not global, so results center on the first match. Turn g on to see multiple matches.
Is the test string sent to a server?
No. Testing runs only in the browser.
Does lookbehind work?
Only as far as the browser supports it. Older engine syntax can throw an error.
Does it also show a replacement result?
It shows matches and groups only. Use the text replace tool to substitute.
Which flag is best for matching Hangul with Unicode?
Turn on the u flag to handle Unicode escapes and Hangul boundaries more easily.
Last reviewed: 2026-09-04