Regex Tester

Test a regular expression against a string with live highlighting, a full match list with capture groups, and a replace preview. Nothing is sent to a server — everything runs in your browser.

Pattern

Test string

Highlight

Match list

Replace

How to use

Enter a pattern: Type a regular expression between the / / and check whichever flags you need (g, i, m, s, u, y). A syntax error in the pattern shows an error message right below it.
Test string: Type the text you want to match against, and matching portions are highlighted live.
Check the match list: See each match's position, its full matched text, and both numbered and named capture group values. Click 📋 next to any value to copy just that.
Preview a replace: Open the "Replace" panel and enter a replacement string (reference capture groups with $1, $2, $<name>) to see the result using the same rules as JavaScript's String.replace(). Without the g flag, only the first match is replaced.

All matching runs on the browser's built-in JavaScript regex engine; the pattern and string you enter are never sent to a server or stored.

Common flags and patterns

Flags: g (global — find every match) · i (ignore case) · m (multiline — ^/$ match at each line boundary) · s (dotAll — . also matches newlines) · u (unicode mode) · y (sticky — matches only at lastIndex).

A few common patterns:

\b[\w.+-]+@[\w-]+\.[a-zA-Z]{2,}\b — email address
^$?\d{3}$?[-.\s]?\d{3}[-.\s]?\d{4}$ — US-style phone number
https?:\/\/[^\s]+ — URL
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$ — password with lower/upper/digit, 8+ chars
(\d{4})-(\d{2})-(\d{2}) — YYYY-MM-DD date (captures year/month/day)

Frequently Asked Questions

Is the pattern or test string sent anywhere?

No. All matching and replacing runs entirely in your browser via JavaScript; nothing is sent to a server.

Which regex flavor does this use?

Whatever your browser's JavaScript engine implements. Some syntax (lookbehind support, named-group syntax) can differ from PCRE (PHP) or Python's re module.

I only see one match even with the g flag on — why?

The highlight and match list always find every match, for exploration purposes. The "Replace" result, however, follows real String.replace() semantics: without the g flag, only the first match is replaced. Turn on g to replace all of them.

Can a bad pattern freeze my browser?

Certain patterns (nested repetition, in particular) can trigger "catastrophic backtracking," where matching time grows exponentially for some inputs. That's a property of regex engines in general, not specific to this tool — if the page seems to hang, simplify the pattern or reload the tab.

Are named capture groups supported?

Yes. Write (?<name>...) and the match list will show it by name; in the replacement string you can reference it with $<name>.