Q: How many matches will it show at once?

When the g flag is on, the tester collects up to 10,000 matches, then stops and reports the limit. This guard prevents a runaway zero-width or pathological pattern from freezing the page. Tighten your pattern or test on a smaller sample if you hit it.

Q: Why does my regex hang or match nothing on a long string?

Nested quantifiers such as (a+)+ or (.*)* cause catastrophic backtracking: on a non-matching input the engine explores exponentially many paths and stalls. Rewrite (a+)+ as a+, use a{2,} for explicit counts, or anchor the pattern. The native engine has no backtrack limit, so a bad pattern can lock the tab.

Q: How do I match the start and end of each line instead of the whole string?

Enable the m (multiline) flag. With m on, ^ matches after every newline and $ matches before every newline, so a pattern like ^\w+ catches the first word of every line. Without m, ^ and $ only anchor to the very start and end of the entire input.

Question 1

Is this regex tester free?

Accepted Answer

Yes. The regex tester is 100% free with no signup and no usage limits. Every feature, including flag toggles, capture-group inspection, and live highlighting, is unlocked for everyone.

Question 2

Does my pattern run in my browser or get uploaded?

Accepted Answer

It runs entirely in your browser. The tool calls the native JavaScript RegExp engine on your device, so your pattern and test text are never sent to a server, logged, or stored. You can disconnect from the internet and it still works.

Question 3

Which regex flavor does this tester use?

Accepted Answer

It uses the JavaScript (ECMAScript) RegExp flavor via your browser's built-in new RegExp(pattern, flags). That means JS-specific rules apply: no possessive quantifiers, no atomic groups, and \d matches Unicode digits only when the u flag is set. Patterns written for PCRE, Python, or .NET may behave differently.

Question 4

Which flags can I toggle?

Accepted Answer

You can toggle five flags: g (global, find all matches), m (multiline, ^ and $ match each line), i (case-insensitive), s (dotAll, lets . match newlines), and u (full Unicode). The default is gm. The sticky (y), hasIndices (d), and unicodeSets (v) flags are not exposed as toggles.

Question 5

Does it support named capture groups and lookbehind?

Accepted Answer

Yes. Because it uses the modern browser engine, ES2018 features work: named groups with (?<name>...), positive and negative lookbehind (?<=...) and (?<!...), and Unicode property escapes like \p{L} under the u flag. Captured groups display as $1, $2, and named groups appear in the match details.

Question 6

How many matches will it show at once?

Accepted Answer

When the g flag is on, the tester collects up to 10,000 matches, then stops and reports the limit. This guard prevents a runaway zero-width or pathological pattern from freezing the page. Tighten your pattern or test on a smaller sample if you hit it.

Question 7

Why does my regex hang or match nothing on a long string?

Accepted Answer

Nested quantifiers such as (a+)+ or (.*)* cause catastrophic backtracking: on a non-matching input the engine explores exponentially many paths and stalls. Rewrite (a+)+ as a+, use a{2,} for explicit counts, or anchor the pattern. The native engine has no backtrack limit, so a bad pattern can lock the tab.

Question 8

How do I match the start and end of each line instead of the whole string?

Accepted Answer

Enable the m (multiline) flag. With m on, ^ matches after every newline and $ matches before every newline, so a pattern like ^\w+ catches the first word of every line. Without m, ^ and $ only anchor to the very start and end of the entire input.

Flag	Name	What it changes
g	global	Find all matches instead of stopping after the first.
m	multiline	`^` and `$` match the start/end of each line, not just the whole string.
i	ignoreCase	Casing differences are ignored when matching.
s	dotAll	Lets `.` match newline characters too.
u	unicode	Treats the pattern as Unicode code points; enables `\p{...}` escapes.

Token	Description	Example Matches
.	Any character (except newline)	a, 1, %
\w	Word character	a-z, A-Z, 0-9, _
\d	Digit	0-9
[abc]	Character set (a, b, or c)	a, b, or c
(?:...)	Non-capturing group (no $n slot)	groups without capturing
+	One or more quantifier	"a" or "aaaa"

Regex Tester — Free Online Regular Expression Debugger

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Online Regex Tester for JavaScript Regular Expressions

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