Regex tester

Regex & strings

Test regex with live highlights, groups and replace preview.

Updated May 17, 2026

Pattern

//g

Test string

The quick brown fox jumps over the lazy dog.

Matches (9)

"The"at 0, length 3
$1:

The
word:

The
"quick"at 4, length 5
$1:

quick
word:

quick
"brown"at 10, length 5
$1:

brown
word:

brown
"fox"at 16, length 3
$1:

fox
word:

fox
"jumps"at 20, length 5
$1:

jumps
word:

jumps
"over"at 26, length 4
$1:

over
word:

over
"the"at 31, length 3
$1:

the
word:

the
"lazy"at 35, length 4
$1:

lazy
word:

lazy
"dog"at 40, length 3
$1:

dog
word:

dog

Replace

Enable

Quick start

How to test a regex in your browser

Type a pattern, flip flags, paste test text, see live highlights and captured groups.

Step 1

Write your pattern

Type the pattern (without the surrounding slashes). Toggle g / i / m / s / u / y flags as needed.
Step 2

Paste test text

Drop the string to test against. Matches highlight live and group values appear in the list below.
Step 3

Try a replacement

Enable the Replace pane to preview replacements with $1, $<name>, $& syntax. Output updates as you type.

In-depth guide

JavaScript regex tester with live highlights

Test JavaScript regular expressions against any input string with live match highlighting, numbered and named capture group inspection, and an optional replace preview.

Flags cheat sheet

g — global. Find every match, not just the first.
i — case-insensitive.
m — multiline. ^ and $ match line boundaries instead of only string boundaries.
s — dotall. . matches newlines.
u — unicode mode. Required for \u{...} and \p{...}.
y — sticky. Match must start at lastIndex.

Capture groups, named groups and back-references

Numbered groups — wrap with parentheses: (\w+). Reference in replacement as $1.
Named groups — (?<name>\w+). Reference as $<name>.
Non-capturing — (?:foo|bar). Groups for alternation without producing a capture.
Back-references — \1 in the pattern matches whatever the first group captured.

Common pitfalls

Catastrophic backtracking — nested quantifiers like (a+)+ can hang the engine on long inputs. Prefer non-greedy or atomic alternatives.
Greedy by default — .* matches as much as possible. Use .*? for the lazy version.
Anchors — without ^ / $, a regex matches anywhere. Add them when validating full-string formats.
Escape special chars — . * + ? ( ) [ ] { } | \ ^ $ all need \ when matched literally.

Frequently asked questions

Does this tool send my regex anywhere?

No. Compilation, matching and replacement all run in your browser using the native RegExp engine. No requests fire.

Which regex dialect is supported?

JavaScript's ECMAScript 2018+ regex. Supports lookahead, lookbehind, named groups (?<name>...), unicode property escapes \p{L} with the u flag, and the s (dotall) flag.

How are flags interpreted?

g = find all matches; i = case-insensitive; m = ^ and $ match line breaks; s = dot matches newlines; u = unicode mode (needed for \u{...} and \p{...}); y = sticky.

What replacement variables can I use?

$& for the full match, $1 / $2 / ... for numbered groups, $<name> for named groups, $$ for a literal dollar sign. Same as String.prototype.replace.

Why is the match list truncated?

We cap at 100 displayed rows to keep the UI responsive on huge inputs. The full match count is shown in the header — if you need all entries, narrow your pattern or use the Replace pane.

Why doesn't a regex from another language work?

Perl/PCRE, Python re and .NET each have slightly different syntax — recursive patterns, possessive quantifiers, embedded modifiers. JavaScript supports a slightly narrower set. Test inside JavaScript before relying on it.

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