Which regex flavor is supported?

JavaScript regex (ECMAScript 2022). Flags g, i, m, s, u, y are all supported. Lookbehinds and named groups work.

Does it support PCRE or POSIX?

No. For PCRE (Perl) or POSIX regex, use a server-side tester. Most ECMAScript regex translates directly to PCRE.

No. Everything runs in your browser - neither the pattern nor the test string is sent anywhere.

Live as you type, with yellow highlight and a match count. The global flag is always added internally so you see all matches.

Highlighting stops after 10,000 matches so a runaway pattern cannot freeze the page. The count you see is capped at that number too.

No. The panel highlights each whole match and reports how many there were; named and numbered groups are not listed separately.

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Which regex flavor is supported?: JavaScript regex (ECMAScript 2022). Flags g, i, m, s, u, y are all supported. Lookbehinds and named groups work.
Does it support PCRE or POSIX?: No. For PCRE (Perl) or POSIX regex, use a server-side tester. Most ECMAScript regex translates directly to PCRE.
Is the test string stored?: No. Everything runs in your browser - neither the pattern nor the test string is sent anywhere.
How are matches highlighted?: Live as you type, with yellow highlight and a match count. The global flag is always added internally so you see all matches.
Is there a limit on how many matches are shown?: Highlighting stops after 10,000 matches so a runaway pattern cannot freeze the page. The count you see is capped at that number too.
Does it show capture groups?: No. The panel highlights each whole match and reports how many there were; named and numbered groups are not listed separately.