Regex Tester

Pattern

/ /

Flags

g — every match, not just the first

i — ignore case

m — ^ and $ match each line

s — . also matches a newline

u — treat the pattern as Unicode

y — match only at lastIndex

Test text

Replacement

$1 is the first group, $<name> a named one, $& the whole match and $$ a literal dollar sign.

Good to know

The pattern runs in a worker thread. Nothing can interrupt a regular expression once it starts — no timeout, no try/catch — so a pattern that goes wrong is killed with the thread it is in, and this page stays alive.
Named groups are shown next to their number, because a named group is also a numbered one: (?<year>\d{4}) is both.
A pattern that can match nothing — like a* — matches at every position. Those zero-length matches are real and are marked, rather than skipped.
This is the JavaScript flavour, where \w, \d and \b are ASCII-only whatever flags you set: /\b\w+\b/ finds "ok" inside "çok". Unicode letters need \p{L} together with the u flag.

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