About the Regex Tester
The Regex Tester runs a JavaScript regular expression against your sample text in real time — match count, every match with its capture groups and position, and clear error messages for invalid patterns.
JavaScript (ECMAScript) — including lookaheads/lookbehinds, named groups and the u flag for Unicode.
g = all matches, i = case-insensitive, m = ^\$ per line, s = dot matches newline, u = Unicode mode.