Live matches and capture groups — plus a warning when a pattern can hang on long input, which is the failure nobody sees coming.
—
Capture groups
About the danger warning. Patterns that nest one quantifier inside
another — (a+)+, (\\w*)*, (.*)* — can take
exponential time on input that almost matches. On a server handling user input
that is not a slow page, it is an outage: one crafted string pins a CPU core. It has taken
down Cloudflare and Stack Overflow, both from a single regex. The check here is a
heuristic, not a proof — it flags the shapes that cause it, and silence is not a guarantee.
Flags: g all matches · i ignore case ·
m ^$ match line ends · s . matches
newline · u unicode · y sticky.