In Regexes§
See primary documentation in context for Non-capturing grouping
The parentheses in regexes perform a double role: they group the regex elements inside and they capture what is matched by the sub-regex inside.
To get only the grouping behavior, you can use square brackets [ ... ] which, by default, don't capture.
if 'abc' ~~ / [a||b] (c) /
If you do not need the captures, using non-capturing [ ... ] groups provides the following benefits:
they more cleanly communicate the regex intent,
they make it easier to count the capturing groups that do match, and
they make matching a bit faster.