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It is customary to initially study extremes.

It does not mean there is no effect at lesser extremes.

s/effect/association/g

The study established association not causality (as is also customary in initial studies).

why do people write regex in comments?

I see it from time to time and struggle to understand it

It's a concise notation for a type of editing drawn from an old Unix program called `sed`, which is still around in Unix's descendants (i.e., Linux).

In this case, GP suggests that GGP replace "effect" with "association".

Strangely they are using the global flag, even though there's only one instance to replace.