The context is that this is a feature cribbed straight from perl, where where it's passed down from perl 4/pre-5.6, where compiled regexen weren't first-class values. Pretty much every use of it this century is a mistake.

Actually, I have a way this would work well. If you’re interpolating a value that comes from configuration and wouldn’t change.

Example: /admin@#{Rails.config.x.domain}/io

But you’re right that a constant would be a lot more clear. “o” is a footgun.

An HTTP application server matching routes based on runtime configiuration (domains and whatnot) is not really that niche or contrived? Loads of other situations where input not changing during the thread/process lifecycle is part of a set of hot regexes large enough that explicitly compiling each is not a great experience.

I, for one, appreciate /o.