As far as I understand pattern

?:something

means something is a pattern to be tested for.

Not quite. What you quoted is incomplete. It should be 

(?:something)

[download]
and it tells the regex engine that the parentheses should be used only for grouping, and not for capturing a submatch in one of $1, $2, etc.

the lowliest monk

In reply to Re^2: Explain output produced by Regexp::Assemble by tlm
in thread Explain output produced by Regexp::Assemble by swift

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