in reply to Re: Explain output produced by Regexp::Assemble
in thread Explain output produced by Regexp::Assemble

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)
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