Ah yes, you need to add parenthesis after the pos (to make sure it is interpreted as a function without parameters):

/fred(?(?{ pos() % 4 != 0 })(?!))(....)/sg

I fixed that locally while testing ikegami's regex but then forgot about it when reporting back, sorry.

In reply to Re^4: A regex that only matches at offset that are multiples of a given N? by smls
in thread A regex that only matches at offset that are multiples of a given N? by BrowserUk

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