Argument MAX with REGex

How to limit actors number to 7 in this example:

index_actor.scrub {regex||\{"FirstName":"([^"]*","LastName":"[^"]*)","Title":"Actor"\}||}

Normally this work but not in regex:

index_actor.scrub {regex(max=7)||\{"FirstName":"([^"]*","LastName":"[^"]*)","Title":"Actor"\}||}

I found this for regex:

{7} Exactly 7
{5,} At least 5
{2,7} 2 to 7

i try several times but no success :-(

Can please somebody help :-)