Here's a solution that doesn't have the arbitrary restriction on the solution size, although since it uses the same regex trick as above, the inputs must still be <32k. It's 75 characters:

($",$n,$_)="}|1{";while($n++<$_[-1]){$_.=1;/^(1{@_})*$/ or$n=0}length()-pop

But the non-regex ideas are interesting to me, as I wasn't sure it was easily possible without the regex logic.