Oooh, found another interesting form for the higher prime powers.

For prime P, ^(?!(x*)(?!(\1{P-1})(\1\2)*$)\1+$) is the same as ^((x+)\2{P-2}(?=\2$))*x$ and ^(x{P-1})?((\1(x+))(?=(\3*)\1*$)\3*(?=\4$\5))*x$ except that the former also matches zero.

Of interest because of the single instance of the base, not because it's beating anything in length.

How silly of me not to notice a much better naming opportunity earlier :) NEO (No Empty Optional) is much better than NEAM (No Empty After Minimum).