Minimum String Length

[mcrowson]

The limit argument allows me to set a maximum length for the returned string, but i have no way to set the minimum. I attempted to use a forward lookahead in the regex pattern, but it seems like it is ignored.

^(?=.{12,16}$)(9*)$

So that should give me strings with a minimum length of 12 and maximum length of 16 made up of 9s. When i run this it spits out a whole lot of random stuff. If i take away the lookahead, it correctly gives me the 9s

^(9*)$

If lookaheads or lookbehinds are not the right way to set string minimums, what is? If they are the correct way, what is going wrong with exrex?

EDIT: I should note that my desired regex pattern is quite a bit more involved than a bunch of 9s, I am just using it here as a simplified example for setting minimum lengths. My desired pattern for which i wish to set a minimum length is

^(2{0,1}3{0,1}4{0,1}6{0,1}7*8*9*|3{0,1}5{0,1}7*9*)$