I also would encourage people not to submit this sort of dynamic expression regexp trick solutions indiscriminately. Having said that, and just for reference, you can push this down to a Cody score of 10... (hint1: use varargin; hint2: use 'command syntax' instead of 'function syntax').

To keep Cody from degenerating into a bunch of unreadable party tricks, we've blocked usage of eval, evalc, and a few others. But regexp is too important for us to block, so this trick will remain available into the foreseeable future. Which means, to back up Alfonso's comment, please don't abuse it. It's funny once, and then it just gets annoying.

function y = bit_reverse(x,n)
x = dec2bin(x);
if n>length(x)
for i=1:n-length(x)
a(i)='0';
end
for i=1:length(x)
a(n-length(x)+i)=x(i);
end
for i=1:n-1
y(i) = a(n-i);
end
else
for i=1:length(x)
y(i) = x(length(x)-i+1);
end
end
y=bin2dec(y);
end