Question

I’m trying to search a big project for all examples of where I’ve declared an array with [48] as the size or any multiples of 48.

Can I use a regular expression function to find matches of 48 * n?

Thanks.

Answer 1

Here you go (In PHP’s PCRE syntax):

^(0*|(1(01*?0)*?1|0)+?0{4})$

Usage:

preg_match('/^(0*|(1(01*?0)*?1|0)+?0{4})$/', decbin($number));

Now, why it works:

Well we know that 48 is really just 3 * 16. And 16 is just 2*2*2*2. So, any number divisible by 2^4 will have the 4 most bits in its binary representation 0. So by ending the regexp with 0{4}$ is equivalent to saying that the number is divisible by 2^4 (or 16). So then, the bits to the left need to be divisible by 3. So using the regexp from this answer, we can tell if they are divisible by 3. So if the whole regexp matches, the number is divisible by both 3 and 16, and hence 48…

QED…

(Note, the leading 0| case handles the failed match when $number is 0). I’ve tested this on all numbers from 0 to 48^5, and it correctly matches each time…