Syracuse / Collatz

import re

rn = re.compile(r'^[0-9]+$')

num = raw_input("Enter a natural number: ")
while not rn.match(num):
    num = raw_input("Enter a natural number: ")
num = int(num)

while num != 1:
    print num
    if (num % 2):
        num = 3 * num + 1
    else:
        num /= 2
print num