placid: This may be a solution:
l1 = ['acXXX1', 'XXX2', 'wXXX3', 'kXXX5'] l2 = [ 'bXXX1', 'xXXX2', 'efXXX3', 'yXXX6', 'zZZZ9'] import re findnum = re.compile(r"[0-9]+$") s1 = set(int(findnum.search(el).group()) for el in l1) s2 = set(int(findnum.search(el).group()) for el in l2) nmax = max(max(s1), max(s2)) # XXXnmax is surely unavailable missing = set(range(1, nmax)) - s1 - s2 print ["XXX%d" % i for i in sorted(missing)] # Output: ['XXX4', 'XXX7', 'XXX8'] If you need more speed you can replace some of those sets (like the range one) with fors. Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
