使用regexp检查模算术条件

Question

I'm writing a script to parse text files (csv to be precise) and I want to pick lines from files based on each line content. There are a number of string conditions to check, so I surmised regexp is the way to go, but I also need to check a number in a beginning of a line against conditions in modulo arithmetics, so far it's n%4==k and n%2==k . It seems however that there are only ad hoc solutions. n%2==k is pretty straightforward, but to check n%4==2 I had to devise something like this:

r'((^\d*[24680]|^)[26]|^\d*[13579][048])[\s;,].*' # more (unrelated) conditions follow

My questions are:

1. Is there a way to simplify the regexp above? Are there any obvious problems with it?
2. If I want to generalize the script to other modulo conditions (eg n%3==k or n%7==k ), is there a feasible way to do it with regexp, or I'd better extract a number from string and write additional code to check such conditions.

Answer 1

This seems to be more accurate for n%4==2 (ref: http://en.wikipedia.org/wiki/Divisibility_rule )

r = r'^[26]$|^\d*[02468][26]$|^\d*[13579][048]$'

# test
for i in xrange(1, 1000):
    m = re.match(r, str(i))
    if i % 4 == 2:
        assert m, [i, i % 4]
    else:
        assert not m, i

For n%3==0 see Regex filter numbers divisible by 3 . I'm not aware of any generic solution for mod n , in any case it would be an interesting but purely theoretical exercise. In real life, just use ints.