Python code - 11 lines

Link:

Python, pasted on Oct 14:

import re

def fn(n):
	s = ''
	while n!=s:
		s = n
		n = re.sub(r'^(-?\d+)(\d{3})', r'\1,\2', str(n))
	return n

print fn(1234567)
print fn(-13579)

Output:

1 2	1,234,567 -13,579