import re
regex = r"\d(?=(\d{3}))"
test_str = "123456789"
matches = re.finditer(regex, test_str)
for match in matches:
print(match.group()+match.group(1));
aW1wb3J0IHJlCnJlZ2V4ID0gciJcZCg/PShcZHszfSkpIgp0ZXN0X3N0ciA9ICIxMjM0NTY3ODkiCm1hdGNoZXMgPSByZS5maW5kaXRlcihyZWdleCwgdGVzdF9zdHIpCgpmb3IgbWF0Y2ggaW4gbWF0Y2hlczoKIHByaW50KG1hdGNoLmdyb3VwKCkrbWF0Y2guZ3JvdXAoMSkpOwo=