Splitting numbers into groups of digits

Solution 1:

This phenomenon is called digit grouping. And you're right, Western cultures always group them in threes, but some Eastern cultures do not:

The groups created by the delimiters tend to follow the use of the local language, which varies. In European languages, large numbers are read in groups of thousands and the delimiter (which occurs every three digits when it is used) may be called a "thousands separator". In East Asian cultures, particularly China, Japan, and Korea, large numbers are read in groups of myriads (10,000s) but the delimiter commonly separates every three digits. The Indian numbering system is somewhat more complex: it groups the rightmost three digits together (till the hundreds place) and thereafter groups by sets of two digits. One trillion would thus be written as 10,00,00,00,00,000 or 10 kharab.

(source: Wikipedia)