Look around and substitutions

The zero-width nature of the look around operation is especially useful in substitutions. Thanks to them, we are able to perform transformations that would otherwise be extremely complex to read and write.

One typical example of look ahead and substitutions would be the conversion of a number composed of just numeric characters, such as 1234567890, into a comma separated number, that is, 1,234,567,890.

In order to write this regular expression, we will need a strategy to follow. What we want to do is group the numbers in blocks of three that will then be substituted by the same group plus a comma character.

We can easily start with an almost naive approach with the following highlighted regular expression:

>>>pattern = re.compile(r'\d{1,3}')
>>>pattern.findall("The number is: 12345567890")
['123', '455', '678', '90']

We have failed in this attempt. We are effectively grouping in blocks of three numbers, but they should be taken from the right to...