Tell vim to add commas to a number, e.g. change 31415926 to 31,415,926 -

I have a very large number (one hundred and one hundred point tall), and I want to use vim to add com I am in a proper way, after each group of three points, goes from right to left. How can I do this efficiently?

derived

The sub-rule that adds the command to the right place

 :% s / \ (\ d \) \ (\ (\ d \ d \ d \) \ + \ d \ @! \) \ @ = / \ 1, / g   
 It uses a zero-width shape to match any number that is not followed by a group of three numbers after a number. (Or 3n + 1 number)  
 The numbers that are in the match are marked with  ^ . After this they are replaced with commas after the match.  
  31415926 ^ ^   
 instead of  
  31,415,926