s/(
   ^[-+]?            # beginning of number.
   \d+?              # first digits before first comma
   (?=               # followed by, (but not included in the match) :
      (?>(?:\d{3})+) # some positive multiple of three digits.
      (?!\d)         # an *exact* multiple, not x * 3 + 1 or whatever.
   )
   |                 # or:
   \G\d{3}           # after the last group, get three digits
   (?=\d)            # but they have to have more digits after them.
)/$1,/xg;