$_ = 123456789;
s/ # begin substitution operator
(\d{1,3}) # match and capture one-to-three digits
(?= # if they are followed by:
(?:\d\d\d)+ # one-or-more groups of three digits
(?!\d) # that are not followed by a digit
) # end lookahead assertion
/$1,/gx; # /g = perform substitution repeatedly
print; #prints: 123,456,789
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