Thank you so much guys. I didn't notice I got such many helps here. ;)

Just share, I got another working regex from perlguru, I changed \d to \w.

$line =~ s/(\w{3}(?!$))/$1-/xg;
[download]

By the way, is there a major processing speed to concern since there are more than one regex to get the same results?

such as:

$string =~ s/(\w\w\w)(?!$)/$1-/g;
from McA

s/(\w{3})\B/$1-/g
from Ken
[download]