Here is another way, based on the RE you used:

The trick is to loop looking for groups of 3 digits. 

while($col=~s/(\d)(\d\d\d)(,|$)/$1,$2/)
 {}

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Notice - we look of either trailing COMMA or, end of line.

"Experience is a wonderful thing. It enables you to recognize a mistake when you make it again."

In reply to Re: simple regex by NetWallah
in thread simple regex by Anonymous Monk

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