This is not really answering the OP as it is not a one-liner nor does it use a regex. But, in the spirit of TIMTOWTDI

use strict;
use warnings;

printf qq{%20s\n}, comma3($_)
   for (12, 1234, 1234567, 1234567890);

sub comma3
{
    my $ct = 0;
    join q{},
       reverse map {++ $ct % 3 ? $_ : ($_, q{,})}
       reverse split m{}, $_[0];
}

[download]

produces

                  12
               1,234
           1,234,567
       1,234,567,890

[download]

Cheers,

JohnGG

Update: Subroutine was erroneously putting commas at the front of numbers with lengths divisble by three, thanks jwkrahn for pointing this out. Revised subroutine (which is, of course, even slower) below

sub comma3
{
    my $ct = 0;
    my $len = length $_[0];
    return join q{},
       reverse map
       {
           ++ $ct % 3
              ? $_
              : $ct == $len
                 ? $_
                 : ($_, q{,})
       }
       reverse split m{}, $_[0];
}

[download]

Benchmarks will be updated in a bit.

In reply to Re: Formatting a number by johngg
in thread Formatting a number by harryf

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