One Benchmark: 
use strict;
use Benchmark;

for ( 1..5 ) # Do five tests.
{
    $_ = int( rand(10_000) ) ** int( rand(3) + 2 );
    print $_, "\n";
    timethese( 1_000_000, {
        'KM'     => sub { 1 while s/^(-?\d+)(\d{3})/$1,$2/ },
        'Merlyn' => sub { s/(\d)(?=(\d{3})+(\D|$))/$1\,/g }
    });
    print "\n", "- " x 39, "-\n";
}

[download]
Results: 
82755409
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  1 wallclock secs ( 1.04 usr +  0.00 sys =  1.04 CPU) @ 958772.77/s
(n=1000000)
    Merlyn:  1 wallclock secs ( 0.49 usr +  0.00 sys =  0.49 CPU) @ 2036659.88/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
293198635825936
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  1 wallclock secs ( 1.05 usr +  0.00 sys =  1.05 CPU) @ 949667.62/s
(n=1000000)
    Merlyn:  0 wallclock secs ( 0.49 usr +  0.00 sys =  0.49 CPU) @ 2036659.88/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
602425897921
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  0 wallclock secs ( 1.05 usr +  0.00 sys =  1.05 CPU) @ 949667.62/s
(n=1000000)
    Merlyn:  0 wallclock secs ( 0.47 usr +  0.00 sys =  0.47 CPU) @ 2123142.25/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1.80935247108226e+015
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  1 wallclock secs ( 1.04 usr +  0.00 sys =  1.04 CPU) @ 958772.77/s
(n=1000000)
    Merlyn:  1 wallclock secs ( 0.46 usr +  0.00 sys =  0.46 CPU) @ 2169197.40/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
106294343553
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  1 wallclock secs ( 1.04 usr +  0.00 sys =  1.04 CPU) @ 958772.77/s
(n=1000000)
    Merlyn:  1 wallclock secs ( 0.48 usr +  0.00 sys =  0.48 CPU) @ 2083333.33/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

In reply to RE: regexp for adding commas to a number by Adam
in thread regexp for adding commas to a number by Kozz

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