For you merlyn, I made it run on a string. Plus, I ran it on a faster machine since mine is busy. 
use strict;
use Benchmark;

for ( 1..5 ) # Do five tests.
{
    $_ = int( rand(10_000) ) ** int( rand(3) + 2 );
    $_ = "For $_ Merlyn " . reverse($_) . " plus the constants ".
         "8634641234541275032000523 and 8,634,641,234,541,275,032,000,
+523";
    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]
Output: 
For 2699449 Merlyn 9449962 plus the constants 8634641234541275032000523 and 8,63
4,641,234,541,275,032,000,523
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  1 wallclock secs ( 1.04 usr +  0.00 sys =  1.04 CPU) @ 960614.79/s
(n=1000000)
    Merlyn:  0 wallclock secs ( 0.51 usr +  0.00 sys =  0.51 CPU) @ 1956947.16/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
For 78836641 Merlyn 14663887 plus the constants 8634641234541275032000523 and 8,
634,641,234,541,275,032,000,523
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  1 wallclock secs ( 1.06 usr +  0.00 sys =  1.06 CPU) @ 942507.07/s
(n=1000000)
    Merlyn:  0 wallclock secs ( 0.58 usr +  0.00 sys =  0.58 CPU) @ 1721170.40/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
For 126128378375 Merlyn 573873821621 plus the constants 863464123454127503200052
3 and 8,634,641,234,541,275,032,000,523
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  2 wallclock secs ( 1.02 usr +  0.00 sys =  1.02 CPU) @ 979431.93/s
(n=1000000)
    Merlyn:  1 wallclock secs ( 0.53 usr +  0.00 sys =  0.53 CPU) @ 1883239.17/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
For 8665653464 Merlyn 4643565668 plus the constants 8634641234541275032000523 an
d 8,634,641,234,541,275,032,000,523
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  2 wallclock secs ( 1.12 usr +  0.00 sys =  1.12 CPU) @ 891265.60/s
(n=1000000)
    Merlyn:  0 wallclock secs ( 0.53 usr +  0.00 sys =  0.53 CPU) @ 1886792.45/s
 (n=1000000)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
For 36300625 Merlyn 52600363 plus the constants 8634641234541275032000523 and 8,
634,641,234,541,275,032,000,523
Benchmark: timing 1000000 iterations of KM, Merlyn...
        KM:  2 wallclock secs ( 0.98 usr +  0.00 sys =  0.98 CPU) @ 1018329.94/s
 (n=1000000)
    Merlyn:  0 wallclock secs ( 0.54 usr +  0.00 sys =  0.54 CPU) @ 1848428.84/s
 (n=1000000)

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

In reply to RE: RE: 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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