I had a hard time thinking of an example where reversing the string before using a regex made sense. I came across this little gem.. 
use strict;
use warnings;

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}

foreach (qw(-1000 23000000))
{
   print "$_ transformed to: ",commify($_),"\n";
}

__END__
-1000 transformed to: -1,000
23000000 transformed to: 23,000,000

[download]
There are of course ways to do this with printf().

In reply to Re^4: My regex works, but I want to make sure it's not blind luck by Marshall
in thread My regex works, but I want to make sure it's not blind luck by SergioQ

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