There are a range of variations on the commify theme. That faq version is probably the worst. It is inefficient as it backtracks to do the job and also chokes on things like $1000 or 'string 123456'. The regex in my sub is one of merlyns creations but it chokes on >3 numbers after the decimal point without handling them separately. The Perl cookbook version:

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}
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Is more more fault tolerant than the one from the faq.