my $n = '4123456789';
$n =~ s/(?<= \d )(?= (?:\d{2})* \d{3} $ )/,/gx;
[download]

Thank you very much!

That does not look like money format. The output would be

4,123,456,789

and possibly followed by a period and two digits for cents.

It is the Indian way of formatting money amounts. See: Indian Numbering System

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

Learned something today, thanks!

Was that a homework question? It looks a bit contrived to do it as a regular expression, when there alternative ways of inserting commas every three digits. For example:

join(',', '4123456789' =~ m/.{3}/g )
[download]

did you mean something like this ? :-)

scalar reverse join(',', (reverse '4123456789') =~ m/.{1,3}/g);
[download]

Maybe :| its probably an interview question, or its a social experiment, or a shared intern account

use warnings;
use strict;
use Number::Format qw(format_picture);

print format_picture(4123456789, '##,##,##,##,##,##,###'), "\n";

__END__

        4,12,34,56,789
[download]

I don't. But then again I'm unfairly biased for reasons that I explained at Re (tilly) 2: decimal placement.

When faced with a problem like that, I write a sub to do it. I fill that subroutine with comments, and I write the routine to be Perfectly Clear. Not Fast, not Short, and especially not Clever.

Oh really? Is that what You Do?