There are several ways.

Here's an explicit way using length and treating the 'number' like a string: 

use strict;
use warnings;

my $number = 123;
my $pad = 9 - length $number;

$number = '0' x $pad . $number;
print "$number\n";

[download]

The sprintf approach is tidier: 

use strict;
use warnings;

my $number = 123;

$number = sprintf "%09u", $number;
print "$number\n";

[download]

Update: When only a RE will do, here's a regex approach with the s/// operator. It's not as clean as the sprintf approach, but you may have your reasons for using it: 

$number = 123;
$number =~ s/(\d{0,9})$/'0' x (9 - length $1) . $1/e;
print "$number\n";

[download]

Update 2: It's been said that it can't be done with a m// regex, and I suppose for a pure regular expression implementation that's true. But I couldn't just leave it at that, so here's a m// regex approach that works by taking liberties with what is being bound to the m// operator: 

my $number = 123;

print "$number\n" if ($number) = ( '0' x 9 . $number ) =~ m/(\d{9})$/;

[download]


Dave

In reply to Re: adding characters via a regular expression by davido
in thread adding characters via a regular expression by jwking

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