$number = sprintf "%09d" => $number;
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Abigail

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";
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The sprintf approach is tidier:

use strict;
use warnings;

my $number = 123;

$number = sprintf "%09u", $number;
print "$number\n";
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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";
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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 . $number ) =~ m/(\d{9})$/;
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Assumung users are entering letters, whitespace and other stuff, and you want only the first numbers (maximal 9), use

$number =~ s/.*?(\d{1,9}).*/sprintf "%09d", "$1"/e;
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This does not work if users enters no digit. The best way to handle this is to perform not just a single check:

my $number = <STDIN>;
if( $number !~ /\d/ ) {
  print "no number";
}
else {
  # clean up and pad input
}
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-- Frank

my $number = 1;
($number) = "000000000$number" =~ m[(.{9})$];
print $number;
000000001
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