Hello viffer,

As others have noted, your question is somewhat under-specified. Here is my take on what you may be looking for:

#! perl
use strict;
use warnings;

my $len = 7;

for my $s ('    999', '   9999', '9999999', '      9', '  99  9', '   
+9')
{
    if (length $s == $len && $s =~ /^\s*\d+$/)
    {
        printf "%-*s matches\n",        $len + 2, "|$s|";
    }
    else
    {
        printf "%-*s does not match\n", $len + 2, "|$s|";
    }
}
[download]

Output:

12:33 >perl 960_SoPW.pl
|    999| matches
|   9999| matches
|9999999| matches
|      9| matches
|  99  9| does not match
|   9|    does not match

12:33 >
[download]

When the size of the field changes, no need for a new regex — just change the value of $len.

Hope that helps,