#!/usr/bin/perl
use strict;
use warnings;

while (<DATA>) {
    chomp;
    print m/\d(?:(?:\d| (?!\d))){19}/ ?
          "$_ matches\n" : "$_ does not match\n";
}

__DATA__
"01234567890123456789"
"123"
"123 123             "
"123c                "
"                    "
"11232424525252423   "