I'm not sure you'll find a regexp-way to solve this in an accurate way.
You might want to process/validate the entry you get with DateTime::Event::Cron, somehow similar to the following:

#!/usr/bin/perl

use strict;
use warnings;

use DateTime::Event::Cron;

my @crontab = ( '*/3 15 1-10 3,4,5 */2',
                '* * * * 1-4,5-7,11,20',
                '* * * 1-4,5-7,11,20 *',
                '* * 1-4,5-7,11,20 * *',
                '* 1-4,5-7,11,20 * * *',
                '1-4,5-7,11,20 * * * *',
              );

my $set;

foreach my $line (@crontab) {
    eval {
       $set = DateTime::Event::Cron->from_cron( $line );
    };
    print "$line: ", ($@ ? $@ : 'OK'), "\n";
}
[download]

The output of which is

*/3 15 1-10 3,4,5 */2: OK
* * * * 1-4,5-7,11,20: Field value (20) out of range (1-7)
* * * 1-4,5-7,11,20 *: Field value (20) out of range (1-12)
* * 1-4,5-7,11,20 * *: OK
* 1-4,5-7,11,20 * * *: OK
1-4,5-7,11,20 * * * *: OK
[download]

which might be easier to handle/debug then the output of a regexp check.

Hth.