Hi Fubra,
You can disavow at a domain level, so no regex is required (and I don't think it will work).
Just add "domain:" before the domain, eg. domain:spammysite.com
Marie Haynes wrote a good guide to using the disavow tool here if you need any further information: https://moz.com/blog/guide-to-googles-disavow-tool
Cheers,
David