There are loads of other possible TLDs (e.g. 'au', 'ca') that your solution is not taking into account. Not to mention cases like the Hungarian news portal called "index.hu".

True but subdomains are quite tricky since checking the length doesn't solve the problem. Your solution passes if the test is e.g. "images.google.com" but fails with e.g. "google.com.au". I don't know how to take that into account but it's an interesting question.

I thought it was a requirement since the description says "No additional characters are allowed except for those mentioned".

I don't understand how it excludes longer strings like "a:-D" or ";)b". Can you please explain this to me?