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Why the Torzon URL only uses letters and the digits 2 to 7
The alphabet of an onion address is smaller than it looks, and that is useful.
The base32 alphabet
Base32 uses a to z and 2 to 7. The digits 0, 1, 8 and 9 never appear in a valid onion address.
Why that helps
If a torzon url contains a 0, 1, 8 or 9, it is not a valid v3 onion address at all and can be discarded immediately.
Case does not matter
Onion addresses are not case-sensitive, but lowercase is the convention. Mixed-case addresses in posts are usually just copied carelessly.
A quick sanity check
Count the characters before .onion and scan for forbidden digits. It takes ten seconds and removes the crudest fakes.
A Torzon URL with 0, 1, 8 or 9 in it is not a real onion address.