On this website, we can see that the the private key 1 is written in this format.
I would like to know what is this format.
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This is the Base58 Wallet Import format.
It is composed of the alphanumeric characters excluding
0OIl (zero, capital O, capital I, and lowercase L). The format includes an error checking code which makes it highly unlikely to mistype a key.
The format is also in use for addresses. Private keys start with a five. Pay-to-pubkey-hash addresses start with a one, pay-to-script-hash addresses start with a three.