Right private keys are Hex but why there is K, w, y and .... in private key ? Aha, they turn 128bit binary to hex then find it's related Characters ! and show set of characters !!
If you see a private key with those kind of characters then that is
not a hexadecimal formatted key, it is a WIF private key encoded in Base58 (or if it begins with "bc1", bech32).
This regex should find Base58 keys:
[123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ]{51,52}
Additionally, the following regex will find bech32
private keys addresses:
bc1[qpzry9x8gf2tvdw0s3jn54khce6mua7l]{6,87}