"it's quite customary to represent hashes in hexadecimal notation (that is, using 0-9 and a-z) - that's what we'll use in this article." -- hexadecimal is 0-9 a

"it's quite customary to represent hashes in hexadecimal notation (that is, using 0-9 and a-z) - that's what we'll use in this article." -- hexadecimal is 0-9 and a-f