Explain what is 0x5A in this code?

Question

C++ / C#

for (int i=0; i= 0x30 && username[i] <= 0x39) ||
(username[i] >= 0x41 && username[i] <= 0x5A) ||
(username[i] >= 0x61 && username[i] <= 0x7A)
Roughly guessing that 0x30, 0x39 is all related to the length of the text, but still, explain what it means..
Thank you!

Answer 1

It's just that someone didn't know about the isalnum function and wrote their own.

Answer 2

These are hexadecimal numbers. They do not refer to the length of the text - this is a check for the occurrence of a character in a range.