I don’t know if reporting this is useful, but I’ll do it anyway just in case.
I assume the most common solution to this problem is:
let userCheck = /^[a-z][a-z]+\d*$|^[a-z]\d\d+$/i;
However, the page never checks for a username that starts with a letter and has more than two numbers after it (e.g., “A123”), making the “+” omissible and giving a false positive with
let userCheck = /^[a-z][a-z]+\d*$|^[a-z]\d\d$/i;
The second false positive is less important since you wouldn’t do this, but this also gives a false positive:
let userCheck =/^[a-z][a-z]+\d*$|^[a-z]\d\w$/i;
Witch shouldn’t be possible since it can’t have a number in the middle of the user name.