What does tr -cd 'a-z0-9' do? What does it mean to clean the commit hash? - because few lines above we are anyway checking if commit hash is valid or not.

-cd is complement + delete: The command deletes all characters that isn't in a-z0-9.

This is definitely redundant, but I recall PSE's having a policy against ever using raw input from the user directly, especially if it has a potential of being later used in a command. However, if I just use tr, users wouldn't get an error if they inputted invalid input. So, the first few lines that check each input against a regex were mostly to notify the user incase they put in anything invalid.

I figured the time spent doing the extra regex check above the tr command (and vice versa) would be negligible, thus went for this instead.

TBH, I think we can just remove this. If user enters an invalid commit id or PR number, we'll just throw an error message and exit