Keep in mind, the lowercase and uppercase letters are in continuous blocks on the ASCII table so you can can use that to verify if a char is a letter without doing an incredible long chain of if else statements.

Depends...how well written the form is. Often stuff like this is pushed to libraries who have covered all the gotchas but you have to be careful not to get into dependency hell. Understand where to use them and not. For example don't use left_pad but also don't make your own encryption.

How easy is it to allow new lines,very easy. The important part is only accepting new lines e.g. /r/n a well made form can include extra functions but anything not defined should be denied.

Also consider you likely should not accept a username with a semi-colon in it...