username patterns

1Only letters (either case), numbers, hyphens, underscores, and periods. (Not the slash character, that is being used to escape the period.) The username must start with a letter and must be between 1 and 20 characters long (inclusive).  [a-zA-Z][a-zA-Z0-9-_.]{1,20} 

1Only letters (either case), numbers, and the underscore; no more than 15 characters.  [A-Za-z0-9_]{1,15} 

1Only lowercase letters and numbers; at least 5 characters, but no limit.  [a-zd.]{5,}