Variable usernameRegexConst

Username regex that tests the following properties:

• exclusive use of the following characters: letters (a-z and A-Z), numbers (0-9), dot (.), dash (-), at (@), and underscore (_)
• at least 2 and at most 32 characters long
• start with a letter
• no consecutive special characters (a special character is a dot (.), dash (-), at (@), or underscore (_))