The stuff between two square brackets, [...], called a character class or character set, will always match a single character.
For details, see: http://www.regular-expressions.info/charclass.html
If you want to negate a string, try a negative-look-ahead:
which will match any string except "SuperAdmin".
A short explanation:
^ // the start of the string
(?!SuperAdmin$) // NOT followed by 'SuperAdmin' followed by the end of the string
.* // matches any character (except new line chars) zero or more times
$ // followed by the end of the string