Special Char

I want to check if a string has special char ( all the special char like .+*@ etc., i want only numbers and letters and nothing else ) so i have try with StringInString but seems not work

$string = "fdaafsdfsafa@"
$a = StringInStr($string, '!"£$%/\(|)=?^ìè+òàù,.-é*ç°§;:_@#')
ConsoleWrite($a)

I don't want to replace it, but give an error if found a special char in the string

Thanks