It has to be a validation and with the help of my teammate..Got a rather simple solution/workaround.

IsAlphaNumeric(
replacestr(replacestr(String,#,,,True),@,,True),
True)

A nested Replace String function to replace all the allowed special characters with "Nothing" and at the end we should/would get a string which is only alpha numeric with spaces. For E.g. In the above expression alllowed special characters are # and @. To Include more special characters expand the nest of ReplaceStr.

If the IsAlphaNumeric produces a False the validation fails, and passes if it produces True.

Hope it made sense.