validate method

@override String? validate()

Implementation

@override
String? validate() {
  if(value.isEmpty){
    return null;
  }
  RegExp exp = RegExp(r"^[a-zA-Z0-9]+$");
  if (!exp.hasMatch(value)) {
    return FV.buildMessage(customMessage ?? FV.alphaNum, attribute);
  }
  return null;
}