class FormFieldValidator{
  static String validate(String value, String message){
    return (value.isEmpty || (value.contains(**SPECIAL CHARACTERS**))) ? message : null;
  }
}

class FormFieldValidator {
  static String validate(String value, String message) {
    return RegExp(r"^[a-zA-Z0-9]+$").hasMatch(value) ? null : message;
  }
}

// alphanumeric
static final  validCharacters = RegExp(r'^[a-zA-Z0-9]+$');

// alphanumeric and &%=
static final validCharacters = RegExp(r'^[a-zA-Z0-9&%=]+$');

// alphanumeric and _-=@,.;
static final validCharacters = RegExp(r'^[a-zA-Z0-9_-=@,.;]+$');

validCharacters.hasMatch('abc123');  // true
validCharacters.hasMatch('abc 123'); // false (spaces not allowed)

void main() {
  final validCharacters = RegExp(r'^[a-zA-Z0-9_-=@,.;]+$');
  print(validCharacters.hasMatch('abc123'));
}