An old snippet from the code of my game Solution.

validInput-ObjC.m

+ (NSCharacterSet *)validCharacters {
  return [NSCharacterSet characterSetWithCharactersInString:@"1234567890+-*()"];
}

+ (BOOL)isStringValid:(NSString *)string {
  if (!string) return NO;
  NSCharacterSet *inputSet = [NSCharacterSet characterSetWithCharactersInString:string];
  if (string.length > 1) {
    if ([[NSCharacterSet decimalDigitCharacterSet] isSupersetOfSet:inputSet]) return YES;
  }else {
    if ([[CDExpression validCharacters] isSupersetOfSet:inputSet]) return YES;
  }
  return NO;
}

- (BOOL)canStringBeAddedToEquation:(NSString *)string {
  if (![CDExpression isStringValid:string]) return NO;
  
  NSString *lastString = ![self.Expression lastObject] ? @"" : [self.Expression lastObject];

  NSCharacterSet *lastSet = [NSCharacterSet characterSetWithCharactersInString:lastString];
  NSCharacterSet *stringSet = [NSCharacterSet characterSetWithCharactersInString:string];
  if (!lastString || [[NSCharacterSet characterSetWithCharactersInString:@"+*-x("] isSupersetOfSet:lastSet]) {
    if ([[NSCharacterSet decimalDigitCharacterSet] isSupersetOfSet:stringSet]) {
      return YES;
    }else if ([string isEqualToString:@"("]) {
      return YES;
    }
  }
  if ([string isEqualToString:@")"]) {
    if (self.parenthesesStack > 0) {
      return YES;
    }
  }
  if ([[NSCharacterSet decimalDigitCharacterSet] isSupersetOfSet:lastSet] && ![lastString isEqualToString:@""]) {
    if ([[NSCharacterSet characterSetWithCharactersInString:@"*-+x"] isSupersetOfSet:stringSet]) {
      return YES;
    }
  }
  if ([lastString isEqualToString:@")"]) {
    if ([[NSCharacterSet characterSetWithCharactersInString:@"*-+x"] isSupersetOfSet:stringSet]) {
      return YES;
    }
  }
  return NO;
}