Check if NSString contains alphanumeric + underscore characters only

You can create your own character set:

NSCharacterSet *s = [NSCharacterSet characterSetWithCharactersInString:@"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890_"];

Once you have that, you invert it to everything that’s not in your original string:

s = [s invertedSet];

And you can then use a string method to find if your string contains anything in the inverted set:

NSRange r = [string rangeOfCharacterFromSet:s];
if (r.location != NSNotFound) {
  NSLog(@"the string contains illegal characters");
}