#define NUM @"0123456789"
#define ALPHA @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
#define ALPHANUM @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
  NSCharacterSet *cs = [[NSCharacterSet characterSetWithCharactersInString:ALPHANUM] invertedSet];
  NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];
  return [string isEqualToString:filtered];
}