注：设置（UITextField＊）的 delegate

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

{

NSLog(@"%s",__FUNCTION__);

NSString *result = [textField.text stringByReplacingCharactersInRange:range withString:string];

if ([result length] == 0) return YES;

NSString *regex = @"^[a-zA-Z0-9_]{0,20}$";

NSPredicate *prd = [NSPredicate predicateWithFormat:@"SELF MATCHES %@",regex];

return [prd evaluateWithObject:result];

}