密码输入框-密码限制：字母，数字，下划线

//数字和字母

#define ALPHANUM @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_"

#pragma mark - UITextFieldDelegate

- (BOOL)textField:(UITextField*)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString*)string
{
    NSCharacterSet *cs = [[NSCharacterSetcharacterSetWithCharactersInString:ALPHANUM]invertedSet];
    NSString *filtered = [[stringcomponentsSeparatedByCharactersInSet:cs]componentsJoinedByString:@""];
    BOOL result = [stringisEqualToString:filtered];
    if (!result) {
        [selfshowHUDInView:self.viewjustWithText:@"密码只能输入数字、字母、下划线"disMissAfterDelay:2.0];
        return result;
    }
    return result;
}