iOS 手动实现精确查找rangeOfString算法

需求：要给输入的长度小于8，大于18的数字标红显示，用自带的rangeOfString方法，经常会找到前面的range，导致显示异常. 所以自己计算出range

 NSArray *arr = [operatedStrM componentsSeparatedByString:@","];
    
    int i = 0;
    for (NSString *str in arr) {
        
        NSInteger length = str.length;
        
        if (length) {
            
            if (!(length >= 8 && length <= 18)) {
                
                self.redCount++;
                int j = i;
                
                if (j > 0) {
                    
                    for (int k = 0; k < textLength; k++) {
                        
                        NSString *subStr = [operatedStrM substringAtRange:NSMakeRange(k, 1)];
                        if ([subStr isEqualToString:@","]) {
                            
                            j--;
                            if (j == 0) {
                                
                                j = k + 1;
                                break;
                            }
                            
                        }
                    }
                }
            
                [attributeStr addAttribute:NSForegroundColorAttributeName value:[UIColor redColor] range:NSMakeRange(j, length)];
                
            }
            
        }
        
        i++;
        
    }