NSMutableString * string = [[NSMutableString alloc] initWithString:@"hello134hello"];
    NSLog(@"%@",[self string:[string mutableCopy] start:2 end:5]);
    NSLog(@"%@",[self string:[string mutableCopy] start:1 end:3]);
    NSLog(@"%@",[self string:[string mutableCopy] start:1 end:2]);


/// 替换字符，将字符串中的部分数据换成另一个
/// @param string 要修改的字符串
/// @param num 替换的起始位置
/// @param end 替换的长度
-(NSString *)string:(NSMutableString *)string start:(NSInteger)num end:(NSInteger)end{
    
    NSRange range = NSMakeRange(num, end);
    
    NSString * str2  = @"";
    for (int i = 0; i < end; i++) {
        str2 = [NSString stringWithFormat:@"%@*",str2];
    }
    
    [string replaceCharactersInRange:range withString:str2];
    
    return string;
}


NSString *strUrl = [urlString stringByReplacingOccurrencesOfString:@" " withString:@""];  去掉空格

NSString *strUrl = [urlString stringByReplacingOccurrencesOfString:@" " withString:@"/"];替换字符