“％[^ \ n] s”和“％[^ \ n] s”有什么区别

Question

#include<stdio.h>
void calculator();
int str_cmp(char *str1, char *str2);

int main()
{

    calculator();
}

void calculator()
{
    char str[255], operation[2];
    int num1, num2, result = 0, exit = 1, count, index;
    char op[6][3] = {{"+"}, {"-"}, {"*"}, {"/"}, {"**"}, {"%"}};

    printf("\n****CALCULATOR****\n"
           "Rules (1) You put a space each expression\n"
           "Rules (2) If you want to exit the calculator, enter '.'\n"
           "(+) Addition\n"
           "(-) Substraction\n"
           "(*) Multiplication\n"
           "(/) Division\n"
           "(**) Power\n"
           "(%%) Modulo\n");

    while (exit)
    {
        scanf("%[^\n]s", str);
        count = sscanf(str, "%s %d %d", operation, &num1, &num2);

        if (str_cmp(operation, ".") == 1)
            exit = 0;

        for (int i = 0; i < 6; i++)
            if (str_cmp(operation, op[i]) == 0)
                index = i;
        printf("index: %d", index);
    }
}

int str_cmp(char *str1, char *str2)
{
    while (*str1 && *str2 && *str1 == *str2)
        ++str1, ++str2;
    return *str1 - *str2;
}

我不理解本节内容：

scanf("%[^\n]s", str); ==>无限循环

scanf(" %[^\n]s", str); ==>不是无限循环

"%[^\n]s"和" %[^\n]s"有什么区别？

其他部分正在工作。但是这部分使我感到困惑。