C++ - Kontrollera formatet på en sträng.

#include <stdlib.h>
#include <stdio.h>

int ipow10(int e)
{
    if (e == 0)
        return 1;
    return 10 * ipow10(e - 1);
}

const char *parse_digit(const char *str, int *digit)
{
    if (str == NULL || *str < '0' || *str > '9')
        return NULL;
    *digit = *str - '0';
    return str + 1;
}

const char *parse_number(const char *str, int *n, int cnt)
{
    int d;
    const char *rest = str;

    *n = 0;
    for (int i = 1; i <= cnt; i++) {
        rest = parse_digit(rest, &d);
        *n += d * ipow10(cnt - i);
    }
    return rest;
}

const char *parse_year(const char *str, int *year)
{
    return parse_number(str, year, 4);
}

const char *parse_month(const char *str, int *month)
{
    return parse_number(str, month, 2);
}

const char *parse_day(const char *str, int *day)
{
    return parse_number(str, day, 2);
}

const char *parse_tkn(const char *str, char tkn)
{
    if (str && *str == tkn)
        return str + 1;
    return NULL;
}

const char *parse_date(const char *str, int *year, int *month, int *day)
{
    const char *rest;
    rest = parse_year(str, year);
    rest = parse_tkn(rest, '-');
    rest = parse_month(rest, month);
    rest = parse_tkn(rest, '-');
    return parse_day(rest, day);
}

int main(int argc, char *argv[])
{
    int year;
    int month;
    int day;

    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s <date-string>\n", argv[0]);
        return EXIT_FAILURE;
    }

    if (!parse_date(argv[1], &year, &month, &day))
    {
        fprintf(stderr, "%s is not a valid date in format yyyy-mm-dd\n", argv[1]);
        return EXIT_FAILURE;
    }

    printf("Year  : %d\nMonth : %d\nDay   : %d\n", year, month, day);
    return EXIT_SUCCESS;
}