C_PRG_LANG_EX_3.3

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

int expand(char *s1, char *s2)
{
    while (!isdigit(*s1) && !isalpha(*s1))
    {
        *s2 = *s1;
        s1++;
        s2++;
    }

    while(*s1)
    {
        if (!(*s2 = *s1)) // End of line
            return 0;

        char start = *s1;
        if (*(s1 + 1) == '-')
        {
            s1 += 2;
            if (!(*s1)) // End of line
            {
                *s2++ = start;
                *s2++ = '-';
                *s2 = *s1;
                return 0;
            }
            if (start <= *s1 && ((isdigit(*s1) && isdigit(start)) ||
                (isalpha(*s1) && isalpha(start))))
            {
                while (start <= *s1)
                {
                    *s2++ = start;
                    start++;
                }
                s1++;
            }
            else
                return -1;  // Incorrect sequence
        }
        else
            *s2++ = *s1++;
    }
    *s2 = *s1;
    return 0;
}

int main(void)
{
    char s1[256] = "a-z";
    char s2[256];

    if (expand(s1, s2))
    {
        printf("Incorrect sequence!\n");
        return -1;
    }
    printf("%s\n", s1);
    printf("%s\n\n", s2);

    strcpy(s1, "0-9");
    if (expand(s1, s2))
    {
        printf("Incorrect sequence!\n");
        return -1;
    }
    printf("%s\n", s2);
    printf("%s\n\n", s2);

    strcpy(s1, "a-z0-9");
    if (expand(s1, s2))
    {
        printf("Incorrect sequence!\n");
        return -1;
    }
    printf("%s\n", s2);
    printf("%s\n\n", s2);

    strcpy(s1, "a-z0-9-");
    if (expand(s1, s2))
    {
        printf("Incorrect sequence!\n");
        return -1;
    }
    printf("%s\n", s2);
    printf("%s\n\n", s2);

    strcpy(s1, "a-z qwerty 0-9-");
    if (expand(s1, s2))
    {
        printf("Incorrect sequence!\n");
        return -1;
    }
    printf("%s\n", s2);
    printf("%s\n\n", s2);

    return 0;
}