static char* put_replacement(char* out, const char* r, int len)
{
    memcpy(out, r, len);
    return out + len;
}

static void replace(const char* in, char* out, int outlen)
{
    const char* r = "NUM";
    const char* q;
    int flag = 0;
    char p;

    while ((p = *in++)) {
        if (flag == 0) {
            if (isdigit(p)) {
                q = in - 1;
                flag = 1;
                continue;
            }
            if (isalpha(p))
                flag = 2;
        }

        if (flag == 1) {
            if (isdigit(p))
                continue;
            if (isalpha(p)) {
                out = put_replacement(out, q, in - 1 - q);
                flag = 2;
            }
            else {
                out = put_replacement(out, r, strlen(r));
                flag = 0;
            }
        }

        if (flag == 2 && !(isalpha(p) || isdigit(p))) {
            flag = 0;
        }

        *out++ = p;
    }

    if (flag == 1)
        out = put_replacement(out, r, strlen(r));

    *out++ = '\0';
}

const char* input = "for (int i = 0; i < 9; i++)";

int main(int argc, char* argv[])
{
    const char* in = input;
    size_t outlen;
    char* out;

    outlen = strlen(in) * 3 / 2 + 1;
    out = malloc(outlen);
    if (!out)
        return ENOMEM;

    replace(in, out, outlen);

    printf("in: %s\n", in);
    printf("out: %s\n", out);

    free(out);
    return 0;
}