Circling back around to go faster.

p1 isolated: 76ms

p2 isolated: 82ms

combined run: 156ms

static void
one(Point rng, usize* total) {
  for (usize i = rng.x; i <= rng.y; i++) {
    c8 pid[PID_BUFSZ] = {};
    snprintf(pid, sizeof(pid), "%zu", i);
    usize len = strlen(pid);
    if (len % 2 != 0) {
      continue;
    }
    usize hlen = len / 2;
    c8 a[PID_BUFSZ] = {};
    memcpy(a, pid, hlen);
    c8 b[PID_BUFSZ] = {};
    memcpy(b, pid + hlen, hlen);
    if (strcmp(a, b) == 0) {
      *total += i;
    }
  }
}

static void
two(Point rng, usize* total) {
  for (usize i = rng.x; i <= rng.y; i++) {
    c8 pid[PID_BUFSZ] = {};
    snprintf(pid, sizeof(pid), "%zu", i);
    usize len = strlen(pid);
    for (usize j = 1; j <= len / 2; j++) {
      if (len % j != 0) {
        continue;
      }
      bool valid = true;
      for (usize k = j; k < len; k++) {
        if (pid[k] != pid[k - j]) {
          valid = false;
          break;
        }
      }
      if (valid) {
        *total += i;
        break;
      }
    }
  }
}

static void
solve(Mode mode) {
  FILE* input = fopen("input", "r");
  c8 line[LINE_BUFSZ] = {};
  fgets(line, sizeof(line), input);
  line[strcspn(line, "\n")] = 0;
  usize total = 0;
  strc tok = strtok(line, ",");
  while (tok) {
    Point rng = {};
    sscanf(tok, "%zu-%zu", &rng.x, &rng.y);
    (mode == MODE_ONE ? one : two)(rng, &total);
    tok = strtok(nullptr, ",");
  }
  fclose(input);
  printf("%zu\n", total);
}

i32
main(void) {
  solve(MODE_ONE);
  solve(MODE_TWO);
}

One last time with threads.

p1 isolated: 12ms

p2 isolated: 13ms

combined run: 25ms

typedef struct job_s {
  Mode mode;
  Point rng;
  usize total;
} Job;

static void*
worker(void* a) {
  Job* job = a;
  (job->mode == MODE_ONE) ? one(job->rng, &job->total)
                          : two(job->rng, &job->total);
  return nullptr;
}

static void
solve(Mode mode) {
  FILE* input = fopen("input", "r");
  c8 line[LINE_BUFSZ] = {};
  fgets(line, sizeof(line), input);
  line[strcspn(line, "\n")] = 0;
  fclose(input);
  usize nrng = 1;
  for (strc s = line; *s; s++) {
    if (*s == ',') {
      nrng++;
    }
  }
  Job* jobs = calloc(nrng, sizeof(*jobs));
  pthread_t* threads = calloc(nrng, sizeof(*threads));
  usize idx = 0;
  strc tok = strtok(line, ",");
  while (tok) {
    sscanf(tok, "%zu-%zu", &jobs[idx].rng.x, &jobs[idx].rng.y);
    jobs[idx].mode = mode;
    tok = strtok(nullptr, ",");
    idx++;
  }
  for (usize i = 0; i < nrng; i++) {
    pthread_create(&threads[i], nullptr, worker, &jobs[i]);
  }
  usize total = 0;
  for (usize i = 0; i < nrng; i++) {
    pthread_join(threads[i], nullptr);
    total += jobs[i].total;
  }
  free(threads);
  free(jobs);
  printf("%zu\n", total);
}

i32
main(void) {
  solve(MODE_ONE);
  solve(MODE_TWO);
}

I like your regexes a lot better than mine. I wonder if there is a perf difference between them. I’ll have to try.

Edit: Yours are faster, 3.5s vs 3.9s