C code to test regex(3)'s behaviour especially about match data for parenthized pattern

regtest.c

#include <stdio.h>
#include <sys/types.h>
#include <regex.h>

#define NMATCH 10

int
main(int argc, char **argv)
{
  regex_t reg;
  regmatch_t md[NMATCH];
  int r, i, j;
  if (1 < argc) {
    r = regcomp(&reg, argv[1], REG_EXTENDED);
    if (r) { puts("regcomp err"); return 2; }
  } else {
    puts("a.out re str ...");
    return 1;
  }
  for (i = 2; i < argc; i++) {
    r = regexec(&reg, argv[i], NMATCH, md, 0);
    if (r == REG_NOMATCH) {
      continue;
    }
    if (r) { puts("regexec err"); return 3; }
    printf("<%s>\n", argv[i]);
    for (j = 0; j < NMATCH; j++) {
      printf("#%d %zd..%zd (%.*s)\n",
        j, (size_t)md[j].rm_so, (size_t)md[j].rm_eo,
        (int)(md[j].rm_eo - md[j].rm_so),
        argv[i] + (int)md[j].rm_so);
    }
  }
  return 0;
}