regex alternation is not commutative

r.c

/* turns out this is wrong, the regex "quic | noise" contains spaces you need to consider */

/*
 *    $ ./r
 *    "quic | noise" matches "high pitched noise coming from car"
 *    $ ./r s
 *    "noise | quic" matches "a quick fix"
 *    "noise | quic" matches "high pitched noise coming from car"
 */
#include <stdio.h>
#include <regex.h>

int main(int argc, char **argv) {
  char *p = "quic | noise";
  char haystack[2][1024] = {
    "a quick fix",
    "high pitched noise coming from car",
  };
  char err[1024];
  regex_t r;
  int s;

  if (argc > 1 && argv[1][0] == 's') {
    p = "noise | quic";
  }

  s = regcomp(&r, p, REG_EXTENDED | REG_NEWLINE | REG_NOSUB | REG_ICASE);

  if (s) {
    regerror(s, &r, err, sizeof(err));
    fprintf(stderr, "regex error: %s\n", err);
    return 1;
  }

  for (int i = 0; i < sizeof(haystack)/sizeof(haystack[0]); i++) {
    s = regexec(&r, haystack[i], 0, NULL, 0);
    if (!s) {
      printf("\"%s\" matches \"%s\"\n", p, haystack[i]);
    }
  }

  return 0;
}