This test passes on NetBSD, the illumos gate, and musl
with https://www.openwall.com/lists/musl/2023/04/20/2;
it's nothing revolutionary and the behaviour it tests
is largely guaranteed by the 4.4BSD-Lite manual;
nevertheless, it currently fails with
  tst-reg-startend.c: ^a: a^@c: no match$
  tst-reg-startend.c: ^a: a^@c: wanted {1, 2}, got {1, 4}$
  tst-reg-startend.c: ^a: abc: no match$
  tst-reg-startend.c: ^a: abc: wanted {1, 2}, got {1, 4}$
  tst-reg-startend.c: ^a.c$: a^@c: no match$
  tst-reg-startend.c: ^a.c$: abc: no match$
  tst-reg-startend.c: ^a.*c$: a^@c: no match$
  tst-reg-startend.c: ^a.*c$: abc: no match$
  tst-reg-startend.c: ^a[^c]c$: a^@c: no match$
  tst-reg-startend.c: ^a[^c]c$: abc: no match$
  tst-reg-startend.c: ^a..: a^@c: no match$
  tst-reg-startend.c: ^a..: abc: no match$
  tst-reg-startend.c: ..c: a^@c: no match$

The test may also be compiled stand-alone (-DSTANDALONE)
and on all platforms that have the interface
(hence the macro to initialise regmatch_ts,
 which start with pointer fields on the illumos gate),
for ease of testing and inclusion in other test suites.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
Resending after a week; clean rebase.

 posix/Makefile           |   1 +
 posix/tst-reg-startend.c | 124 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 125 insertions(+)
 create mode 100644 posix/tst-reg-startend.c

diff --git a/posix/Makefile b/posix/Makefile
index cc77e939ad..24aeb781ca 100644
--- a/posix/Makefile
+++ b/posix/Makefile
@@ -295,6 +295,7 @@ tests := \
   tst-posix_spawn-setsid \
   tst-preadwrite \
   tst-preadwrite64 \
+  tst-reg-startend \
   tst-regcomp-truncated \
   tst-regex \
   tst-regex2 \
diff --git a/posix/tst-reg-startend.c b/posix/tst-reg-startend.c
new file mode 100644
index 0000000000..c3bfac0359
--- /dev/null
+++ b/posix/tst-reg-startend.c
@@ -0,0 +1,124 @@
+/* Permission to use, copy, modify, and/or distribute this software for any
+   purpose with or without fee is hereby granted.
+
+   THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+   WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+   MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+   ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+   WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+   ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.  */
+
+#include <assert.h>
+#include <locale.h>
+#include <string.h>
+#include <regex.h>
+#include <stdio.h>
+#include <stdbool.h>
+
+
+#define M(s, e) (regmatch_t) {.rm_so = s, .rm_eo = e}
+#define MEQ(l, r) ((l).rm_so == (r).rm_so && (l).rm_eo == (r).rm_eo)
+
+static const regmatch_t bound = M(1, 4);
+
+static const char *const regex_ac[] =
+  {"^a", "c$", "^a.c$", "^a.*c$", "^a[^c]c$", "^a..", "..c", "[^z]c", NULL};
+static const char *const regex_aa[] =
+  {"^", "^a", "a$", "^\\(a\\).\\1$", "^a[^a]*", NULL};
+static const char *const data_ac[] = {"_a\0cdef", "_abcdef"};
+static const char *const data_aa[] = {"_a\0adef", "_abadef"};
+static const regmatch_t results_ac[] =
+  {M(1, 2), M(3, 4), M(1, 4), M(1, 4), M(1, 4), M(1, 4), M(1, 4), M(2, 4)};
+static const regmatch_t results_aa[] =
+  {M(1, 1), M(1, 2), M(3, 4), M(1, 4), M(1, 3)};
+static_assert(sizeof(regex_ac) / sizeof(*regex_ac) - 1 ==
+              sizeof(results_ac) / sizeof(*results_ac), "");
+static_assert(sizeof(regex_aa) / sizeof(*regex_aa) - 1 ==
+              sizeof(results_aa) / sizeof(*results_aa), "");
+
+
+static bool
+testbunch (const char *const *regexes, const char *const data[static 2],
+           const regmatch_t *results)
+{
+#define BASEERR(data)                              \
+  err = true,                                      \
+    fprintf (stdout, __FILE__ ": %s: ", *regexes), \
+    fwrite (data[i] + bound.rm_so, 1, bound.rm_eo - bound.rm_so, stdout)
+
+  bool err = false;
+  for (; *regexes; ++regexes, ++results)
+    {
+      regex_t rgx;
+      assert (!regcomp (&rgx, *regexes, 0));
+
+      for (size_t i = 0; i < 2; ++i)
+        {
+          regmatch_t match = bound;
+          if (regexec (&rgx, data[i], 1, &match, REG_STARTEND))
+            BASEERR(data), fputs (": no match\n", stdout);
+
+          if (!MEQ(match, *results))
+            BASEERR(data), fprintf (stdout, ": wanted {%d, %d}, got {%d, %d}\n",
+                                    (int)results->rm_so, (int)results->rm_eo,
+                                    (int)match.rm_so, (int)match.rm_eo);
+        }
+
+      regfree(&rgx);
+    }
+
+  return err;
+}
+
+
+static const char *const mb_data[2] = {"_aaćdef", "_aćdef"};
+static const bool mb_exp[] = {false, true};
+
+static bool
+testmb (void)
+{
+  bool err = false;
+  regex_t rgx;
+  const char *const regexes[] = {"ać"};
+  assert (!regcomp (&rgx, *regexes, 0));
+
+  for (size_t i = 0; i < 2; ++i)
+    {
+      regmatch_t match = bound;
+      if (regexec (&rgx, mb_data[i], 1, &match, REG_STARTEND) == mb_exp[i])
+        BASEERR(mb_data), fprintf (stdout, ": %s match\n",
+                                   mb_exp[i] ? "no" : "yes");
+
+      if (!MEQ(match, bound))
+        BASEERR(mb_data), fprintf (stdout, ": wanted {%d, %d}, got {%d, %d}\n",
+                                   (int)bound.rm_so, (int)bound.rm_eo,
+                                   (int)match.rm_so, (int)match.rm_eo);
+    }
+
+  regfree(&rgx);
+  return err;
+}
+
+
+static int
+do_test (int argc, char **argv)
+{
+  (void) argc, (void) argv;
+  assert (setlocale (LC_ALL, "C.UTF-8"));
+
+  return testbunch (regex_ac, data_ac, results_ac) ||
+         testbunch (regex_aa, data_aa, results_aa) ||
+         testmb ();
+}
+
+
+#ifndef STANDALONE
+#include "../test-skeleton.c"
+#else
+int
+main(int argc, char **argv)
+{
+  return do_test(argc, argv);
+}
+#endif
-- 
2.30.2