tested on rawhide, with glibc-2.11.90-10.x86_64:

$ cat regex-check.c
#include <config.h>
#include <limits.h>
#include <regex.h>
#include <string.h>

int
main (int argc, char **argv)
{
  char *regexp = (2 <= argc ? argv[1] : "a[b-a]");
  struct re_pattern_buffer regex;
  /* Ensure that [b-a] is diagnosed as invalid. */
  re_set_syntax (RE_SYNTAX_POSIX_EGREP);
  memset (&regex, 0, sizeof regex);
  const char *s = re_compile_pattern (regexp, strlen (regexp), &regex);
  return s == NULL;
}
$ echo '#define _GNU_SOURCE 1' > config.h
$ gcc -g -Wall -W -Wextra regex-check.c -I.
$ ./a.out
[Exit 1]

It looks like the code intends to diagnose that condition with REG_ERANGE,
but something is not working:

      start_collseq = lookup_collation_sequence_value (start_elem);
      end_collseq = lookup_collation_sequence_value (end_elem);
      /* Check start/end collation sequence values.  */
      if (BE (start_collseq == UINT_MAX || end_collseq == UINT_MAX, 0))
	return REG_ECOLLATE;
      if (BE ((syntax & RE_NO_EMPTY_RANGES) && start_collseq > end_collseq, 0))
	return REG_ERANGE;