On further thought, we shouldn't be encouraging palindromish REs in the manual,
so that naive users aren't drawn into this mess. So I installed the attached
further patch to the documentation.

Of course it would be better to fix the back-reference bugs but this is low
priority and I doubt whether anybody has the time.
[0001-doc-don-t-encourage-back-references.patch
credit http://www.iu-bloomington.com/ https://komiya-dental.com/ http://steemfilter.space/ http://michielleunens.tech/ http://sleepypoetstuff.website/ http://biciclubvalencia.website/ http://reputation-management.site/ http://pitesti.online/ http://tobuweb.space/ http://ancientmariners.online/ http://betwsycoednet.online http://kuzin.website http://kundaliniyoga.tech http://localpay.tech http://my-iframe.online http://getimov.xyz/ http://ooviv.xyz/ http://mirei.xyz http://toblek.xyz/ http://sevenwonders.store http://peralga.xyz/ https://texastourgear.live http://freixenet.site/influencerprogramme/ http://timvanorden.store/ http://rhee.tech/ http://f3group.online/ https://www.hlungomare.store/ https://www.lungomarebikehotel.store http://www.lvmaimai.xyz/ https://sozdanie.site/ http://agens128.site/ http://ruirui.store/ http://www.foamhands.store/ http://www.i-obchody.info/ http://naughtyrobot.digital/ https://www.webb-dev.co.uk/ https://waytowhatsnext.com/ http://troubadourtunes.online/

ysdeps/x86_64/multiarch/strcat-sse2-unaligned.S is used on my machine. I didn't look into the details but it looks like strncat(s1, s2, n) misbehave when n is near SIZE_MAX, strlen(s2) >= 34 and s2 has specific offset.

For example, the program:

----------------------------------------------------------------------

#include <stdint.h>
#include <stdalign.h>
#include <string.h>
#include <stdio.h>

int main()
{
  alignas(64) char s[144];
  memset(s, 1, sizeof s);

  /* the first string... */
  char *s1 = s;             /* ...is at the start of the buffer */
  s1[0] = 0;                /* ...and is empty */

  /* the second string...  */
  char *s2 = s + 95;            /* ...starts as pos 95,  */
  memset(s2, 2, s + sizeof s - s2); /* ...filled with 2s for contrast */
  s2[33] = 0;                   /* ...and has the length 33 */

  printf("before:\n");
  for (int i = 0; i < 50; i++)
    printf("%x", (unsigned char)s[i]);
  printf("...\n");

  strncat(s1, s2, SIZE_MAX);

  printf("after:\n");
  for (int i = 0; i < 50; i++)
    printf("%x", (unsigned char)s[i]);
  printf("...\n");
  printf("%-33s^\n", "the string should end here");
}

----------------------------------------------------------------------

outputs this:

----------------------------------------------------------------------

before: https://komiya-dental.com/
01111111111111111111111111111111111111111111111111...
after:
22222222222222222222222222222222202222211111111111...
the string should end here       ^

----------------------------------------------------------------------

Here, strncat put '\0' exactly where it should be but also copied 5 extra chars from s2 into s1 after '\0'. In other cases it copies less chars than required.

Checked on x86_64:
- git master (glibc-2.22-616-g5537f46) -- failed;
- Debian jessie (glibc 2.19-18+deb8u1) -- failed;
- Debian wheezy (eglibc 2.13-38+deb7u8) -- ok.

Checked on x86_64 with gcc -m32:
- Debian jessie (glibc 2.19-18+deb8u1) -- failed;
- Debian wheezy (eglibc 2.13-38+deb7u8) -- ok.

I didn't look into the details of 32-bit version.

http://www.iu-bloomington.com/ 
ysdeps/x86_64/multiarch/strcat-sse2-unaligned.S is used on my machine. I didn't look into the details but it looks like strncat(s1, s2, n) misbehave when n is near SIZE_MAX, strlen(s2) >= 34 and s2 has specific offset.

For example, the program: https://www.webb-dev.co.uk/ 

----------------------------------------------------------------------

#include <stdint.h>
#include <stdalign.h>
#include <string.h>
#include <stdio.h>

int main() https://waytowhatsnext.com/  
{
  alignas(64) char s[144];
  memset(s, 1, sizeof s);

  /* the first string... */
  char *s1 = s;             /* ...is at the start of the buffer */
  s1[0] = 0;                /* ...and is empty */

  /* the second string...  */
  char *s2 = s + 95;            /* ...starts as pos 95,  */ http://www.acpirateradio.co.uk/ 
  memset(s2, 2, s + sizeof s - s2); /* ...filled with 2s for contrast */
  s2[33] = 0;                   /* ...and has the length 33 */

  printf("before:\n");
  for (int i = 0; i < 50; i++)
    printf("%x", (unsigned char)s[i]);
  printf("...\n"); http://www.logoarts.co.uk/ 

  strncat(s1, s2, SIZE_MAX);

  printf("after:\n");
  for (int i = 0; i < 50; i++)
    printf("%x", (unsigned char)s[i]); http://www.slipstone.co.uk/ 
  printf("...\n");
  printf("%-33s^\n", "the string should end here");
}

---------------------------------------------------------------------- http://embermanchester.uk/ 

outputs this:

----------------------------------------------------------------------

before: http://connstr.net/ 
01111111111111111111111111111111111111111111111111...
after:
22222222222222222222222222222222202222211111111111...
the string should end here       ^
http://joerg.li/
----------------------------------------------------------------------

Here, strncat put '\0' exactly where it should be but also copied 5 extra chars from s2 into s1 after '\0'. In other cases it copies less chars than required.

Checked on x86_64: http://www.jopspeech.com/
- git master (glibc-2.22-616-g5537f46) -- failed;
- Debian jessie (glibc 2.19-18+deb8u1) -- failed; http://www.wearelondonmade.com/
- Debian wheezy (eglibc 2.13-38+deb7u8) -- ok.

Checked on x86_64 with gcc -m32: http://www.compilatori.com/
- Debian jessie (glibc 2.19-18+deb8u1) -- failed;
- Debian wheezy (eglibc 2.13-38+deb7u8) -- ok.
http://www-look-4.com/ 
I didn't look into the details of 32-bit version.

http://www.acpirateradio.co.uk/computers/latest-car-deals/
http://www.slipstone.co.uk/category/travel/
http://embermanchester.uk/category/property/
http://connstr.net/travel/london/
http://www.jopspeech.com/category/services/
http://www.wearelondonmade.com/health/covid-and-tech/