I think I have confirmed the bug. Consider this bash-script:

   #! /bin/bash
   
   for x in A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e
   f g h i j k l m n o p q r s t u v w x y z; do
     echo "[ ${x}XXtxt1 ]" | sed -n 's|^\[\s\+\([^\s]\+[0-
   9]\+\)\s\+\]\s*$|\1|p'
   done
   
sed is printing a result in all cases, except when the 's' is starting
the text that is captured in the sed-script.

Regards,
Vidar Hanto
Norway

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----------------------------------------
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This seems to qualify as a bug:

The sed version included in my Linux seems to be unstable when using
the '\s' and/or '\S' regex extensions:

Example:

   id@pc:~$ echo '[ subCA2 ]' | sed -n 's|^\[\s\+\([^\s]\+[0-
   9]\+\)\s\+\]\s*$|\1|p'
   id@pc:~$ echo '[ subCA2 ]' | sed -n 's:^\[\s\+\([\S]\+[0-
   9]\+\)\s\+\]\s*$:\1:p'
   id@pc:~$ echo '[ rootCA1 ]' | sed -n 's|^\[\s\+\([^\s]\+[0-
   9]\+\)\s\+\]\s*$|\1|p'
   rootCA1
   
If I replace '\s' with '[ \t]' (and '\S' with '[^ \t]') things work as
expected:
   id@pc:~$ echo '[ subCA2 ]' | sed -n 's:^\[[ \t]\+\([^ \t]\+[0-9]\+\)[
   \t]\+\][ \t]*$:\1:p'
   subCA2
   id@pc:~$ echo '[ rootCA2 ]' | sed -n 's:^\[[ \t]\+\([^ \t]\+[0-9]\+\)[
   \t]\+\][ \t]*$:\1:p'
   rootCA2

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My Linux version: 
Debian 12.10 as per 2025-04-02, terminal session in an Xfce4 desktop
environment, fully updated: 

My sed version:
   id@pc:~$ sed --version
   sed (GNU sed) 4.9
   Packaged by Debian
   Copyright (C) 2022 Free Software Foundation, Inc.
   License GPLv3+: GNU GPL version 3 or later
   <https://gnu.org/licenses/gpl.html>.
   This is free software: you are free to change and redistribute it.
   There is NO WARRANTY, to the extent permitted by law.
   
   Written by Jay Fenlason, Tom Lord, Ken Pizzini,
   Paolo Bonzini, Jim Meyering, and Assaf Gordon.
   
   This sed program was built with SELinux support.
   SELinux is disabled on this system.
   
   GNU sed home page: <https://www.gnu.org/software/sed/>.
   General help using GNU software: <https://www.gnu.org/gethelp/>.
   E-mail bug reports to: <bug-sed@gnu.org>.
   id@pc:~$ 
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Regards,
Vidar Hanto
Norway