bug#62605: `[\d]` does not work with PCRE

[Tukusej’s Sirs]

Hi, everyone,

I have just noticed that `grep -P '[\d]' <<< 'asdf 1234 asdf'` does not work, 
while `grep -P '[0-9]' <<< 'asdf 1234 asdf'` works as expected. When `\d` is 
used outside of square brackets, it works as expected.
I have no idea why `[\d]` does not work, but after I open this SO question, it 
seems to be a problem at least in `grep@3.10` or `3.7+` (excluding `3.7`).
Note that I use Arch Linux and I have installed `grep` from the official 
repositories.
`grep` version I currently use:

    grep (GNU grep) 3.10
    Copyright (C) 2023 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 Mike Haertel and others; see
    <https://git.savannah.gnu.org/cgit/grep.git/tree/AUTHORS>.


How this helps someone to find the bug in the code. :)

Tukusej’s Sirs

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