The version referenced in the linked message are very old.
There is no more issue for some time.
$ cat /etc/redhat-release
Fedora release 17 (Beefy Miracle)
$ rpm -q php
php-5.4.9-0.1.RC1.fc17.remi (x86_64)
$ php -r "var_dump(preg_match('/^[[:alnum:]]+$/u', 'Régis'));"
int(1)
$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5.8 (Tikanga)
$ rpm -q php
php-5.4.8-1.el5.remi
$ php -r "var_dump(preg_match('/^[[:alnum:]]+$/u', 'Régis'));"
int(1)
$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.3 (Santiago)
$ rpm -q php
php-5.3.18-1.el6.remi.x86_64
$ php -r "var_dump(preg_match('/^[[:alnum:]]+$/u', 'Régis'));"
int(1)
Laptop: Fedora 38 + rpmfusion + remi (SCL only)
x86_64 builder: Fedora 39 + rpmfusion + remi-test
aarch64 builder: RHEL 9 with EPEL
Hosting Server: CentOS 8 Stream with EPEL, rpmfusion, remi