The discussion you point out is more a suggested workaround to use Perl in a custom command than any actual discussion of adding such a facility to Geany.
The Glib regular expression library used by Geany allows such replacements, but it isn't used by Geany for replacements because it seems to only do [replace all](https://developer.gnome.org/glib/stable/glib-Perl-compatible-regular-express...). Somebody could provide a well written pull request to change Geany to use this.