I am running Realnames v0.3.1 with the default options.
When navigating to a sub-page of a user's page (e.g. /User:somename/Subpage) an 'Unknown modifier' error is generated by preg_replace_callback() in lookForBare().
I have fixed this in our installation by changing the delimiters for the regex from '/' to '~' in the following two places in Realnames.body.php:
268 - $out->setPagetitle(static::lookForBare($out->getPageTitle(),'/'.static::getNamespacePrefixes().'\s*('.$title->getText().')(?:\/.+)?/')); 268 + $out->setPagetitle(static::lookForBare($out->getPageTitle(),'~'.static::getNamespacePrefixes().'\s*('.$title->getText().')(?:/.+)?~'));
334 - $pattern = '/'.static::getNamespacePrefixes().'([^ \t]+)(:\/.+)?/'; 334 + $pattern = '~'.static::getNamespacePrefixes().'([^ \t]+)(:/.+)?~';
I have not yet noticed this breaking anything else but will post again if I notice anything.