For anyone who sees this error:
Warning: preg_match() [function.preg-match]: Compilation failed: PCRE does not support \L, \l, \N, \P, \p, \U, \u, or \X at ...
As this manual page says, you need PHP 5.1.0 and the /u modifier in order to enable these features, but that isn't the only requirement! It is possible to install later versions of PHP (we have 5.1.4) while linking to an older PCRE install. A quick look at the PCRE changelog suggests that you probably need at least PCRE 5; we're running 4.5, while the latest is 7.1. You can find out your PCRE version by checking phpinfo().
I suspect this ancient PCRE version is included in some officially-supported Red Hat Enterprise package which is probably why we are running it so might also affect other people.
