I've seen uses (grep -r "preg.*\/u" *) of the /u modifier since ages ago in Moodle (and don't remember any report about ppl having problems with it), but the /pL thing (grep -r '\\pL' *) is the newer one, used by backup recently and immediately detected by Sean and others. So perhaps it's a different thing.
Perhaps Sean, as we know that you have a "buggy regexp server" could try one script containing different tests (/u modifier vs /pL classes). That will help to confirm the problem better.
For future reference, Petr has already created MDL-19071.
Ciao
PS: In any case, regexp in general (no matter of this unicode problem), should be a hard requirement. 100%. There is at last one bug in the Tracker about that.