Some lookbehind assertions were accepted when they should not have been.

Fixes #4309.

[SVN r62563]

libs/regex

@@ -1 +1 @@
-Subproject commit eaaf00a92491cf5dc321aa87ade0e9edee96af52
+Subproject commit 528cf2abb9d8f44afc4e264f6c31d5e735a0956a