My frenzied hacking through the unexplored territory of Expression Filters was derailed (to mix my metaphors) by an internal error when using the Evaluate() function in a rather complex query that uses an inline view, analytic functions etc..
The following was extracted from the alert log:
ORA-07445: exception encountered: core dump [kkqsCkLegalEqvExpCB()+199] [SIGSEGV] [Address not mapped to object] [0x000000000] [] []
The mnemonic beginning “kkqs” in the first parameter apparantly indicates a fault raised from the query rewrite source module. Because query rewrite is not helpful to the execution of the query I coded an alter session to set query_rewrite_enabled to false, and executed the problem query again.
Success.
Interestingly, adding a NO_REWRITE hint to the query did not prevent the error occurring. A shame, because I would have preferred that approach.
According to the website query rewrite problems might also be indicated by any of the codes kkqg, kkqs, kkqs1, kkqs2, kkqs3, kkqu, kkqv, and kkqw, so disabling query rewrite might also be a good workaround for internal errors that reference mnemonics beginning with any of those strings.
It also prompts the thought that in some cases it might be possible to get guidance on working around internal erros based on the mnemonic. For example an error in “kkfd” probably indicates a parallel query fault, and the remediation might be to disable parallel query. “kxtr” … trigger problem?
And there I was thinking that all of these kernel code things were for nerds.
The Oracle Sponge 
Do you know if regex is an operator you can set a filter expression on?
I’m pretty sure there aren’t restrictions of that type since you can use user defined functions. However there are restrictions on what can effectively be indexed, and if your expression can’t be indexed it is a very big performance hit.
http://download.oracle.com/docs/cd/E11882_01/appdev.112/e14919/exprn_expconcepts.htm#BABGCHJB
My expressions mostly cannot be indexed :(