PL/SQL :: Join Between Table With User Based Content And Base Table

I stumbled about some weird 11gR2 behavior (running on AIX).When I performed a join between a table with user based content (parts belonging to an sourcing scope) and a base table (parts available) whereas the parts have to fulfill a special regular expression, it showed that the same query is faster when using outer join than inner join (about 0.7sec vs. 20sec; which makes me believe that regexp_like works wrong when involved in an inner join).

i tried the same statement with a standard like (but not fulfilling the same condition).This time performance was as expected (inner join outperforming outer join).

Oracle version information
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE     11.2.0.2.0     Production
TNS for IBM/AIX RISC System/6000: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
[code]...

I can see it, the execution plan for the "inner join" doesn't show so much more costs than the one for the outer (but why at all is does an inner join cost more?) ...The execution plan for both "not like" is the same and (surprisingly ;-) ) similar to "outer-regexp".

I hope sample data are not needed as there would be needed a lot...this is the second time I came across the "plan worse but execution time better" phenomenon.