From the California Vehicle Code:

CVC Section 21714 States:

21714. The driver of a vehicle described in subdivision (f) of Section 27803 shall not do either of the following:
(a) Operate the vehicle in any lane established under Section 21655.5 as an exclusive or preferential use lane for high-occupancy vehicles.
(b) Operate the vehicle in either of the following:
(1) The area on, or immediately adjacent to, the striping or other markers designating adjacent traffic lanes.
(2) The area between two or more vehicles that are traveling in adjacent traffic lanes.

This the ONLY reference in CA Law about Lane Splitting.