Step 1: Define a regulator.
A regulatory body is set up to supervise and frame rules for a sector, watching compliance and protecting users.
Step 2: Check SEBI.
SEBI oversees the securities market, brokers and listed firms. It is a regulator, so A is correct.
Step 3: Check IRDAI.
IRDAI supervises insurers and protects policyholders. It is a regulator, so B is correct.
Step 4: Check SIDBI.
SIDBI is a development finance body that supports small industries with funding. It does not regulate a sector, so C is wrong.
Step 5: Check CERC.
CERC sets tariffs and rules for the central power sector. It is a regulator, so D is correct.
\[ \boxed{(A),\ (B)\text{ and }(D)} \]