I've overheard that trainee pilots hired by airlines are not required to pass the PPL exam before attempting the CPL exam. Is this allowed by regulation?
You've tagged your question with 3 different sets of regulations, I can't answer all of them but here are the FAA ones.
The answer depends on what you mean by "exam". If you mean the written test, then anyone can take it if they have a graduation certificate or a logbook endorsement:
An airman applicant may present one or more of the following acceptable forms of authorization for all tests listed above [...]
- Certificate of graduation or a statement of accomplishment certifying the satisfactory completion of the ground school portion of a course for the certificate or rating sought. The certificate or statement may be issued by an FAA certificated pilot school or an agency, such as a high school, college, adult education program, Civil Air Patrol, or Reserve Officers Training Corps (ROTC) flight training school.
- Written statement or logbook endorsement from an authorized ground or flight instructor certifying that the applicant completed an applicable ground training or home study course and is prepared for the knowledge test. (AC 61-65E) (14 CFR § 61.35)
At least in theory, you could get a certificate or endorsement to take the commercial written test without ever having taken any other FAA test. But in practice I suspect that it's very unlikely: why would an organization or CFI endorse you as ready for the commercial written if you haven't already done the private one?
However, if "exam" means the actual checkride then the situation is much clearer: it isn't possible to do it unless you already have a private certificate, at least for civilian pilots. 14 CFR 61.123 says:
§61.123 Eligibility requirements: General.
To be eligible for a commercial pilot certificate, a person must:
(h) Hold at least a private pilot certificate issued under this part or meet the requirements of §61.73
61.73 lets you use military experience to apply for a civilian certificate.
Finally, some people use the term "commercial pilot" when "ATP" would be more accurate. I'm not saying that you're using it incorrectly, but since I have no idea how EASA and CASA define a CPL it may be worth mentioning. In the US, ATP multi-engine candidates need a certificate for the written exam, and a commercial certificate with instrument rating (or military equivalent) for the checkride. See 61.153.
For EASA, having a PPL is not required, however having an ICAO compliant PPL allows 50% of previously logged hours—up to a limit—to count toward the EASA CPL.
For CASA, having a PPL is not required, however PPL experience will count toward the required hours—depends on the type of hours.
If you only have CPL and want to exercise PPL privileges, you only need a Class 2 medical certificate—my interpretation: say after retiring.