25 Pa. Stat. § 292
In all election districts in cities of the first, second, and third classes, where personal registration is required, any elector desiring to participate in the primaries of any party shall, at the time of registration, state under oath to the registering officers the name of the party of which he is a member, and whose ballot he desires to vote at the primaries; and it shall then be the duty of the proper registering officers to write the name of such party opposite the name of the elector on the registration list, in a column which it is hereby made the duty of the Registration Commissioners to provide in the registration books for that purpose. If any elector about to register and enroll as a member of a party is challenged, he shall not be registered as a member of that party unless he makes oath before the said registrars that, at the last preceding November election at which he voted, he voted for a majority of the candidates of that party: Provided, however, That the group of Presidential electors, shall be counted as two candidates: And provided further, That any qualified voter may enroll as a member of any party which has secured, by order of the court, the exclusive use of a party name or political appellation for the ensuing primary. If a voter does not desire to vote at a primary election he shall not be required to answer as to his party affiliation or membership, and such party affiliation or membership shall not be recorded.
When an elector in cities of the first, second, and third classes registers his political party at the fall registration, he may vote the ballot of that party at the succeeding fall and spring primary. Should he change his political affiliation by voting for a majority of candidates of some other party at the November election, and desire to change his party vote at the spring primary, he may appear before the registrars of his division at the spring registration, and upon taking oath that at the preceding November election he voted for a majority of candidates of some other party, it shall be the duty of the registrars to change his political designation on the register, and if otherwise qualified he may vote the ballot of the said party at the spring primary. Any elector who has registered at the fall registration, but did not designate his political party, may appear before the registrars of his division at the spring registration, and, under oath, designate the party of which he is a member and for whom he desires to vote at the ensuing spring primary; provided he takes oath, if challenged, that, at the last preceding November election at which he voted, he voted for a majority of the candidates of that party, and if otherwise qualified he may vote the ballot of said party at the spring primary: Provided, however, That the group of Presidential electors shall be counted as two candidates: And provided further, That any qualified elector may enroll as a member of any party which has secured, by order of the court, the exclusive use of a party name or political appellation for the ensuing primary.
25 P.S.§ 292