The Regency is designated in Article 59 of the Constitution.
Casa de Su Majestad el Rey de España - La Corona Hoy - El Orden de Sucesión
The Royal Household of His Majesty the King_ Welcome
Art. 59
1. In the event of the King being under age,
the father or mother of the King or, in default thereof, the relative of legal age who is nearest in succession to the Crown, according to the order established in the Constitution, shall immediately assume the office of Regent, which he shall exercise during the King's minority.
2. If the King becomes incapacitated for the exercise of his authority, and this incapacity is recognized by the Cortes Generales, the Crown Prince shall immediately assume the powers of the Regency, if he is of age. If he is not, the procedure outlined in the foregoing clause shall be followed until the coming of age of the Crown Prince.
3. If there is no person entitled to assume the Regency, the latter shall consist of one, three or five persons.
4. In order to exercise the Regency, it is necessary to be Spanish and legally of age.
5. The Regency shall be exercised by constitutional mandate, and always on behalf of the King.
The Constitution also designates the line of succession for "successors" of Juan Carlos de Borbón, however, it does not explicitly limit it to his descendants.
Art. 57
1. The Crown of Spain shall inherited by
the successors of H.M. Juan Carlos I de Borbon, the legitimate heir of the historic dynasty. Succession to the throne shall follow the regular order of primogeniture and representation, in the following order of precedence: the earlier shall precede the more distant; within the same degree, the male shall precede the female; and for the same sex, the older shall precede the younger.
2. The Crown Prince, from the time of his birth or the event conferring this position upon him, shall hold the title of Prince of Asturias and the other titles traditionally held by the heir to the Crown of Spain.
3. Should all the lines designated by law become extinct, the Cortes Generales shall provide for the succession to the Crown in the manner most suited to the interests of Spain.
4. Those persons with a right of succession to the Throne who marry against the express prohibition of the King and the Cortes Generales, shall be excluded from succession to the Crown, as shall their descendants.
5. Abdications and renunciations and any doubt concerning a fact or the law that may arise in connection with the succession to the Crown shall be resolved by an organic law.