A phrase structure rule has the following form:
S → NP VP;
The previous rule reads: An S (sentence) consists of an NP (noun phrase) followed by a VP (verb phrase).
Phrase structure rules can have optional constituents. A constituent is optional if enclosed in paretheses.
NP → (DET) N (PP);
Constituents may also occur in repetition by using the Kleene star or the kleene plus operators.
VP → V (NP)+ (PP)*;
The previous rule reads: A VP (verb phrase) consists of a V (verb) followed by one or more NP (noun phrase) and zero or more PP (prepositional phrase). The use of parentheses is optional so you can also write
VP → V NP+ PP*;