Phrase structure rules

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*;