5.1.3 A Little Grammar of English

Phrase structure rules and lexical rules.

The above ideas adapt straightforwardly to natural languages. But with natural languages it is useful to draw a distinction between rules which have syntactic categories on the right hand side, and rules which have only words on the right hand side. The rules are known as phrase structure rule s, and lexical rule s, respectively.

A Phrase Structure Grammar

Here's a simple so-called phrase structure grammar (PSG) of English. We have phrase structure rules:

...and lexical rules:

So in this grammar, the terminal symbols are English words. There is a special word for the symbols (such as N, PN, and Det) which occur in lexical rules: they are called preterminal symbol s.

This grammar is unambiguous. That is no string has two distinct parse trees. (Incidentally, this means that it is not a realistic grammar of English: all natural languages are highly ambiguous.) But it does display an interesting (and troublesome) phenomenon called local ambiguity .

Local Ambiguity

Consider the sentence ``The robber knew Vincent shot Marsellus''. This has a unique parse tree in this grammar. But now look at the first part of it: ``The robber knew Vincent''. This is also a sentence according to this grammar --- but not when considered as a part of ``The robber knew Vincent shot Marsellus''. This can be a problem for a parser. Locally we have something that looks like a sentence --- and a parser may prove that this part really is a sentence according to the grammar. But this information does not play a role in analyzing the larger sentence of which it is a part. Keep this in mind. It will become important again when we build a parser using this grammar in Section 6.1.