2.12 Thematic Takeaways

• The identifiers and numbers in programming languages can be described by a regular grammar.

• The nested expressions and blocks in programming languages can be described by a context-free grammar.

• Neither a regular nor a context-free grammar can describe the rule that a variable must be declared before it is used.

• Grammars are language recognition devices as well as language generative devices.

• An ambiguous grammar poses a problem for language recognition.

• Two parse trees for the same sentence from a language are sufficient to prove that the grammar for the language is ambiguous.

• Semantic properties, including precedence and associativity, can be modeled in a context-free grammar.