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lecture05 - Outline Regular languages revisited...

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1 Prof. Aiken CS 143 Lecture 5 1 Introduction to Parsing Lecture 5 Prof. Aiken CS 143 Lecture 5 2 Outline Regular languages revisited Parser overview Context-free grammars (CFG’s) • Derivations • Ambiguity Prof. Aiken CS 143 Lecture 5 3 Languages and Automata Formal languages are very important in CS Especially in programming languages Regular languages The weakest formal languages widely used Many applications We will also study context-free languages, tree languages Prof. Aiken CS 143 Lecture 5 4 Beyond Regular Languages Many languages are not regular Strings of balanced parentheses are not regular: ( ) | 0 i i i Prof. Aiken CS 143 Lecture 5 5 What Can Regular Languages Express? Languages requiring counting modulo a fixed integer Intuition: A finite automaton that runs long enough must repeat states Finite automaton can’t remember # of times it has visited a particular state Prof. Aiken CS 143 Lecture 5 6 The Functionality of the Parser Input: sequence of tokens from lexer Output: parse tree of the program (But some parsers never produce a parse tree . . .)
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