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# Finite State Automata

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Speech and Language Processing (Jurafsky & Martin, 2009)

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### Finite State Automata

1. 1. 2. Regular Expressions and Automata Jurafsky & Martin (2009)
2. 2. Outline ● ● ● ● ● What is Regular Expression How to Implement Regular Expression Finite-State Automata FSA to Model Morphological Knowledge Finite-State Transducer
3. 3. What is Regular Expression? ● How to recognize a sheep language? baa! baaa! baaaa! ... ● /ba+!/
4. 4. How to Implement the Regular Expression? ● Use Finite-State Automata (有限狀態機) to implement the regular expression
5. 5. Finite-State Automata ● a finite set of states ● start state (q0) ● final state (q4)
6. 6. Finite-State Automata (cont.) 1. First, start in the start state (q0) 2. Check the next letter of the input 3. If it matches the symbol on an arc leaving the current state, then cross that arc, move to the next state, and also advance one symbol in the input.
7. 7. Finite-State Automata (cont.) 4. If we are in the accepting state (q4) when we run out of input, the machine has successfully recognized the input as some legitimate sheep language. 5. If the machine never gets to the final state, it rejects the input.
8. 8. FSA to Model Morphological Knowledge ● How to recognize an English noun? ● The Problem of Nominal Inflection: cat / cats mouse / mice
9. 9. FSA to Accept Nominal Inflection reg-noun plural irreg-sg-noun irreg-pl-noun cat s mouse mice
10. 10. Finite-State Transducer (FST)
11. 11. Four Ways of Thinking about FST 1. 2. 3. 4. FST as recognizer FST as generator FST as translator FST as set relater
12. 12. How to make Siri understand language 從三月到五月為止，要定每天早上六點的鬧鐘起床 (Lin, 2010:159)
13. 13. Finite-State Transducer
14. 14. FSA to Accept Nominal Inflection