Implementation of Lexical Analysis

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1 Implementation of Lexical Analysis

2 Outline Specifying lexical structure using regular expressions Finite automata Deterministic Finite Automata (DFAs) Non-deterministic Finite Automata (NFAs) Implementation of regular expressions RegExp NFA DFA Tables Compiler Design (20) 2

3 Notation For convenience, we use a variation (allow userdefined abbreviations) in regular expression notation Union: A + B A B Option: A + A? Range: a + b + + z [a-z] Excluded range: complement of [a-z] [^a-z] Compiler Design (20) 3

4 Regular Expressions in Lexical Specification Last lecture: a specification for the predicate s L(R) But a yes/no answer is not enough! Instead: partition the input into tokens We will adapt regular expressions to this goal Compiler Design (20) 4

5 Regular Expressions Lexical Spec. (). Select a set of tokens Integer, Keyword, Identifier, OpenPar, Write a regular expression (pattern) for the lexemes of each token Integer = digit + Keyword = if + else + Identifier = letter (letter + digit)* OpenPar = ( Compiler Design (20) 5

6 Regular Expressions Lexical Spec. (2) 3. Construct R, matching all lexemes for all tokens R = Keyword + Identifier + Integer + = R + R 2 + R 3 + Facts: If s L(R) then s is a lexeme Furthermore s L(R i ) for some i This i determines the token that is reported Compiler Design (20) 6

7 Regular Expressions Lexical Spec. (3) 4. Let input be x x n (x... x n are characters) For i n check x x i L(R)? 5. It must be that x x i L(R j ) for some j (if there is a choice, pick a smallest such j) 6. Remove x x i from input and go to previous step Compiler Design (20) 7

8 How to Handle Spaces and Comments?. We could create a token Whitespace Whitespace = ( + \n + \t ) + We could also add comments in there An input \t\n 5555 is transformed into Whitespace Integer Whitespace 2. Lexer skips spaces (preferred) Modify step 5 from before as follows: It must be that x k... x i L(R j ) for some j such that x... x k- L(Whitespace) Parser is not bothered with spaces Compiler Design (20) 8

9 Ambiguities () There are ambiguities in the algorithm How much input is used? What if x x i L(R) and also x x K L(R) Rule: Pick the longest possible substring The maximal munch Compiler Design (20) 9

10 Ambiguities (2) Which token is used? What if x x i L(R j ) and also x x i L(R k ) Rule: use rule listed first (j if j < k) Example: R = Keyword and R 2 = Identifier if matches both Treats if as a keyword not an identifier Compiler Design (20) 0

11 Error Handling What if No rule matches a prefix of input? Problem: Can t just get stuck Solution: Write a rule matching all bad strings Put it last Lexer tools allow the writing of: R = R R n + Error Token Error matches if nothing else matches Compiler Design (20)

12 Summary Regular expressions provide a concise notation for string patterns Use in lexical analysis requires small extensions To resolve ambiguities To handle errors Good algorithms known (next) Require only single pass over the input Few operations per character (table lookup) Compiler Design (20) 2

13 Regular Languages & Finite Automata Basic formal language theory result: Regular expressions and finite automata both define the class of regular languages. Thus, we are going to use: Regular expressions for specification Finite automata for implementation (automatic generation of lexical analyzers) Compiler Design (20) 3

14 Finite Automata A finite automaton is a recognizer for the strings of a regular language A finite automaton consists of A finite input alphabet Σ A set of states S A start state n A set of accepting states F S A set of transitions state input state Compiler Design (20) 4

15 Finite Automata Transition Is read s a s 2 In state s on input a go to state s 2 If end of input (or no transition possible) If in accepting state accept Otherwise reject Compiler Design (20) 5

16 Finite Automata State Graphs A state The start state An accepting state A transition a Compiler Design (20) 6

17 A Simple Example A finite automaton that accepts only Compiler Design (20) 7

18 Another Simple Example A finite automaton accepting any number of s followed by a single 0 Alphabet: {0,} 0 Compiler Design (20) 8

19 And Another Example Alphabet {0,} What language does this recognize? Compiler Design (20) 9

20 And Another Example Alphabet still { 0, } The operation of the automaton is not completely defined by the input On input the automaton could be in either state Compiler Design (20) 20

21 Epsilon Moves Another kind of transition: -moves A B Machine can move from state A to state B without reading input Compiler Design (20) 2

22 Deterministic and Non-Deterministic Automata Deterministic Finite Automata (DFA) One transition per input per state No -moves Non-deterministic Finite Automata (NFA) Can have multiple transitions for one input in a given state Can have -moves Finite automata have finite memory Enough to only encode the current state Compiler Design (20) 22

23 Execution of Finite Automata A DFA can take only one path through the state graph Completely determined by input NFAs can choose Whether to make -moves Which of multiple transitions for a single input to take Compiler Design (20) 23

24 Acceptance of NFAs An NFA can get into multiple states 0 Input: 0 0 Rule: NFA accepts an input if it can get in a final state Compiler Design (20) 24

25 NFA vs. DFA () NFAs and DFAs recognize the same set of languages (regular languages) DFAs are easier to implement There are no choices to consider Compiler Design (20) 25

26 NFA vs. DFA (2) For a given language the NFA can be simpler than the DFA NFA 0 0 DFA DFA can be exponentially larger than NFA Compiler Design (20) 26

27 Regular Expressions to Finite Automata High-level sketch NFA Regular expressions DFA Lexical Specification Table-driven Implementation of DFA Compiler Design (20) 27

28 Regular Expressions to NFA () For each kind of reg. expr, define an NFA Notation: NFA for regular expression M M For For input a a Compiler Design (20) 28

29 Regular Expressions to NFA (2) For AB A B For A + B B A Compiler Design (20) 29

30 Regular Expressions to NFA (3) For A* A Compiler Design (20) 30

31 Example of Regular Expression NFA conversion Consider the regular expression (+0)* The NFA is A B C D 0 E F G H I J Compiler Design (20) 3

32 NFA to DFA. The Trick Simulate the NFA Each state of DFA = a non-empty subset of states of the NFA Start state = the set of NFA states reachable through -moves from NFA start state Add a transition S a S to DFA iff S is the set of NFA states reachable from any state in S after seeing the input a considering -moves as well Compiler Design (20) 32

33 NFA to DFA. Remark An NFA may be in many states at any time How many different states? If there are N states, the NFA must be in some subset of those N states How many subsets are there? 2 N - = finitely many Compiler Design (20) 33

34 NFA to DFA Example A B ABCDHI 0 C D 0 E F FGABCDHI 0 EJGABCDHI G H I J 0 Compiler Design (20) 34

35 Implementation A DFA can be implemented by a 2D table T One dimension is states Other dimension is input symbols For every transition S i a S k define T[i,a] = k DFA execution If in state S i and input a, read T[i,a] = k and skip to state S k Very efficient Compiler Design (20) 35

36 Table Implementation of a DFA S 0 T 0 U 0 S T U 0 T T T U U U Compiler Design (20) 36

37 Implementation (Cont.) NFA DFA conversion is at the heart of tools such as lex, ML-Lex or flex But, DFAs can be huge In practice, lex/ml-lex/flex-like tools trade off speed for space in the choice of NFA and DFA representations Compiler Design (20) 37

38 Theory vs. Practice Two differences: DFAs recognize lexemes. A lexer must return a type of acceptance (token type) rather than simply an accept/reject indication. DFAs consume the complete string and accept or reject it. A lexer must find the end of the lexeme in the input stream and then find the next one, etc. Compiler Design (20) 38

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