Regular Expressions andFinite State Automata
Gul Agha2104 SChttp://www.cs.uiuc.edu/class/fa05/cs421/current/
Based in part on slides by Mattox Beckman, as updated by Vikram Adve and Elsa Gunter
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Language Syntax
Syntax is the description of which strings of symbols are meaningful expressions in a languageIt takes more than syntax to understand a language; need meaning (semantics) tooSyntax is the entry point
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Elements of Syntax
Character set – previously always ASCII, now often 64 character setsKeywords – usually reservedSpecial constants – cannot be assigned toIdentifiers – can be assigned toOperator symbolsDelimiters (parenthesis, braces, brackets,)Blanks (aka white space)
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Elements of Syntax
ExpressionsType expressionsDeclarations (in functional languages)Statements (in imperative languages)Subprograms
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Elements of Syntax
ModulesInterfacesClasses (for object-oriented languages)
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Formal Language Descriptions
Regular expressions, regular grammars, finite state automataContext-free grammars, BNF grammars, syntax diagrams
Whole family more of grammars and automata – covered in automata theory
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Grammars
Grammars are formal descriptions of which strings over a given character set are in a particular languageLanguage designers write grammarLanguage implementers use grammar to know what programs to acceptLanguage users use grammar to know how to write legitimate programs
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Regular Expressions
Start with a given character set –a, b, c…
Each character is a regular expressionThe regular expression b represents the set { b }
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Regular Expressions
If x and y are regular expressions, then xy is a regular expression
xy represents the set of all strings made concatenating a string in x with a string in y
If x and y are regular expressions, then x | yis a regular expression
x | y represents the set of strings in x or in y (or both)
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Regular Expressions
If x is a regular expression, then so is ( x )( x ) represents the same thing as x
If x is a regular expression, then so is x*x* represents strings made by concatenating zero or more strings from x
εrepresents the empty set
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Example Regular Expressions
(0|1)*1The set of all strings of 0’s and 1’s ending in 1, {1, 01, 11,…}
a*b(a*)The set of all strings of a’s and b’s with exactly one b
((01) |(10))*You tell me
Regular expressions (equivalently, regular grammars) important for lexing, breaking strings into recognized words
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Example: Lexing
Regular expressions good for describing lexemes (words) in a programming language
Identifier = (a | b | … | z | A | B | … | Z) (a | b |… | z | A | B | … | Z | 0 | 1 | … | 9)*Digit = (0 | 1 | … | 9)Number = (1 | … | 9)(0 | … | 9) | ~ (1 | … | 9)(0 | … | 9) Keywords: if = if, while = while,…
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Implementing Regular Expressions
Regular expressions can generate strings in a languageNot so good for recognizing when a string is in languageRegular expressions: which option to choose, how many repetitions to makeAnswer: finite state automata
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Finite State Automata
A finite state automata over an alphabet is:a directed graphedges are labeled with elements of alphabet,
or empty stringsome nodes (or states), marked as finalone node marked as start state
Syntax of FSA
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Example FSA
0 11
0Start State
Final State
Final State
1
0
10
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Deterministic FSA’s
If FSA has for every state exactly one edge for each letter in alphabet then FSA is deterministic
No edge labeled with ε
In general FSA in non-deterministic.NFSA also allows edges labeled by ε
Deterministic FSA special kind of non-deterministic FSA
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DFSA Language Recognition
Think of a DFSA as a board game; DFSA is boardYou have string as a deck of cards; one letter on each cardStart by placing a disc on the start state
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DFSA Language Recognition
Move the disc from one state to next along the edge labeled the same as top card in deck; discard top card
When you run out of cards:if you are in final state, you win; string is in languageif you are not in a final state, you lose; string is not in language
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DFSA Language Recognition -Summary
Given a string (over the alphabet)Start at start stateMove over edge labeled with first letter to new stateRemove first letter from stringRepeat until string goneIf end in final state then string in language
Semantics of FSA
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Example DFSA
Regular expression: (0 | 1)* 1Deterministic FSA
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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Example DFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0 1
1
0
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NFSA generalize DFSA in two ways:Include edges labeled by ε
Allows process to non-deterministically change state
Each state can have zero, one or more edges labeled by each letter
Given a letter, non-deterministically choose an edge to use
Non-deterministic FSA’s
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NFSA Language Recognition
Play the same game as with DFSAFree move: move across an edge with empty string label without discarding cardWhen you run out of letters, if you are in final state, you win; string is in languageYou can take one or more moves back and try againIf have tried all possible paths without success, then you lose; string not in language
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FSA Language RecognitionMove the disc from one state to next if edge between labeled the same as top card in deck; discard top card Free move: move across an edge with empty string label without discarding cardWhen you run out of cards, if you are in final state, you win; string is in languageYou can take a move back and try anotherIf you have cards left and you have tried all possible edges without success, then you lose; string not in language
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Example NFSA
Regular expression: (0 | 1)* 1Non-deterministic FSA
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Guess
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Backtrack
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Guess again
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Guess
0
1
1
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Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Backtrack
Example NFSA
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Guess again
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1
0
1
1
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Example NFSA
Regular expression: (0 | 1)* 1Accepts string 0 1 1 0 1Guess (Hurray!!)
0
1
1
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Rule Based Execution
SearchWhen stuck backtrack to last point with choices remainingExecuting the NFSA in last example was example of rule based executionFSA’s are rule-based programs; transitions between states (labeled edges) are rules; set of all FSA’s is programming language
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