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Unformatted text preview: CS 373: Theory of Computation Gul Agha Mahesh Viswanathan Fall 2010 1 Regular Expressions and Regular Languages Why do they have such similar names? Theorem 1. L is a regular language if and only if there is a regular expression R such that L ( R ) = L i.e., Regular expressions have the same expressive power as finite automata. Proof. Given regular expression R , can construct NFA N such that L ( N ) = L ( R ) Given DFA M , will construct regular expression R such that L ( M ) = L ( R ) 1 Converting a DFA to an Equivalent Regular Expression DFA to Regular Expression Given DFA M , will construct regular expression R such that L ( M ) = L ( R ). In two steps: Construct a Generalized NFA (GNFA) G from the DFA M And then convert G to a regex R 1.1 Generalized NFA Generalized NFA A GNFA is similar to an NFA, but: There is a single accept state. The start state has no incoming transitions, and the accept state has no outgoing tran sitions. * These are cosmetic changes: Any NFA can be converted to an equivalent NFA of this kind. The transitions are labeled not by characters in the alphabet, but by regular expressions . * For every pair of states ( q 1 ,q 2 ), the transition from q 1 to q 2 is labeled by a regular expression ( q 1 ,q 2 ). Generalized NFA because a normal NFA has transitions labeled by , elements in (a union of elements, if multiple edges between a pair of states) and (missing edges). Generalized NFA 2 Transition: GNFA nondeterministically reads a block of characters from the input, chooses an edge from the current state q 1 to another state q 2 , and if the block of symbols matches the regex ( q 1 ,q 2 ), then moves to q 2 . Acceptance: G accepts w if there exists some sequence of valid transitions such that on starting from the start state, and after finishing the entire input, G is in the accept state. Generalized NFA: Example q q 1 q 2 * 10 * * * 10 * 10 * Figure 1: Example GNFA G Accepting run of G on 11110100 is q 1 G q 1 11 G q 1 101 G q 1 00 G q 2 Generalized NFA: Definition Definition 2. A generalized nondeterministic finite automaton (GNFA) is G = ( Q, ,q ,q F , ), where Q is the finite set of states is the finite alphabet q...
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 Fall '08
 Viswanathan,M

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