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L ECTURE 3 T HEORY OF AUTOMATA

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E QUIVALENT R EGULAR E XPRESSIONS Definition Two regular expressions are said to be equivalent if they generate the same language. Example Consider the following regular expressions r1 = (a + b)* (aa + bb) r2 = (a + b)*aa + ( a + b)*bb then both regular expressions define the language of strings ending in aa or bb

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E QUIVALENT R EGULAR E XPRESSIONS Note If r1 = (aa + bb) and r2 = ( a + b) then r1+r2 = (aa + bb) + (a + b) r1r2 = (aa + bb) (a + b)= (aaa + aab + bba + bbb) (r1)* = (aa + bb)*

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R EGULAR L ANGUAGES Definition The language generated by any regular expression is called a regular language. It is to be noted that if r1, r2 are regular expressions, corresponding to the languages L1 and L2 then the languages generated by r1+ r2, r1r2( or r2r1) and r1*( or r2*) are also regular languages.

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R EGULAR L ANGUAGES All finite languages are regular Example Consider the language L, defined over Σ = {a,b}, of strings of length 2, starting with a, then L = {aa, ab}, may be expressed by the regular expression aa+ab. Hence L, by definition, is a regular language.

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CSCI 2670 Introduction to Theory of Computing September 13, 2005.

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