GATE CS 2018 Set 1 — Question 47
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Compiler Design → Lexical Analysis → Regular Expressions for Tokens
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Question
A lexical analyzer uses the following patterns to recognize three tokens , , and over the alphabet .
Note that ‘x?’ means 0 or 1 occurrence of the symbol x. Note also that the analyzer outputs the token that matches the longest possible prefix.If the string
Note that ‘x?’ means 0 or 1 occurrence of the symbol x. Note also that the analyzer outputs the token that matches the longest possible prefix.If the string
bbaacabc is processed by the analyzer, which one of the following is the sequence of tokens it outputs?Correct answer
(D) T₃ T₃
Solution
The lexical analyzer uses the "longest possible prefix" rule (maximal munch) to identify tokens.Step 1: Process input string
We check the longest match for each pattern:
Remaining input:
Remaining input: empty.The sequence of tokens is .
bbaacabc from the beginning.We check the longest match for each pattern:
- Matches
bba: , , ends with . (Length 3) - Cannot extend to
bbaabecause the middle part cannot contain . - Matches
bb: , , ends with . (Length 2) - Cannot extend further because the middle part cannot contain .
- Matches
bbaac: , , ends with . (Length 5) - Cannot extend to
bbaacabcbecause the middle part cannot contain .
bbaac.Remaining input:
abc.Step 2: Process remaining input abc.- Matches
a: , , ends with . (Length 1) - Matches
ab: , , ends with . (Length 2) - Matches
abc: , , ends with . (Length 3)
abc.Remaining input: empty.The sequence of tokens is .
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