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Steps to use Flex

Ravi ChotraniNew York University

Reviewed ByProf. Mohamed Zahran

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Outline What is Flex Structure of a Flex File Steps to use Flex Example Summary

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What is Flex Flex (The Fast Lexical Analyzer) It is a tool for generating programs that

perform pattern-matching on text. Flex is a free implementation of the

original Unix lex program.

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What is Flex(Contd.)

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Structure of a Flex File Format:

    

The definitions section: "name definition"The rules section: "pattern action"The user code section: "yylex() routine"

definitions %% rules %% user code

Required

Optional

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Steps to use Flex Create a lex file which has lexical

specification rules for the lexer generator. It also has a main method within the file itself which repeatedly invokes the lexer and prints out the token and lexeme pairs.

Compile this file by running the following command

flex lexicalAnalyzer.l

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Steps to use Flex(Contd.) A new file called lex.yy.c will be created

in the same folder in which the above file was present.

Compile this file by running the command:

gcc lex.yy.c to generate a.out executable. Test the executable using : ./a.out < "lexer_test.pas"

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Example

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Example(Contd.)

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Example(Contd.)

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Example(Contd.)

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Summary Tool for generating programs that perform pattern-

matching on text Input file is a lex file(.l file) and output file is a c file. Input file is divided into 3 sections:

Definitions Rules User Code