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Lex program to take input from file and remove multiple spaces, lines and tabs

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  • Difficulty Level : Easy
  • Last Updated : 14 Feb, 2023
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FLEX (Fast Lexical Analyzer Generator) is a tool/computer program for generating lexical analyzers (scanners or lexers) written by Vern Paxson in C around 1987. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language. The function yylex() is the main flex function which runs the Rule Section. Prerequisite: FLEX (Fast Lexical Analyzer Generator) Example:

hello      how


Welcome      to
Geeks    for


Approach: Open input file in read mode and whenever parser encounters newline (\n), space ( ) or tab (\t) remove it and write all the other characters in output file. Input File: Input.txt (Input File used in this program)  
Below is the implementation program: 


/*Lex program to take input from file and
remove multiple spaces, newline and tab
and write output in a separate file*/
% {
    /*Definition section */
/* Rule: whenever space, tab or
newline is encountered, remove it*/
% %
[ \n\t]+ {fprintf(yyout, "%s");}
.       { fprintf(yyout, "%s", yytext); }
% %
// driver code
int main()
    /* yyin and yyout as pointer
    of File type */
    extern FILE *yyin, *yyout;
    /* yyin points to the file input.txt
    and opens it in read mode*/
    yyin = fopen("Input.txt", "r");
    /* yyout points to the file output.txt
    and opens it in write mode*/
    yyout = fopen("Output.txt", "w");
    return 0;




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