Lex code to replace a word with another word in a file

Last Updated : 30 Sep, 2022

Given a text file as input, the task is to replace a given word with another word in the file.

Lex is a computer program that generates lexical analyzers and was written by Mike Lesk and Eric Schmidt.
Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

Prerequisite: Flex (Fast lexical Analyzer Generator)

Approach:
As we know, yytext holds the value of current matched token, we can compare it with word to replace. if the value of yytext and word to replace are same, replace the word with another word and write it to the output file otherwise simply copy the content of input file to output file.

Input File: input.txt

Below is the implementation of above approach:

/* LEX code to replace a word with another 
   taking input from file */
  
/* Definition section */
/* variable replace_with and replace can be  
   accessed inside rule section and main() */
  
%{ 
#include<stdio.h> 
#include<string.h> 
  
char replace_with [] = "Best"; 
char replace [] ="A"; 
  
  
%} 
  
/* Rule Section */
/* Rule 1 compares the matched token with the 
   word to replace and replaces with another word 
   on successful match else simply writes the 
   content of input file to output file */
/* Rule 2 matches everything other than string 
   like whitespace, digits, special symbols etc  
   and writes it to output file */
  
%% 
[a-zA-Z]+    { if(strcmp(yytext, replace)==0) 
                   fprintf(yyout, "%s", replace_with); 
                else
                    fprintf(yyout, "%s", yytext);} 
.            fprintf(yyout, "%s", yytext); 
%% 
  
  
int yywrap() 
{ 
    return 1; 
} 
  
/* code section */
int main() 
{ 
        extern FILE *yyin, *yyout; 
          
        /* open the input file 
           in read mode */
    yyin=fopen("input.txt", "r"); 
   
        /* open the output file 
           in write mode */
    yyout=fopen("output.txt", "w"); 
      
        yylex(); 
} 