Lex program to search a word in a file

Problem: Write a Lex program to search a word in a file.

Explanation:
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.

Examples:

Input: welcome to geeksforgeeks 
Output: welcome 
FOUND
 
for 
NOT FOUND

geeksforgeeks
FOUND

Approach:
To search a word, I have stored words in file named “input.txt” and used lex to receive words from keyboard. On receiving each word, I check it with the words in the file. If the word is found, the program prints “FOUND” else “NOT FOUND”.

Input File: input.txt (Input File used in this program)

Below is the implementation of program:

/* Lex program to search a word in a file */
  
%{ 
 /* Definition section */
#include<string.h> 
void check(char *); 
%} 
  
/* Rule Section */
%% 
[a-zA-Z]+ check(yytext); 
%% 
  
// driver code 
int main() 
{ 
    // The function that starts the analysis  
    yylex(); 
    return 0; 
} 
void check(char *str) 
{ 
      /* fp as pointer  
    of File type */
    FILE *fp; 
    char temp[30]; 
     
       
    /* fp points to the file input.txt  
    and opens it in read mode */
    fp=fopen("input.txt", "r"); 
    while((fscanf(fp, "%s", temp))!=EOF) 
    { 
        if(!(strcmp(temp, str))) 
        { 
            printf("FOUND\n"); 
            return; 
        } 
         
    } 
    printf("NOT FOUND\n"); 
        return; 
} 

Output:

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