Program to create Lexical Analyser for C Programming Language using Lex/Flex

Flex/Lex is a compiler construction tool that can be used to design a compiler.In this example flex is used to create a sample lexical analyzer for c programming language,it can recognize the valid symbols in c programming language including valid programming constructs.
The following is a complete source code Flex/Lex program to implement a Lexical analyzer for C programming language

 letter [a-zA-Z]  
 digit[0-9]  
 %%  
 {digit}+("E"("+"|"-")?{digit}+)? printf("\n%s\tis real number",yytext);  
 {digit}+"."{digit}+("E"("+"|"-")?{digit}+)? printf("\n%s\t is floating pt no ",yytext);  
 "if"|"else"|"int"|"char"|"scanf"|"printf"|"switch"|"return"|"struct"|"do"|"while"|"void"|"for"|"float" printf("\n%s\t is keywords",yytext);  
 "\a"|"\\n"|"\\b"|"\t"|"\\t"|"\b"|"\\a" printf("\n%s\tis Escape sequences",yytext);  
 {letter}({letter}|{digit})* printf("\n%s\tis identifier",yytext);  
 "&&"|"<"|">"|"<="|">="|"="|"+"|"-"|"?"|"*"|"/"|"%"|"&"|"||" printf("\n%s\toperator ",yytext);  
 "{"|"}"|"["|"]"|"("|")"|"#"|"'"|"."|"\""|"\\"|";"|"," printf("\n%s\t is a special character",yytext);  
 "%d"|"%s"|"%c"|"%f"|"%e" printf("\n%s\tis a format specifier",yytext);  
 %%  
 int yywrap()  
 {  
 return 1;  
 }  
 int main(int argc,char *argv[])  
 {  
 yyin=fopen(argv[1],"r");  
 yylex();  
 fclose(yyin);  
 return 0;  
 }  
 /*INPUT PROGRAM  
 #include<stdio.h>  
 void main()  
 {  
  printf("\nhai\n");  
 }  
/*
OUTPUT

#  is a special character
include is identifier
< operator 
stdio is identifier
.  is a special character
h is identifier
> operator 

void  is keywords 
main is identifier
(  is a special character
)  is a special character

{  is a special character
 
printf  is keywords
(  is a special character
"  is a special character
\n is Escape sequences
hai is identifier
\n is Escape sequences
"  is a special character
)  is a special character
;  is a special character

}  is a special character
*/