Lex is a tool for generating lexical analyzers (programs that recognize lexical tokens in input streams) which are commonly known as scanners or tokenizers. The lex tool takes as input a file with the extension .l that contains regular expressions and C code. It generates as output a C source file (.yy.c) that contains functions for matching patterns in the input against the regular expressions and performing actions specified in the C code. This C file can then be compiled along with a main program to create an executable that recognizes tokens in an input stream according to the lexical analysis specification. The document provides an example lex program that counts vowels in an input string and explains how to compile and run a program generated with lex.

1. lex

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lex or flex
compiler
lex
source
program
lex.l
lex.yy.c
input
stream
C
compiler
a.out
sequence
of tokens
lex.yy.c
a.out
Creating a Lexical Analyzer with Lex and Flex

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13. Pre-defined Functions
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18. Sample program to count vowels in
input
%{
#include<stdio.h>
int vowel=0
%}
%%
"a"|"e"|"i"|"o"|"u"|"A"|"E"|"I"|"O"|"U" {printf("is a VOWEL");vowel++;}
%%
int yywrap()
{
return 1;
}
int main()
{
printf("Enter Stringn");
yylex();
printf("vowel=%d “, vowel);
return 0;
}
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19. Compilation & Execution of your
Program:
• Open Command prompt and switch to your working
directory where you have stored your lex file (“.l“)
• Let your lex file be “hello.l”
• Now, follow the preceding steps to compile and run
your program.
flex hello.l
gcc lex.yy.c
./a.out
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