Flex (Fast Lexical Analyzer Generator) (original) (raw)

Last Updated : 11 Apr, 2026

Flex (Fast Lexical Analyzer Generator), or simply Flex, is a tool for generating lexical analyzers scanners or lexers. Written by Vern Paxson in C, circa 1987,

Role of Flex in Compiler Design

Flex takes a specification file (with extension ****.l**) containing regular expressions and actions, and automatically generates a C program that performs lexical analysis.

**Installing Flex on Ubuntu:

sudo apt-get update
sudo apt-get install flex

**Note: If the update command has not been executed on the system for a long time, it is recommended to run it first. This ensures that the latest package versions are installed, since older versions may be incompatible with newly installed packages or may no longer be available in the repository.

compiler

Workflow of Flex

How Flex Works (Workflow)

  1. Lexical specification file lex.l is written using Flex syntax.
  2. Flex compiler converts lex.l into a C file named lex.yy.c.
  3. C compiler compiles lex.yy.c into an executable (usually a.out).
  4. Generated executable reads the input characters and converts them into a sequence of tokens.

**Structure of a Flex Program

**1. Definition Section: This section contains:

Anything written inside %{ %} is copied directly into lex.yy.c.

**Syntax:

%{
// Definitions
%}

**2. Rules Section: This section defines patterns and actions.

**Syntax:

%%
pattern action
%%

**Examples: Table below shows some of the pattern matches.

Pattern It can match with
[0-9] all the digits between 0 and 9
[0+9] either 0, + or 9
[0, 9] either 0, ', ' or 9
[0 9] either 0, ' ' or 9
[-09] either -, 0 or 9
[-0-9] either - or all digit between 0 and 9
[0-9]+ one or more digit between 0 and 9
[^a] all the other characters except a
[^A-Z] all the other characters except the upper case letters
a{2, 4} either aa, aaa or aaaa
a{2, } two or more occurrences of a
a{4} exactly 4 a's i.e, aaaa
. any character except newline
a* 0 or more occurrences of a
a+ 1 or more occurrences of a
[a-z] all lower case letters
[a-zA-Z] any alphabetic letter
w(x | y)z wxz or wyz

**3. User Code Section: This section contains:

Linked with the generated lexical analyzer.

Basic Program Structure:

%{
// Definitions
%}

%%
Rules
%%

User code section

**How to run the program:

First save the file with the extension .l or .lex. After saving the file, execute the following commands in the terminal to compile and run the program.

**Step 1: flex filename.l or flex filename.lex depending on the extension file is saved with
**Step 2: gcc lex.yy.c
**Step 3: ./a.out
**Step 4: Provide input to program if it is required

**Note: To stop the input, press **Ctrl + D (EOF) in the terminal, or define a rule in the program to terminate input. You can refer to the output examples of the programs below if you face difficulty while running them.

**Example 1: Count the number of characters in a string

C `

/*** Definition Section has one variable which can be accessed inside yylex() and main() ***/ %{ int count = 0; %}

/*** Rule Section has three rules, first rule matches with capital letters, second rule matches with any character except newline and third rule does not take input after the enter***/ %% [A-Z] {printf("%s capital letter\n", yytext); count++;} . {printf("%s not a capital letter\n", yytext);} \n {return 0;} %%

/*** Code Section prints the number of capital letter present in the given input***/ int yywrap(){} int main(){

// Explanation: // yywrap() - wraps the above rule section /* yyin - takes the file pointer which contains the input*/ /* yylex() - this is the main flex function which runs the Rule Section*/ // yytext is the text in the buffer

// Uncomment the lines below // to take input from file // FILE *fp; // char filename[50]; // printf("Enter the filename: \n"); // scanf("%s",filename); // fp = fopen(filename,"r"); // yyin = fp;

yylex(); printf("\nNumber of Capital letters " "in the given input - %d\n", count);

return 0; }

`

If the user enters:

GFG123gfg

**Output:

G capital letter
F capital letter
G capital letter
1 not capital letter
2 not capital letter
3 not capital letter
g not capital letter
f not capital letter
g not capital letter

Number of capital letters in the given input - 3

**Example 2: Count the number of characters and number of lines in the input

C `

/* Declaring two counters one for number of lines other for number of characters */ %{ int no_of_lines = 0; int no_of_chars = 0; %}

/rule 1 counts the number of lines, rule 2 counts the number of characters and rule 3 specifies when to stop taking input/ %% \n ++no_of_lines; . ++no_of_chars; end return 0; %%

/*** User code section***/ int yywrap(){} int main(int argc, char **argv) {

yylex(); printf("number of lines = %d, number of chars = %d\n", no_of_lines, no_of_chars );

return 0; }

`

**Input Given:

Geeks
for
Geeks
end

**Output:

number of lines = 3, number of chars = 13

**Advantages