Lex/Flex tool overview in Compiler Design - Step-by-Step Execution

Concept Flow - Lex/Flex tool overview

Write Lex/Flex rules

↓

Run Lex/Flex tool

↓

Generate C source code

↓

Compile generated code

↓

Use lexer in program

↓

Input text scanned

↓

Tokens produced for parser

The Lex/Flex tool reads rules, generates C code for a lexer, which is compiled and used to scan input text into tokens.

Execution Sample

Compiler Design

%{
#include <stdio.h>
%}
%%
[0-9]+  { printf("NUMBER\n"); }
%%

This Lex/Flex code matches numbers and prints 'NUMBER' when found.

Analysis Table

Step	Action	Input	Matched Pattern	Output/Result
1	Start scanning	123abc	None yet	No output
2	Match digits	123abc	[0-9]+	Print 'NUMBER'
3	Advance input	abc	No match	No output
4	End scanning		No more input	Stop

💡 Input fully scanned; no more tokens to produce.

State Tracker

Variable	Start	After Step 2	After Step 3	Final
Input Pointer	0 (start)	3 (after '123')	6 (after 'abc')	6 (end)
Current Token	None	NUMBER	None	None

Key Insights - 2 Insights

Why does the lexer print 'NUMBER' only after matching digits?

Because the pattern [0-9]+ matches one or more digits, as shown in execution_table step 2 where input '123' matches and triggers the action.

What happens when input characters do not match any pattern?

The lexer skips or stops scanning as in step 3, where 'abc' does not match the digit pattern, so no output is produced.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what pattern is matched?

A [A-Z]+

B [a-z]+

C [0-9]+

D No pattern matched

💡 Hint

Check the 'Matched Pattern' column at step 2 in execution_table.

At which step does the input pointer reach the end of input?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look at the 'Input Pointer' variable in variable_tracker and the 'Action' in execution_table.

If the input was '456xyz', what would be the output at step 2?

A Print 'WORD'

B Print 'NUMBER'

C No output

D Error

💡 Hint

The pattern [0-9]+ matches digits at the start, as shown in execution_table step 2.

Concept Snapshot

Lex/Flex reads pattern-action rules.
Generates C code for a lexer.
Lexer scans input text.
Matches patterns produce tokens.
Tokens used by parser in compiler.

Full Transcript

Lex/Flex is a tool used in compiler design to create lexical analyzers. The process starts by writing rules that define patterns and actions. Running Lex/Flex generates C source code for a lexer. This code is compiled and used in a program to scan input text. The lexer reads the input, matches patterns like numbers or words, and produces tokens. These tokens are then passed to the parser for further processing. For example, a rule matching digits prints 'NUMBER' when digits are found. The lexer moves through the input, matching patterns step-by-step until all input is processed.