Lexical Analysis With Flex, for Flex 2.6.5-rc1

flex

This manual describes flex, a tool for generating programs that perform pattern-matching on text. The manual includes both tutorial and reference sections.

This edition of The flex Manual documents flex version 2.6.5-rc1. It was last updated on 30 October 2026.

This manual was written by Vern Paxson, Will Estes and John Millaway.

Table of Contents

1 Copyright
2 Reporting Bugs
3 Introduction
4 Some Simple Examples
5 Format of the Input File
- 5.1 Format of the Definitions Section
- 5.2 Format of the Rules Section
- 5.3 Format of the User Code Section
- 5.4 Comments in the Input
6 Patterns
7 How the Input Is Matched
8 Actions
9 The Generated Scanner
10 Start Conditions
11 Multiple Input Buffers
12 End-of-File Rules
13 Miscellaneous Controls
14 Values Available To the User
15 Interfacing with Yacc
16 Scanner Options
- 16.1 Options for Specifying Filenames
- 16.2 Options Affecting Scanner Behavior
- 16.3 Code-Level And API Options
- 16.4 Options for Scanner Speed and Size
- 16.5 Debugging Options
- 16.6 Miscellaneous Options
17 Performance Considerations
18 Generating C++ Scanners
19 Reentrant C Scanners
- 19.1 Uses for Reentrant Scanners
- 19.2 An Overview of the Reentrant API
- 19.3 Reentrant Example
- 19.4 The Reentrant API in Detail
- 19.5 Functions Available in Reentrant Scanners
20 Incompatibilities with Lex and Posix
21 Memory Management
- 21.1 The Default Memory Management
- 21.2 Overriding The Default Memory Management
- 21.3 A Note About yytext And Memory
22 Serialized Tables
- 22.1 Creating Serialized Tables
- 22.2 Loading and Unloading Serialized Tables
- 22.3 Tables File Format
23 Diagnostics
24 Limitations
25 Additional Reading
FAQ
- When was flex born?
- How do I expand backslash-escape sequences in C-style quoted strings?
- Why do flex scanners call fileno if it is not ANSI compatible?
- Does flex support recursive pattern definitions?
- How do I skip huge chunks of input (tens of megabytes) while using flex?
- Flex is not matching my patterns in the same order that I defined them.
- My actions are executing out of order or sometimes not at all.
- How can I have multiple input sources feed into the same scanner at the same time?
- Can I build nested parsers that work with the same input file?
- How can I match text only at the end of a file?
- How can I make yyreject() cascade across start condition boundaries?
- Why can’t I use fast or full tables with interactive mode?
- How much faster is -F or -f than -C?
- If I have a simple grammar can’t I just parse it with flex?
- Why doesn’t yyrestart() set the start state back to INITIAL?
- How can I match C-style comments?
- The ’.’ isn’t working the way I expected.
- Can I get the flex manual in another format?
- Does there exist a "faster" NDFA->DFA algorithm?
- How does flex compile the DFA so quickly?
- How can I use more than 8192 rules?
- How do I abandon a file in the middle of a scan and switch to a new file?
- How do I execute code only during initialization (only before the first scan)?
- How do I execute code at termination?
- Where else can I find help?
- Can I include comments in the "rules" section of the file?
- I get an error about undefined yywrap().
- How can I change the matching pattern at run time?
- How can I expand macros in the input?
- How can I build a two-pass scanner?
- How do I match any string not matched in the preceding rules?
- I am trying to port code from AT&T lex that uses yysptr and yysbuf.
- Is there a way to make flex treat NUL like a regular character?
- Whenever flex can not match the input it says "flex scanner jammed".
- Why doesn’t flex have non-greedy operators like perl does?
- Memory leak - 16386 bytes allocated by malloc.
- How do I track the byte offset for lseek()?
- How do I use my own I/O classes in a C++ scanner?
- How do I skip as many chars as possible?
- deleteme00
- Are certain equivalent patterns faster than others?
- Is backing up a big deal?
- Can I fake multi-byte character support?
- deleteme01
- Can you discuss some flex internals?
- yyunput() messes up yyatbol
- The | operator is not doing what I want
- Why can’t flex understand this variable trailing context pattern?
- The ^ operator isn’t working
- Trailing context is getting confused with trailing optional patterns
- Is flex GNU or not?
- ERASEME53
- I need to scan if-then-else blocks and while loops
- ERASEME55
- ERASEME56
- ERASEME57
- Is there a repository for flex scanners?
- How can I conditionally compile or preprocess my flex input file?
- Where can I find grammars for lex and yacc?
- I get an end-of-buffer message for each character scanned.
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- Why do I get "conflicting types for yylex" error?
- How do I access the values set in a Flex action from within a Bison action?
Appendix A Appendices
- A.1 Makefiles and Flex
- A.2 C Scanners with Bison Parsers
- A.3 M4 Dependency
- A.4 Common Patterns
- A.5 Retargeting Flex
- A.6 Deprecated interfaces, , Retargeting Flex
Indices
- Concept Index
- Index of Functions
- Index of Variables
- Index of Data Types
- Index of Hooks
- Index of Scanner Options