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Fast lexical analyser generator.

Flex (The Fast Lexical Analyzer) is a tool for generating programs that recognize lexical patterns in text. There are many applications for Flex, including writing compilers in conjunction with GNU Bison. Flex is a free implementation of the well known Lex program. It features a Lex compatibility mode, and also provides several new features such as exclusive start conditions.

Flex was never part of the GNU Project , although the GNU Project helped distribute it and wrote its manual.


The Texinfo source of the current Flex manual is found in the Flex source package at https://github.com/westes/flex/.

The old version of the initial manual written by the GNU Project is kept at https://ftp.gnu.org/old-gnu/Manuals/flex-2.5.4/ for historical reference only.

Selection from the FSF shop

Free as in Freedom 2.0, by Richard Stallman

Helping the GNU Project and the Free Software Movement

Related Projects



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2 January 2019

Leaders and contributors

W. L. Estes Maintainer

Resources and communication

AudienceResource typeURI
Debian (Ref)https://tracker.debian.org/pkg/flex
Mailing Listhttps://lists.sourceforge.net/mailman/listinfo/flex-announce
Mailing Listhttps://lists.gnu.org/mailman/listinfo/help-flex/
Mailing Listhttps://lists.sourceforge.net/mailman/listinfo/flex-help
Mailing Listhttps://lists.gnu.org/mailman/listinfo/gnu-system-discuss/
Debian (Ref) (R)https://tracker.debian.org/pkg/flex-old
VCS Repository Webviewhttps://github.com/westes/flex/

Software prerequisites

Required to buildgcc
Required to usegettext for i18n support
Required to buildbash
Required to usehelp2man to generate the flex man page
Required to buildm4
Required to usetar, gzip, etc. for packaging of the source distribution
Required to usebison to generate parse.c from parse.y
Required to usetexinfo to build and test the flex manual
Required to useautoconf for handling the build system
Required to useindent for indenting the flex source the way we want it done
Required to useautomake for Makefile generation

This entry (in part or in whole) was last reviewed on 6 April 2020.


"Debian (Ref) (R)" is not in the list (General, Help, Bug Tracking, Support, Developer) of allowed values for the "Resource audience" property.

"Debian (Ref)" is not in the list (General, Help, Bug Tracking, Support, Developer) of allowed values for the "Resource audience" property.

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The copyright and license notices on this page only apply to the text on this page. Any software or copyright-licenses or other similar notices described in this text has its own copyright notice and license, which can usually be found in the distribution or license text itself.