Editing Flex (lexical analyser generator) (section)

{{Short description|UNIX program for lexical analysis}}
{{Infobox software
| name                   = flex
| logo                   =
| developer              = [[Vern Paxson]]
| released               = around {{Start date and age|1987}}<ref name=releasehistorypage9>{{cite book |last=Levine |first=John |author-link=John R. Levine |title=flex & bison |isbn=978-0-596-15597-1 |publisher=O'Reilly Media |date=August 2009 |page=9 |quote=In about 1987, Vern Paxson of the Lawrence Berkeley Lab took a version of lex written in ratfor (an extended Fortran popular at the time) and translated it into C, calling it flex, for {{'}}''F''ast ''Lex''ical Analyzer Generator.{{'}} |url=https://books.google.com/books?id=3Sr1V5J9_qMC&q=%22In+about+1987%2C+Vern+Paxson%22}}</ref>
| latest release version = 2.6.4
| latest release date    = {{Start date and age|2017|5|6}}
| operating system       = [[Unix-like]]
| genre                  = [[Lexical analysis|Lexical analyzer]] generator
| license                = [[BSD license]]
| website                = {{URL|https://github.com/westes/flex}}
}}
'''Flex''' (fast [[lexical analyzer]] generator) is a [[free and open-source software]] alternative to [[lex programming tool|lex]].<ref>{{cite book |last1=Levine |first1=John R. |authorlink1=John R. Levine |last2=Mason |first2=Tony |last3=Brown |first3=Doug |title=lex & yacc |publisher=[[O'Reilly Media|O'Reilly]] |year=1992 |edition=2nd |isbn=1-56592-000-7 |page=279 |quote=A freely available version of lex is ''flex''. |url=https://books.google.com/books?id=YrzpxNYegEkC&q=%22a+freely+available+version+of+lex+is+flex%22}}</ref> 
It is a [[computer program]] that generates [[lexical analysis|lexical analyzer]]s (also known as "scanners" or "lexers").<ref>{{cite book |last1=Levine |first1=John R. | authorlink1=John R. Levine |last2=Mason |first2=Tony |last3=Brown |first3=Doug |title=lex & yacc |pages=1–2 |publisher=[[O'Reilly Media|O'Reilly]] |year=1992 |edition=2nd |isbn=1-56592-000-7 |url=https://books.google.com/books?id=YrzpxNYegEkC}}</ref><ref>{{cite book |last=Levine |first=John |author-link=John Levine |title=flex & bison |publisher=O'Reilly Media |date=August 2009 |isbn=978-0-596-15597-1 |page=304 |url=http://oreilly.com/catalog/9780596155988}}</ref>
It is frequently used as the lex implementation together with [[Berkeley Yacc]] [[parser generator]] on [[Berkeley Software Distribution|BSD]]-derived operating systems (as both <code>lex</code> and <code>yacc</code> are part of [[POSIX]]),<ref>{{cite web |url=http://bxr.su/o/usr.bin/lex/ |title=src/usr.bin/lex/ |website=BSD Cross Reference |author=OpenBSD |author-link=OpenBSD |date=2015-12-11 |access-date=2015-12-26 |quote="This is flex, the fast lexical analyzer generator."}}</ref><ref>{{cite web |url=http://mdoc.su/-/flex.1 |title=<code>flex(1)</code> |website=*BSD [[man page]]s}}</ref><ref>{{cite web |url=http://mdoc.su/-/yacc.1 |title=<code>yacc(1)</code> |website=*BSD [[man page]]s}}</ref> or together with [[GNU bison]] (a version of [[yacc]]) in [[Ports collection|*BSD ports]]<ref>{{cite web |url=http://ports.su/devel/bison |title=bison-3.0.4 – GNU parser generator |work=[[OpenBSD ports]] |date=2015-11-15 |access-date=2015-12-26}}</ref> and in Linux distributions.  Unlike Bison, flex is not part of the [[GNU Project]] and is not released under the [[GNU General Public License]],<ref>[https://westes.github.io/flex/manual/Is-flex-GNU-or-not_003f.html#Is-flex-GNU-or-not_003f Is flex GNU or not?], flex FAQ</ref> although a manual for Flex was produced and published by the Free Software Foundation.<ref>{{Cite web|url=https://ftp.gnu.org/old-gnu/Manuals/flex-2.5.4/flex.html|title=Flex - a scanner generator - Table of Contents - GNU Project - Free Software Foundation (FSF)|website=ftp.gnu.org|access-date=2019-12-05}}</ref>