View source for Software:Lex

{{Short description|Standard UNIX utility}}
{{Infobox software
| name                   = Lex
| logo                   = 
| screenshot             = 
| screenshot size        = 
| caption                = 
| author                 = [[Biography:Mike Lesk|Mike Lesk]], Eric Schmidt
| developer              = 
| released               = {{Start date and age|1975}}
| latest release version = 
| latest release date    = 
| programming language   = [[C (programming language)|C]]
| operating system       = [[Software:Unix|Unix]], [[Unix-like]], [[Software:Plan 9 from Bell Labs|Plan 9]]
| platform               = Cross-platform
| genre                  = [[Command (computing)|Command]]
| license                = Plan 9: [[Software:MIT License|MIT License]]
| website                = 
}}
'''Lex''' is a [[Computer program|computer program]] that generates [[Lexical analysis|lexical analyzer]]s ("scanners" or "lexers").<ref>{{cite book |last1=Levine |first1=John R. | last2=Mason |first2=Tony |last3=Brown |first3=Doug |title=lex & yacc |pages=[https://archive.org/details/lexyacc00levi/page/1 1]–2 |publisher=O'Reilly |year=1992 |edition=2 |isbn=1-56592-000-7 |url=https://archive.org/details/lexyacc00levi|url-access=registration }}</ref><ref>{{Cite book | last=Levine | first=John | author-link=John Levine | title =flex & bison | publisher=O'Reilly Media | date=August 2009 | page=304 | url=http://oreilly.com/catalog/9780596155988 | isbn=978-0-596-15597-1}}</ref>

Lex is commonly used with the [[Software:Yacc|yacc]] parser generator. Lex, originally written by [[Biography:Mike Lesk|Mike Lesk]] and Eric Schmidt<ref>{{cite web|first1=M.E. |last1=Lesk |first2=E. |last2=Schmidt |title=Lex – A Lexical Analyzer Generator|archive-url=https://archive.today/20120728112736/http://dinosaur.compilertools.net/lex/index.html|archive-date=2012-07-28|url-status=dead|url=http://dinosaur.compilertools.net/lex/index.html|access-date=August 16, 2010}}</ref> and described in 1975,<ref>{{cite web|url=http://epaperpress.com/lexandyacc/download/lex.pdf |title=Lex – A Lexical Analyzer Generator |first1=M.E. |last1=Lesk |first2=E. |last2=Schmidt |date=July 21, 1975| work=UNIX TIME-SHARING SYSTEM:UNIX PROGRAMMER’S MANUAL, Seventh Edition, Volume 2B |publisher=bell-labs.com|access-date= Dec 20, 2011}}</ref><ref>{{cite journal |last1=Lesk |first1=M.E. |date=October 1975 |title=Lex – A Lexical Analyzer Generator |journal=Comp. Sci. Tech. Rep. No. 39 |location=Murray Hill, New Jersey |publisher=Bell Laboratories}}</ref> is the standard lexical analyzer generator on many [[Software:Unix|Unix]] systems, and an equivalent tool is specified as part of the [[POSIX]] standard.<ref>[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/lex.html The Open Group Base Specifications Issue 7, 2018 edition § Shell & Utilities § Utilities § lex]</ref>

Lex reads an input [[Stream (computing)|stream]] specifying the lexical analyzer and writes [[Source code|source code]] which implements the lexical analyzer in the [[C (programming language)|C programming language]].

In addition to C, some old versions of Lex could generate a lexer in [[Ratfor]].<ref>{{cite book|title=Lex & Yacc |url=https://archive.org/details/lexyacc00levi |url-access=registration |author1=John R. Levine |author2=John Mason |author3=Doug Brown |publisher=O'Reilly |date=1992 |isbn=9781565920002 }}</ref>

==Open source==
Although originally distributed as proprietary software, some versions of Lex are now [[Open-source software|open-source]]. Open-source versions of Lex, based on the original proprietary code, are now distributed with open-source operating systems such as [[Software:OpenSolaris|OpenSolaris]] and [[Software:Plan 9 from Bell Labs|Plan 9 from Bell Labs]]. One popular open-source version of Lex, called flex, or the "fast lexical analyzer", is not derived from proprietary coding.

==Structure of a Lex file==
The structure of a Lex file is intentionally similar to that of a yacc file: files are divided into three sections, separated by lines that contain only two percent signs, as follows:
*The '''definitions''' section defines [[Macro (computer science)|macros]] and imports header files written in [[C (programming language)|C]]. It is also possible to write any C code here, which will be copied verbatim into the generated source file.
*The '''rules''' section associates [[Regular expression|regular expression]] patterns with C statements. When the lexer sees text in the input matching a given pattern, it will execute the associated C code.
*The '''C code''' section contains C statements and functions that are copied verbatim to the generated source file. These statements presumably contain code called by the rules in the rules section. In large programs it is more convenient to place this code in a separate file linked in at [[Compiler|compile]] time.

==Example of a Lex file==
The following is an example Lex file for the flex version of Lex. It recognizes strings of numbers (positive integers) in the input, and simply prints them out.

<source lang="c">/*** Definition section ***/

%{
/* C code to be copied verbatim */
#include <stdio.h>
%}

%%
    /*** Rules section ***/

    /* [0-9]+ matches a string of one or more digits */
[0-9]+  {
            /* yytext is a string containing the matched text. */
            printf("Saw an integer: %s\n", yytext);
        }

.|\n    {   /* Ignore all other characters. */   }

%%
/*** C Code section ***/

int main(void)
{
    /* Call the lexer, then quit. */
    yylex();
    return 0;
}</source>

If this input is given to <code>flex</code>, it will be converted into a C file, {{nowrap | <code>lex.yy.c</code>}}. This can be compiled into an executable which matches and outputs strings of integers. For example, given the input:
 abc123z.!&*2gj6
the program will print:
 Saw an integer: 123
 Saw an integer: 2
 Saw an integer: 6

==Using Lex with other programming tools==

===Using Lex with parser generators===

Lex, as with other lexical analyzers, limits rules to those which can be described by regular expressions. Due to this, Lex can be implemented by a finite state automata as shown by the [[Chomsky hierarchy]] of languages. To recognize more complex languages, Lex is often used with parser generators such as [[Software:Yacc|Yacc]] or [[Software:GNU bison|Bison]]. Parser generators use a [[Formal grammar|formal grammar]] to parse an input stream.

It is typically preferable to have a parser, one generated by Yacc for instance, accept a stream of tokens (a "token-stream") as input, rather than having to process a stream of characters (a "character-stream") directly. Lex is often used to produce such a token-stream.

[[Scannerless parsing]] refers to parsing the input character-stream directly, without a distinct lexer.

===Lex and make===
[[Software:Make|make]] is a utility that can be used to maintain programs involving Lex. Make assumes that a file that has an extension of <code>.l</code> is a Lex source file. The make internal macro <code>LFLAGS</code> can be used to specify Lex options to be invoked automatically by make.<ref>
{{Cite journal
 |url=http://www.opengroup.org/onlinepubs/009695399/utilities/make.html
 |title=make
 |journal=The Open Group Base Specifications 
 |issue=6
 |publisher=The IEEE and The Open Group
 |year=2004
 |id=IEEE Std 1003.1, 2004 Edition
}}</ref>

==See also==
*Flex lexical analyser
*[[Software:Yacc|Yacc]]
*[[Software:Ragel|Ragel]]
*[[Software:PLY (Python Lex-Yacc)|PLY (Python Lex-Yacc)]]
*[[Software:Comparison of parser generators|Comparison of parser generators]]

==References==
{{Reflist}}

==External links==
*[http://www.mactech.com/articles/mactech/Vol.16/16.07/UsingFlexandBison/ Using Flex and Bison at Macworld.com]
* {{man|1|lex|Solaris}}
* {{man|1|lex|Plan 9}}

{{Unix commands}}

[[Category:Finite automata]]

{{Sourceattribution|Lex (software)|1}}