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| {{Infobox software
| | The author's name is Christy. Some time in the past she chose to live in Alaska and her parents reside nearby. I am an invoicing officer and I'll be promoted quickly. She is really fond of caving but she doesn't have the time lately.<br><br>my web page - psychic phone readings ([http://www.caelumen.com/GOdum caelumen.com]) |
| | name = flex
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| | logo =
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| | developer = [[Vern Paxson]]
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| | latest release version = 2.5.37
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| | latest release date = {{release_date|2012|08|03}}
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| | operating system = [[Unix-like]]
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| | genre = [[Lexical analysis|Lexical analyzer]] generator
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| | license = [[BSD license]]
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| | website = {{URL|http://flex.sourceforge.net/}}
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| }}
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| '''Flex''' (fast [[lexical analyzer]] generator) is a [[free 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=2 |isbn=1-56592-000-7 |page=279 |quote=A freely available version of lex is ''flex''. |url=http://books.google.co.in/books?id=YrzpxNYegEkC&printsec=frontcover#v=onepage&q=%22a%20freely%20available%20version%20of%20lex%20is%20flex%22&f=false}}</ref> It is frequently used with the free [[GNU bison|Bison]] [[parser generator]]. Unlike Bison, flex is not part of the [[GNU Project]].<ref>[http://flex.sourceforge.net/manual/Is-flex-GNU-or-not_003f.html#Is-flex-GNU-or-not_003f Is flex GNU or not?], flex FAQ</ref>
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| ==History==
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| {{incomplete|section|date=November 2012}}
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| Flex was written in [[C (programming language)|C]] by [[Vern Paxson]] around 1987.<ref>{{Cite book | last=Levine | first=John | authorlink=John R. Levine | title=flex & bison | publisher = O'Reilly Media | date=August 2009 | url=http://oreilly.com/catalog/9780596155988 | isbn=978-0-596-15597-1 | 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=http://books.google.com/books?id=3Sr1V5J9_qMC&printsec=frontcover&dq=flex+and+bison&hl=en&sa=X&ei=4lipUJ76Kuu80QGGi4HwCg&ved=0CD0Q6AEwAA#v=snippet&q=%22In%20about%201987%2C%20Vern%20Paxson%22&f=false}}</ref> He was translating a [[Ratfor]] generator, which had been led by [[Jef Poskanzer]].<ref>[http://flex.sourceforge.net/manual/When-was-flex-born_003f.html#When-was-flex-born_003f When was flex born?], flex FAQ</ref>
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| == Example lexical analyzer ==
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| This is an example of a Flex scanner for the instructional programming language [[PL/0]].
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| The tokens recognized are: '<code>+</code>', '<code>-</code>', '<code>*</code>', '<code>/'</code>, '<code>=</code>', '<code>(</code>', '<code>)</code>', '<code>,</code>', '<code>;</code>', '<code>.</code>', '<code>:=</code>', '<code><</code>', '<code><=</code>', '<code><></code>', '<code>></code>', '<code>>=</code>'; | |
| numbers: <code>0-9 {0-9}</code>; identifiers: <code>a-zA-Z {a-zA-Z0-9}</code> and keywords: <code>begin</code>, <code>call</code>, <code>const</code>, <code>do</code>, <code>end</code>, <code>if</code>, <code>odd</code>, <code>procedure</code>, <code>then</code>, <code>var</code>, <code>while</code>.
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| <source lang="c">
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| %{
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| #include "y.tab.h"
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| %}
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| digit [0-9]
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| letter [a-zA-Z]
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| %%
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| "+" { return PLUS; }
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| "-" { return MINUS; }
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| "*" { return TIMES; }
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| "/" { return SLASH; }
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| "(" { return LPAREN; }
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| ")" { return RPAREN; }
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| ";" { return SEMICOLON; }
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| "," { return COMMA; }
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| "." { return PERIOD; }
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| ":=" { return BECOMES; }
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| "=" { return EQL; }
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| "<>" { return NEQ; }
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| "<" { return LSS; }
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| ">" { return GTR; }
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| "<=" { return LEQ; }
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| ">=" { return GEQ; }
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| "begin" { return BEGINSYM; }
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| "call" { return CALLSYM; }
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| "const" { return CONSTSYM; }
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| "do" { return DOSYM; }
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| "end" { return ENDSYM; }
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| "if" { return IFSYM; }
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| "odd" { return ODDSYM; }
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| "procedure" { return PROCSYM; }
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| "then" { return THENSYM; }
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| "var" { return VARSYM; }
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| "while" { return WHILESYM; }
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| {letter}({letter}|{digit})* {
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| yylval.id = strdup(yytext);
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| return IDENT; }
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| {digit}+ { yylval.num = atoi(yytext);
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| return NUMBER; }
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| [ \t\n\r] /* skip whitespace */
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| . { printf("Unknown character [%c]\n",yytext[0]);
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| return UNKNOWN; }
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| %%
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| int yywrap(void){return 1;}
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| </source>
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| The hand-written C code equivalent would likely be at least twice as many lines, and considerably harder to read.{{citation-needed|date=June 2013}}
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| ==Internals==
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| {{main|Lexical analysis}}
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| These programs perform character parsing, and tokenizing via the use of a [[deterministic finite automaton]] (DFA). A DFA is a theoretical machine accepting [[regular language]]s. These machines are a subset of the collection of [[Turing machine]]s. DFAs are equivalent to [[read-only right moving Turing machines]]. The syntax is based on the use of [[regular expressions]]. See also [[nondeterministic finite automaton]].
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| == Issues ==
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| === Time complexity ===
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| A Flex lexical analyzer usually has time complexity <math>O(n)</math> in the length of the input. That is, it performs a constant number of operations for each input symbol. This constant is quite low: [[GNU Compiler Collection|GCC]] generates 12 instructions for the DFA match loop. Note that the constant is independent of the length of the token, the length of the regular expression and the size of the DFA.
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| However, one optional feature of Flex can cause Flex to generate a scanner with non-linear performance: The use of the REJECT macro in a scanner with the potential to match extremely long tokens. In this case, the programmer has explicitly told flex to "go back and try again" after it has already matched some input. This will cause the DFA to backtrack to find other accept states. The REJECT feature is not enabled by default, and because of its performance implications its use is discouraged in the Flex manual.<ref name="performance">{{cite web|url=http://flex.sourceforge.net/manual/Performance.html |title=Performance - Lexical Analysis With Flex, for Flex 2.5.37 |publisher=Flex.sourceforge.net |date= |accessdate=2013-02-25}}</ref>
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| === Reentrancy ===
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| By default the scanner generated by Flex is not [[Reentrant (subroutine)|reentrant]]. This can cause serious problems for programs that use the generated scanner from different threads. To overcome this issue there are options that Flex provides in order to achieve reentrancy. A detailed description of these options can be found in the Flex manual.<ref>{{cite web|url=http://flex.sourceforge.net/manual/Reentrant.html |title=Reentrant - Lexical Analysis With Flex, for Flex 2.5.37 |publisher=Flex.sourceforge.net |date= |accessdate=2013-02-25}}</ref>
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| === Usage under non-Unix environments ===
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| Normally the generated scanner contains references to ''unistd.h'' header file which is [[Unix-like|Unix]] specific. To avoid generating code that includes ''unistd.h'', ''%option nounistd'' should be used. Another issue is the call to ''[[Not a typewriter|isatty]]'' (a Unix library function), which can be found in the generated code. The ''%option never-interactive'' forces flex to generate code that doesn't use ''isatty''. These options are detailed in the Flex manual.<ref>{{cite web|url=http://flex.sourceforge.net/manual/Code_002dLevel-And-API-Options.html |title=Code-Level And API Options - Lexical Analysis With Flex, for Flex 2.5.37 |publisher=Flex.sourceforge.net |date= |accessdate=2013-02-25}}</ref>
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| ===Using flex from other languages===
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| Flex can only generate code for [[C (programming language)|C]] and [[C++]]. To use the scanner code generated by flex from other languages a [[language binding]] tool such as [[SWIG]] can be used.
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| ==Flex++==
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| {{cleanup-section|date=November 2010}}
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| A similar lexical scanner for [[C++]] is '''flex++''', which is included as part of the flex package. At the moment, flex supports generating code only for C and C++. The generated code does not depend on any [[Runtime library|runtime]] or external [[Library (computing)|library]] except for a memory allocator ([[malloc]] or a user-supplied alternative) unless the input also depends on it. This can be useful in [[Embedded system|embedded]] and similar situations where traditional [[operating system]] or [[C standard library|C runtime]] facilities may not be available.
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| The flex++ classes and code require a C++ compiler to create lexical and pattern-matching programs. The flex++ generated C++ scanner includes the header file <code>FlexLexer.h</code>, which defines the interfaces of the two C++ generated classes.
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| == See also ==
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| {{Portal|Free software}}
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| *[[Lex programming tool|Lex]]
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| *[[Ragel]]
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| *[[Quex]]
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| *[[yacc]]
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| *[[GNU Bison]]
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| *[[Berkeley Yacc]]
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| ==References==
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| {{Reflist}}
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| ==Further reading==
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| *{{Cite book | last=Levine | first=John | authorlink=John R. Levine | title=flex & bison | publisher = O'Reilly Media | date=August 2009 | url=http://oreilly.com/catalog/9780596155988 | isbn=978-0-596-15597-1}}
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| *M. E. Lesk and E. Schmidt, ''LEX - Lexical Analyzer Generator''
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| *Alfred Aho, Ravi Sethi and Jeffrey Ullman, ''Compilers: Principles, Techniques and Tools'', Addison-Wesley (1986). Describes the pattern-matching techniques used by flex (deterministic finite automata)
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| ==External links==
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| * {{Official website|http://flex.sourceforge.net/}}
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| * [http://flex.sourceforge.net/manual/ Flex Manual]
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| * [http://www.quut.com/c/ANSI-C-grammar-l-1998.html ANSI-C Lex Specification]
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| * [http://www.jflex.de/ JFlex: Fast Scanner Generator for Java]
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| * [http://dinosaur.compilertools.net/ Brief description of Lex, Flex, YACC, and Bison]
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| {{DEFAULTSORT:Flex Lexical Analyser}}
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| [[Category:Free compilers and interpreters]]
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| [[Category:Compiling tools]]
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| [[Category:Parser generators]]
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| [[Category:Free software programmed in C]]
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The author's name is Christy. Some time in the past she chose to live in Alaska and her parents reside nearby. I am an invoicing officer and I'll be promoted quickly. She is really fond of caving but she doesn't have the time lately.
my web page - psychic phone readings (caelumen.com)