1. Open source

2. Structure of a Lex file

3. Example of a Lex file

4. Using Lex with other programming tools
    Using Lex with parser generators  Lex and make 

5. See also

6. References

7. External links

Lex is commonly used with the yacc parser generator. Lex, originally written by Mike Lesk and Eric Schmidt^[3] and described in 1975,^[4][5] is the standard lexical analyzer generator on many Unix systems, and an equivalent tool is specified as part of the POSIX standard.{{Citation needed|date=July 2010}}

Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

Open source

Though originally distributed as proprietary software, some versions of Lex are now open source. Open source versions of Lex, based on the original AT&T code are now distributed as a part of open source operating systems such as OpenSolaris and Plan 9 from Bell Labs.{{clarify|date=February 2017|reason=This sentence needs to be more clear as it. seems to be more confusing than being informative}} 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 definition section defines macros and imports header files written in 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 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 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.

/*** Definition section ***/

%{

/* C code to be copied verbatim */

1. include

%}

/* This tells flex to read only one input file */

%option noyywrap

%%

[0-9]+ {

            /* yytext is a string containing the matched text. */            printf("Saw an integer: %s\", yytext);        }

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

%%

/*** C Code section ***/

int main(void)

    /* Call the lexer, then quit. */    yylex();    return 0;

}

If this input is given to flex, it will be converted into a C file, {{nowrap | lex.yy.c}}. This can be compiled into an executable which matches and outputs strings of integers. For example, given the input:

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 and parser generators, such as Yacc or Bison, are commonly used together. Parser generators use a formal grammar to parse an input stream, something which Lex cannot do using simple regular expressions (Lex is limited to simple finite state automata). {{Clarify|reason=mixes together grammars, regular expressions and FSMs into a confusing result|date=May 2012}}

It is typically preferable to have a (Yacc-generated, say) parser be fed a token-stream as input, rather than having it consume the input 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

See also

• Flex lexical analyser
• Yacc
• Ragel
• PLY (Python Lex-Yacc)
• Comparison of parser generators

References

External links

• Using Flex and Bison at Macworld.com

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